Genetic and Biological Insights from Long-Term Divergent Selection for Serum IGF-I Concentration in Angus Beef Cattle

1. Introduction

Insulin-like growth factor I (IGF-I) is a central component of the somatotropic axis and plays an important role in regulating growth, tissue development, nutrient utilization, and metabolism. Growth hormone stimulates IGF-I synthesis through the growth hormone receptor, and IGF-I exerts its effects through endocrine, paracrine, and autocrine pathways. Because circulating IGF-I is associated with physiological processes underlying economically important traits, researchers have long investigated its potential as a biological indicator of growth, feed efficiency, body composition, and reproductive performance in livestock.

The value of serum IGF-I as a selection criterion depends on more than its physiological importance. A useful indicator trait must show genetic variation, be measured reliably, and have favorable genetic relationships with traits in the breeding objective. Age, sex, nutrition, season, health status, and other environmental factors also influence circulating IGF-I concentration. Consequently, phenotypic associations between IGF-I and performance measures do not necessarily mean that selecting for IGF-I will produce favorable genetic change in economically important traits.

Although numerous studies reported relationships between circulating IGF-I and measures of growth, body composition, feed utilization, and reproduction, important questions remained unanswered. It was unclear whether serum IGF-I would respond to sustained directional selection and whether such selection would produce consistent correlated responses in economically important traits. Addressing these questions requires more than short-term observational studies. Most investigations of endocrine traits in livestock rely on measurements collected over limited periods or on associations observed within a single population. While these studies help identify relationships and generate hypotheses, they cannot fully demonstrate the long-term consequences of selection on an endocrine phenotype.

A long-term divergent selection experiment more directly tests the biological and genetic significance of a potential indicator trait. Establishing high and low lines from a common foundation population and maintaining selection across generations allows evaluation of direct response in the selected trait, correlated responses in other economically important traits, and changes in the biological systems associated with the selection criterion. Few livestock experiments have maintained divergent selection on a physiological phenotype for multiple decades, making such populations a unique resource for examining the consequences of selection and the genetic architecture of complex traits.

We initiated a divergent selection experiment for serum IGF-I concentration in Angus beef cattle at The Ohio State University in 1989. The experiment followed preliminary research linking serum IGF-I to postweaning performance and aimed to determine whether circulating IGF-I was heritable, responsive to selection, and genetically associated with economically important traits [1,2,3]. Over nearly three decades, the research program generated information on serum IGF-I concentration at various ages; body weight, weight gain, and skeletal growth; feed intake and efficiency; ultrasound and carcass traits; male and female reproductive performance; IGF-binding proteins; and molecular variation in genes of the growth hormone-IGF axis.

The results demonstrated that circulating IGF-I is a moderately heritable phenotype that responds to selection. However, correlated responses were more complex than would be expected if serum IGF-I were a simple proxy for growth or overall biological merit. Relationships with growth, efficiency, body composition, and reproduction depended on the trait evaluated, the animal’s age and sex, and the analytical approach used. Molecular and protein studies further indicated that circulating IGF-I reflects a broader regulatory network rather than the action of a single gene or pathway. In addition, the extensive pedigree and performance records generated throughout the experiment enabled estimation of direct and maternal genetic effects and characterization of genetic relationships between IGF-I and growth traits across data collected from 1989 through 2017 [4,5].

Herein we synthesize the genetic and biological findings from our long-term divergent selection experiment. We describe the scientific background and experimental design, evaluate the direct response to selection and correlated responses in production and reproductive traits, integrate evidence from candidate-gene and IGF-binding-protein studies, and assess the strengths and limitations of serum IGF-I as a selection criterion. More broadly, this review examines what a long-term selection experiment can reveal about the genetic architecture and biological regulation of complex traits, and the opportunities and challenges of using a circulating endocrine phenotype in livestock improvement programs.

2. Historical and Scientific Background

2.1. Development of Interest in Circulating IGF-I

In the 1980s, interest grew in insulin-like growth factor I (IGF-I) as an endocrine mediator of growth and development in domestic animals. IGF-I is an important component of the somatotropic axis, through which growth hormone and its receptor influence tissue growth, nutrient use, and metabolism. Circulating IGF-I was therefore considered a potentially useful indicator of genetic differences in growth traits. For beef cattle breeding, the central question was whether serum IGF-I merely reflected an animal’s age, nutritional status, and recent growth or whether it also represented a heritable phenotype that could respond to selection and produce predictable changes in economically important traits.

The research program that eventually led to divergent selection for serum IGF-I concentration in Angus beef cattle grew from interactions among faculty and graduate students in the Department of Animal Sciences at The Ohio State University in the late 1980s. Dr. Frank Simmen and Dr. Rosalia Simmen were growth physiologists/molecular biologists whose research and teaching emphasized the hormonal regulation of growth. Dr. Michael Bishop, a PhD student in animal breeding and genetics, became interested in IGF-I through coursework and discussions with the Simmens. Conversations among Michael Bishop, Frank and Rosalia Simmen, and Michael Davis brought together complementary expertise in growth physiology, endocrine measurement, quantitative genetics, and beef cattle breeding. These discussions provided the intellectual foundation for evaluating serum IGF-I first as a biological indicator of performance and subsequently as a selection criterion in beef cattle.

An early step was Bishop et al. [1], who examined the relationship between circulating IGF-I and postweaning performance in Angus beef cattle. That investigation helped establish that serum IGF-I could be measured in growing cattle and evaluated against performance traits under a structured postweaning testing program. More importantly, it sharpened the distinction between association and genetic response. A useful selection criterion must show sufficient genetic variation, be measured with acceptable repeatability and precision, and have favorable genetic relationships with traits in the breeding objective. Phenotypic correlations alone could not establish whether selection for IGF-I would improve growth, efficiency, carcass composition, or reproduction.

2.2. Selection Experiments in Mice as a Precedent

In the late 1980s, Australian researchers working with laboratory mice conducted innovative experiments involving selection for circulating IGF-I. Studies by Blair and colleagues documented variation in plasma IGF-I and its covariation with live weight, and subsequent experiments showed that divergent selection could alter circulating IGF-I [6,7]. Related developmental work by Siddiqui et al. [8] and replicated selection research by Baker et al. [9] further strengthened the evidence that circulating IGF-I possessed a genetic basis and could respond to selection. These studies supplied an important proof of principle that an endocrine phenotype could be treated as a quantitative trait rather than solely as a transient physiological measurement.

The mouse experiments also raised questions that only a livestock population could address. Beef cattle have longer generation intervals, smaller family sizes, and much higher phenotyping and selection-line maintenance costs than model species such as mice. Moreover, researchers needed to evaluate the consequences of altering serum IGF-I concentration across a broad range of economically important traits, including weight and weight gain, feed efficiency, carcass composition, and male and female reproduction. The mouse results therefore provided important background information for a cattle experiment but could not predict whether selection for serum IGF-I concentration would be biologically or economically favorable in beef production.

2.3. Rationale for Divergent Selection in Angus Beef Cattle

The Angus breed was well suited for a long-term selection experiment because of its economic importance to the U.S. beef industry, the availability of extensive performance and pedigree records, and the feasibility of maintaining a purebred research herd under relatively uniform management conditions. Divergent selection offered several advantages over analyses based solely on field data. Selection of high and low lines from a common foundation population was designed to generate increasing divergence in serum IGF-I concentrations while allowing correlated responses in production and reproductive traits to be evaluated across generations. In addition, repeated IGF-I measurements during a standardized postweaning performance test reduced the influence of short-term biological variation and provided a more reliable phenotype for selection.

We hypothesized that serum IGF-I concentration was sufficiently heritable to respond to selection and that long-term divergence would reveal its genetic and biological relationships with performance traits. The direction and biological significance of correlated responses were not assumed before the onset of the selection experiment. Although higher serum IGF-I concentration could be associated with accelerated growth or altered body composition, selection on a single endocrine trait could also produce neutral or unfavorable effects on efficiency and reproductive performance. Davis and Bishop [10] highlighted this concern, considering the potential consequences of single-trait selection for IGF-I in beef heifers and emphasizing the importance of monitoring a broad range of correlated traits rather than evaluating the experiment solely on divergence in IGF-I concentration.

The resulting selection experiment was therefore conceived as both a quantitative genetics study and a biological experiment. Its objective was not only to establish divergent lines for serum IGF-I concentration but also to determine how sustained selection on a circulating growth factor affected growth, body composition, feed efficiency, and reproductive performance in beef cattle. The combination of long-term selection, repeated endocrine measurements, and evaluation of correlated responses across generations created a valuable resource for physiological, quantitative genetics, and molecular genetics analyses.

3. Scope and Organization of the Review

3.1. Scope and Evidence Base

This review synthesizes findings from a long-term divergent-selection experiment at The Ohio State University involving serum insulin-like growth factor I (IGF-I) concentration in Angus beef cattle. The primary evidence base consists of publications from the selection experiment, beginning with studies that established the biological and quantitative genetic basis and extending through the final analysis of records collected from 1989 through 2017. The review also includes associated investigations of growth, feed efficiency, carcass composition, male and female reproduction, IGF-binding proteins, and polymorphisms in genes of the growth hormone-IGF axis.

The objective is not to provide an exhaustive review of the extensive literature on IGF-I biology in cattle. Rather, the experiment serves as a unique long-term selection model for evaluating the consequences of sustained genetic change in a circulating endocrine phenotype, providing insight into genetic variation, correlated responses, and biological regulation. We selectively include findings from other cattle populations and species when they provide historical context, independent support, or a meaningful contrast with results from the Ohio State experiment.

3.2. Organization of Evidence

The publications from the Ohio State experiment differ in the years represented, the number and types of animals analyzed, the traits measured, and the statistical methods used. We therefore synthesize the results by biological question rather than summarizing one study at a time, because many of our publications analyze overlapping populations, traits, and time periods and cannot be treated as independent sources of evidence. Early studies are particularly valuable for documenting the establishment of the selection lines, nongenetic sources of variation, and the initial demonstration of genetic variation in circulating IGF-I. Studies from the experiment’s middle phases characterize correlated responses and possible biological mechanisms, whereas later analyses benefit from larger datasets, greater statistical power, and a more complete view of long-term selection response. Differences among studies should also be interpreted in light of changes in analytical methods and available data over the experiment’s duration.

Section 4 describes the experiment’s design and conduct. Section 5 through 9 synthesize the evidence for direct response to selection, correlated changes in production and reproductive traits, molecular variation in the growth hormone-IGF axis, and differences in circulating IGF-binding proteins. Section 10 integrates the quantitative-genetic, physiological, and molecular findings, and Section 11 considers the principal conclusions, limitations, and priorities for future research.

3.3. Principles Used in Interpretation of Results

Four distinctions are important throughout this review. First, differences between the high and low lines show correlated responses to selection but do not, by themselves, estimate genetic correlations between IGF-I and other traits. Second, phenotypic or environmental associations, even when biologically meaningful, should not be assumed to reflect additive-genetic relationships. Third, the precision of maternal genetic effects and direct-maternal genetic correlations varies substantially across analyses, and estimates with large standard errors require cautious interpretation. Fourth, correlated responses observed in the selection lines may reflect the combined effects of pleiotropy, linkage disequilibrium, and changes in allele frequencies at multiple loci. The experiment was not designed to distinguish among these mechanisms. Maintaining these distinctions helps ensure that selection responses, genetic parameter estimates, and biological interpretations are evaluated within their appropriate inferential frameworks, thereby providing a more accurate understanding of the genetic architecture and biological consequences of variation in circulating IGF-I.

4. Design and Conduct of the Divergent Selection Experiment

4.1. Foundation Population and Establishment of Selection Lines

Divergent selection for serum IGF-I concentration was initiated in purebred Angus cattle at the Eastern Ohio Resource Development Center (EORDC), Belle Valley, Ohio, which was renamed the Eastern Agricultural Research Station (EARS) in 2004. The spring-calving herd was established in 1989 with 100 cows. Because serum IGF-I concentrations had not been measured in the foundation cows, 50 cows were randomly assigned to the high and low selection lines. In 1990, a comparable fall-calving herd was established with 100 cows, with 50 cows randomly assigned to each selection line. Including both spring- and fall-calving herds increased the number of animals available for selection and allowed us to evaluate selection responses under different seasonal conditions.

The two lines were maintained as distinct breeding populations. We selected animals in the high line to increase serum IGF-I concentration, whereas we selected animals in the low line to decrease the concentration. The foundation design aimed to establish comparable lines at the outset while preserving sufficient genetic variation to sustain selection over multiple generations. Because the experiment lacked a contemporaneous unselected control line, long-term evaluations focused on divergence between the high and low lines, supplemented by estimates of breeding values and genetic parameters. This design feature should be considered when interpreting selection responses, as temporal changes in management, assay methodology, and environmental conditions cannot be fully separated from genetic change without an unselected control line. The fall-calving herd was dispersed for financial reasons after the 2004 calf crop, and only the spring-calving herd remained through 2017.

4.2. Definition and Measurement of the Selection Criterion

Through the 2006 breeding season, the selection criterion was based on serum IGF-I concentration measured repeatedly during a 140-day postweaning performance test. Blood samples were obtained on days 28, 42, and 56 of the test, producing measurements designated IGF28, IGF42, and IGF56, respectively. The mean of these three concentrations was used as the primary postweaning IGF-I phenotype. Averaging repeated measurements reduced the influence of transient physiological and analytical variation and provided a more representative estimate of circulating IGF-I concentration during the early postweaning period. Selection decisions were based on adjusted residual values derived from this phenotype.

Serum IGF-I concentrations were initially measured by radioimmunoassay in the laboratory of Dr. Rosalia Simmen at The Ohio State University. After Dr. Simmen relocated to the University of Florida, Gainesville, Florida, USA, in 1989, her laboratory continued to perform the IGF-I assays for the selection experiment through 2003. Consequently, despite the change in institutional affiliation, the same investigator directed assay measurements throughout this period using consistent laboratory procedures, providing methodological continuity.

From 2004 through 2017, blood samples were collected on PrimeGRO™ IGF-I blood-spot cards, and IGF-I concentrations were determined using an enzyme-linked immunosorbent assay licensed to PrimeGRO Technologies (Corowa, NSW, Australia). Adopting the blood-spot system enabled continued large-scale phenotyping in the later years of the experiment but introduced discontinuity in assay methodology. Consequently, analyses spanning the entire selection experiment required adjustment for year and contemporary-group effects, and direct comparisons of absolute IGF-I concentrations across the assay transition should be interpreted cautiously.

Beyond assay-related variation, previous studies showed that several systematic nongenetic factors influenced serum IGF-I. Calf age, dam age, season, management, and climatic variables contributed to observed variation [2,11]. Accordingly, selection was based on residual IGF-I values adjusted for relevant fixed effects rather than unadjusted concentrations. This approach was essential because the objective was to alter genetic differences in serum IGF-I concentration while minimizing spurious selection resulting from age, management, or environmental variation.

4.3. Selection of Bulls and Replacement Females

During the active IGF-I selection phase through the 2006 breeding season, selection was conducted independently within the high and low lines. Each year, approximately four bulls with the highest residual IGF-I values and four bulls with the lowest residual values were retained as sires for the respective lines in the spring- and fall-calving herds. Yearling bulls were used for breeding, thereby maintaining a relatively short generation interval within the practical constraints of beef-cattle production. To limit family representation and preserve genetic diversity, no more than two sons from a given sire or dam were kept for breeding when feasible.

During this phase of the selection experiment, replacement heifers were selected using the same principle: the high line retained females with the highest residual IGF-I values, while the low line retained those with the lowest values. Approximately eight cows were removed from each line annually because of physical unsoundness, reproductive failure, or advanced age. They were replaced with approximately eight pregnant heifers with the highest residual IGF-I values in the high line and the lowest residual values in the low line, with records adjusted for calf age and dam age. All available heifers were bred, and selection was performed among those that conceived.

Matings among half-sibs and more closely related animals were avoided to slow the accumulation of inbreeding. These restrictions balanced selection intensity against the need to control inbreeding in the relatively small experimental populations. As in any long-term divergent-selection experiment involving relatively small populations, random genetic drift and accumulated inbreeding must be considered alongside selection when interpreting divergence between lines and correlated responses.

We relaxed selection pressure in the 2007 and 2008 breeding seasons. Selection for IGF-I concentration was discontinued, and cows and heifers were mated to either Angus or Simmental bulls. Therefore, the animals in the herd were no longer purebred Angus. Beginning with the 2009 breeding season, the selection criterion in the IGF-I selection lines was changed from serum IGF-I concentration to maintenance energy expected progeny difference (ME EPD), as provided by the Red Angus Association of America. Cows and heifers in the high IGF-I selection line were mated to bulls with high ME EPDs, and cows and heifers in the low IGF-I line were mated to bulls with low ME EPDs. We continued to measure IGF-I concentration in the serum throughout the remainder of the experiment, but this was no longer the selection criterion.

4.4. Calf Management and Postweaning Testing

Calf management reflected the seasonal production system used throughout the selection experiment. Spring-born calves remained with their dams until approximately seven months of age, whereas fall-born calves were weaned at approximately 140 days. These differences reflected seasonal management requirements and contributed to variation in early-life nutritional environments that could influence circulating IGF-I concentrations.

During the years in which postweaning testing was conducted, calves entered structured postweaning management and performance-testing programs after weaning. Bulls were tested at EORDC, whereas heifers were managed at the North Appalachian Experimental Watershed near Coshocton, Ohio. Management procedures and feeding programs evolved during the nearly three decades of the experiment, as expected in a long-term research program, although animals within a contemporary group were managed similarly.

Postweaning feeding programs changed over time, progressing from corn-soybean concentrate diets and, during some years, urea-treated corn silage to more conventional corn-soybean meal-based diets. Because nutritional status influences circulating IGF-I concentrations, accounting for year, season, location, and management effects was essential for both selection decisions and subsequent statistical analyses. Studies conducted under different dietary intake levels also helped determine whether relationships between IGF-I and feed efficiency were consistent across nutritional environments [12], strengthening confidence in the robustness of these associations.

Calves born from 2005 through 2017 were sold following weaning and therefore did not undergo a postweaning performance test. For these calves, available traits were limited to birth weight, weaning weight, and serum IGF-I concentration at weaning.

4.5. Traits Recorded and Expansion of the Research Program

In addition to repeated IGF-I measurements, the experiment generated an extensive set of performance records. Growth-related traits included birth weight, weaning weight, on-test weight, interim test weights collected during the postweaning evaluation period, off-test weight, and postweaning weight gain. Off-test hip height provided an indicator of skeletal growth. Additional records included feed intake and feed efficiency, ultrasound measures of backfat thickness and longissimus muscle area, carcass characteristics, and male and female reproductive performance. Serum IGF-I measurements obtained at weaning during the later years of the experiment also allowed evaluation of genetic continuity between preweaning and postweaning IGF-I concentrations. Collectively, these data provided opportunities to examine relationships between IGF-I and a broad range of economically important traits.

The breadth of phenotyping enabled the research program to expand beyond its primary objective of determining whether serum IGF-I would respond to selection. Early studies characterized environmental sources of variation and estimated genetic parameters for IGF-I. Subsequent investigations examined associations with feed efficiency, carcass composition, ultrasound traits, reproductive performance, and IGF-binding proteins. Candidate-gene studies involving IGF-I, GH, GHR, PIT1, and IGF2 extended the research from quantitative genetics into molecular investigations of the biological mechanisms underlying variation in IGF-I and related traits. Later analyses quantified accumulated response to selection across generations, and the complete dataset collected from 1989 through 2017 was ultimately used to estimate direct and maternal genetic effects and genetic correlations among IGF-I and performance traits. Measurement of serum IGF-I concentration at weaning began with calves born in the fall of 2003, because Australian research suggested that weaning IGF-I might be an effective selection criterion [13].

5. Direct Response to Selection and Genetic Architecture of IGF-I

5.1. Early Evidence of Heritability and Genetic Response

Our earliest quantitative-genetic analyses showed that postweaning serum IGF-I concentration was heritable and therefore a feasible selection target. Using records from the experiment’s initial generations, Davis and Simmen [3] estimated direct heritabilities of 0.42 ± 0.13, 0.53 ± 0.15, and 0.71 ± 0.16 for IGF28, IGF42, and IGF56, respectively, and 0.48 ± 0.13 for mean IGF-I concentration. Maternal heritability estimates were much smaller, ranging from 0.02 to 0.12, and maternal permanent environmental effects accounted for essentially none of the phenotypic variance in the postweaning IGF-I measures. Although the estimates were based on only 731 animals with mean IGF-I records and consequently had relatively large standard errors, they provided the first direct evidence that circulating IGF-I could respond to selection in beef cattle.

The early analysis also suggested negative genetic relationships between IGF-I and growth. Genetic correlations of the IGF-I measures with weaning weight, postweaning weights, and postweaning weight gain ranged from −0.21 to −0.54 and averaged −0.38, whereas the corresponding environmental correlations were positive and averaged 0.22. This contrast implied that animals in growth-favorable environments tended to have somewhat higher serum IGF-I, while additive-genetic differences associated with lower IGF-I might favor growth. The result supported the biological relevance of the selection experiment but also showed why phenotypic associations could not substitute for genetic-parameter estimates.

5.2. Accumulated Divergence and the Direction of Response

The clearest evidence of direct response came from Huang et al. [4], which included 2,507 calves born from 1989 through 2005. Estimated breeding values for mean IGF-I diverged steadily between the selection lines. In the 2005 calf crop, the mean direct breeding value was 66.92 ± 4.40 ng/mL in the high line and −40.82 ± 5.18 ng/mL in the low line, a difference of approximately 108 ng/mL (P < 0.001). Regression of breeding value on year indicated annual responses of 5.18 ng/mL in the high line and −3.76 ng/mL in the low line. Thus, response occurred in both directions but was not symmetrical, with the rate of increase in the high line exceeding the rate of decrease in the low line.

Heritability estimates from the same analysis remained moderate: 0.44 ± 0.07, 0.43 ± 0.07, and 0.35 ± 0.06 for IGF28, IGF42, and IGF56, respectively, and 0.48 ± 0.07 for mean IGF-I. These values were somewhat lower and more uniform than the previous estimates, particularly for IGF56, but they reinforced the central conclusion that substantial additive-genetic variation persisted after multiple generations of selection. The continuing response also indicated that selection had not exhausted the available genetic variation by 2005.

The moderate heritability estimates from the Ohio State selection experiment align with estimates from independent beef cattle populations. Estimates of heritability for circulating IGF-I concentration in Australian Angus cattle ranged from approximately 0.30 to 0.43 when IGF-I was measured from birth through approximately 22 months of age [13,14,15,16,17]. These estimates are similar to the 0.34 to 0.46 range obtained in the final analysis of the Ohio State population. Thus, although IGF-I concentration is affected by age, nutritional status, management, and assay procedures, studies in independent populations indicate that a substantial proportion of the phenotypic variation in circulating IGF-I concentration is genetically determined.

Correlated genetic trends in growth were trait-specific. Preweaning gain showed no significant trend in either line. In contrast, birth weight and postweaning gain declined significantly in the high-IGF-I line and increased significantly in the low-IGF-I line. These trends were consistent with the practical implication of the 1997 analysis—that selection for lower postweaning IGF-I could be accompanied by increased birth weight and postweaning gain—but they should not be interpreted as evidence that all growth traits responded similarly. Line trends are not identical to genetic correlations. Trends describe accumulated changes under each line’s selection history, whereas genetic correlations estimate the underlying additive genetic association between traits in the population under consideration.

5.3. Refinement of Genetic-Parameter Estimates

As additional generations and records accumulated, estimates of the genetic relationships between IGF-I and growth became more nuanced. The 2006 analysis [18] included 1,761 animals with IGF-I records and yielded direct heritability estimates of 0.44 ± 0.07, 0.51 ± 0.08, 0.42 ± 0.07, and 0.52 ± 0.08 for IGF28, IGF42, IGF56, and mean IGF-I, respectively. Maternal heritabilities ranged from 0.10 ± 0.05 to 0.20 ± 0.06, while maternal permanent environmental effects again were essentially zero. In contrast to the generally negative estimates reported in 1997, genetic correlations of IGF-I with weaning and postweaning weights ranged from 0.07 ± 0.12 to 0.32 ± 0.11. Averaged across IGF-I measures, genetic correlations were 0.14 with preweaning gain and 0.29 with postweaning gain. Environmental and phenotypic correlations with weights and gains were generally positive but small.

The final analysis of records collected from 1989 through 2017 provided the most comprehensive estimates from the experiment [5]. Direct heritabilities for the postweaning IGF-I measures ranged from 0.34 ± 0.07 for IGF56 to 0.46 ± 0.07 for mean IGF-I. Although modestly lower than some earlier estimates, these values remained firmly within the moderate range and confirmed that serum IGF-I retained an important additive-genetic component in the complete dataset. Direct heritabilities for the accompanying performance traits were also moderate, ranging from 0.32 ± 0.05 for birth weight to 0.39 ± 0.07 for off-test weight; estimates were 0.37 ± 0.07 for postweaning gain and 0.49 ± 0.07 for off-test hip height.

The long-term estimates clarify why conclusions about correlated growth response depend on the particular trait and age considered. Additive-genetic correlations of IGF-I with birth weight were consistently negative, ranging from −0.08 to −0.29. Correlations with weaning weight were small: −0.12 for IGF-I measured at weaning and 0.11 to 0.19 for the postweaning IGF-I measures. In comparison, genetic correlations of IGF-I with postweaning gain and off-test hip height ranged from 0.12 to 0.45. Thus, the complete dataset did not support a single, uniformly negative relationship between IGF-I and postweaning performance. Instead, it indicated modest, age-dependent genetic relationships that differed among birth weight, later weights, gain, and skeletal growth.

Studies have reported inconsistent relationships between circulating IGF-I concentration and growth traits. Genetic correlations reported in Australian Angus cattle were generally small or negative for birth weight, postweaning test weights, and average daily gain [15,16,17]. In contrast, the final analysis of the Ohio State population produced small positive genetic correlations between IGF-I concentration and most postweaning weight and growth traits, although the correlations with birth weight were negative. Collectively, these results indicate that serum IGF-I concentration is heritable but not a consistently strong genetic predictor of growth. Differences among studies may reflect the animal’s age and physiological status at blood collection, management and nutritional conditions, selection history, and the growth interval evaluated.

Differences among the 1997, 2006, 2011, and 2024 results likely reflect larger datasets, additional generations, alternative trait definitions, and changes in the selected population’s covariance structure. Therefore, the successive estimates should be viewed as stages in the experiment’s maturation rather than independent replications. The final heritability estimates were based on all relevant records collected from 1989 through 2017. However, the strongest direct evidence of response to selection for serum IGF-I remains the divergence documented through the 2005 calf crop by Huang et al. [4]. Subsequent records extended the evaluation of genetic variation but did not represent an uninterrupted continuation of selection for serum IGF-I, as the selection criterion shifted from serum IGF-I concentration to ME EPD beginning with the 2009 breeding season.

5.4. Serum IGF-I Concentration Across Ages and Maternal Components

Repeated sampling demonstrated that postweaning serum IGF-I concentration was a highly consistent genetic phenotype. In the final analysis, genetic correlations among IGF28, IGF42, IGF56, and mean IGF-I were 0.89 or greater. Selection on the average of the three postweaning samples therefore targeted a common underlying genetic signal while reducing the influence of short-term biological and analytical variation. The high correlations also explain why each postweaning measure produced the same broad conclusion concerning heritability, even though their individual estimates differed.

IGF-I measured at weaning was genetically related to the later measurements but was not identical to the postweaning selection phenotype. Its genetic correlation was 0.90 with IGF28, declined to 0.63 with IGF56, and was 0.69 with mean postweaning IGF-I. This pattern suggests substantial continuity across ages together with increasing differentiation as calves progressed through the postweaning test. Nutritional transition, age-dependent endocrine regulation, and the removal of maternal influence are plausible contributors to that differentiation.

Maternal genetic contributions to IGF-I were generally small to moderate and were estimated less precisely than direct effects. Maternal heritability was near zero for IGF-I measured at weaning (0.02 ± 0.11) and ranged from approximately 0.10 to 0.17 for the postweaning measures. Maternal permanent environmental effects were negligible for IGF-I. Several estimated direct–maternal genetic correlations for postweaning IGF-I were strongly negative, approaching −0.90, but their large standard errors make a strong biological interpretation unwarranted. These estimates are best treated as an indication that direct and maternal components may not be statistically separable with high precision in this population, rather than as firm evidence of biological antagonism.

5.5. Overall Interpretation

These studies provide strong evidence that circulating IGF-I is a moderately heritable endocrine phenotype that responds to selection over time in beef cattle. The divergence documented through 2005 constitutes the strongest direct demonstration, while the 2024 analysis confirms that substantial additive-genetic variation remained across the full 1989–2017 experiment. At the same time, correlated growth responses were neither universally favorable nor consistent across ages and analyses. Birth weight tended to be genetically associated with lower IGF-I, whereas relationships with postweaning gain and skeletal growth were generally modest and, in the final analysis, positive. Serum IGF-I concentration should therefore be interpreted as a biologically informative intermediate phenotype rather than as a substitute for body weight or growth rate. Its value as a selection criterion depends on the breeding objective and on simultaneous evaluation of feed efficiency, body composition, and reproduction, as discussed in the following sections.

6. Correlated Responses in Growth, Feed Efficiency, and Body Composition

6.1. Body Weight, Weight Gain, and Skeletal Growth

Because IGF-I plays central roles in nutrient use and tissue growth, the selection experiment aimed to determine whether divergence in circulating IGF-I produced corresponding changes in body weight and growth. Accumulated evidence indicates that the relationship between circulating IGF-I and growth is complex, varying by developmental stage, growth trait, and dataset, rather than showing a uniformly favorable or unfavorable association. An early analysis by Davis and Simmen [3] suggested negative additive genetic correlations between postweaning serum IGF-I concentration and weaning and postweaning weights and postweaning weight gain. However, with additional generations and larger datasets, these estimates weakened and became generally more positive. In the 2006 analysis, estimated additive genetic associations of the IGF-I measures with weaning and postweaning weights ranged from 0.07 to 0.32. Averaged across IGF-I measures, the correlations were 0.14 with preweaning gain and 0.29 with postweaning gain [18]. Thus, later analyses did not support the earlier indication that lower IGF-I consistently favored subsequent growth.

Huang et al. [4] provided a complementary perspective by estimating genetic trends in the divergently selected lines through the 2005 calf crop. Preweaning gain showed no significant trend in either line, whereas birth weight and postweaning gain decreased in the high IGF-I line and increased in the low IGF-I line. These correlated responses should not be interpreted as direct estimates of genetic correlations. Genetic trends represent cumulative outcomes of selection, drift, and realized covariance over generations, whereas genetic correlations quantify additive genetic associations within a population. Consequently, the observed line divergence demonstrates biologically meaningful change in growth without implying uniformly antagonistic genetic relationships between IGF-I and growth traits.

Analysis of the complete 1989-2017 dataset provided the most comprehensive assessment of these relationships and largely confirmed their trait-specific nature [5]. Direct heritability estimates were moderate for birth, weaning, and postweaning weights and postweaning gain, and were somewhat higher for off-test hip height. Additive-genetic correlations between IGF-I and birth weight were consistently negative (−0.08 to −0.29), whereas associations with weaning weight were generally weak, ranging from −0.12 for IGF-I measured at weaning to 0.11–0.19 for the postweaning IGF-I measures. Genetic correlations with postweaning weight gain and off-test hip height ranged from 0.12 to 0.45. In contrast, several environmental relationships between weaning IGF-I and body weights were appreciably positive. These findings support two conclusions. First, higher serum IGF-I concentration cannot be treated as a simple genetic proxy for greater weight at every age. Second, favorable environments may raise both circulating IGF-I and observed weight even when their additive genetic relationship is weak. Long-term selection for serum IGF-I concentration altered growth trajectories, but the direction and magnitude of response depended on developmental stage and the specific growth trait evaluated. Positive additive genetic correlations between IGF-I and off-test hip height suggest that circulating IGF-I may be more consistently related to skeletal development than to body weight traits, although this relationship was also moderate.

6.2. Feed Intake and Feed Efficiency

Feed efficiency was of particular interest because an endocrine indicator of weight gain relative to feed utilization would be valuable in genetic selection programs. Stick et al. [12] evaluated the association between serum IGF-I concentration and postweaning feed efficiency in crossbred cattle offered three dietary intake levels. Across nutritional treatments, serum IGF-I concentration was positively associated with both average daily gain and feed efficiency. A 1 ng/mL increase in serum IGF-I concentration was associated with a 0.00135 kg/d increase in average daily gain and a 0.0001 kg gain/kg feed increase. This controlled-intake study showed that circulating IGF-I provided information on growth performance and efficiency across different planes of nutrition. However, the results were based on contemporaneous phenotypic associations in crossbred cattle rather than comparisons of the Angus selection lines, and the study preceded the widespread adoption of residual feed intake (RFI) as a measure of feed efficiency.

Lancaster et al. [19] subsequently compared Angus bulls and heifers from the high and low IGF-I selection lines in two individual-feeding studies. One study used a roughage-based diet and the other a grain-based diet, allowing evaluation of growth performance, dry matter intake (DMI), feed conversion ratio, and residual feed intake (RFI). On the roughage-based diet, the selection lines did not differ in initial or final body weight, average daily gain, DMI, or feed conversion ratio. However, calves from the low line tended to have lower RFI than calves from the high line (−0.26 vs. 0.24 ± 0.31 kg/d; P < 0.10), suggesting a possible efficiency advantage because lower RFI indicates less feed consumed than expected based on maintenance requirements and growth. Under the grain-based diet, the selection line did not significantly affect growth, intake, or efficiency traits. Lancaster et al. [19] observed a weak line-by-sex interaction for RFI, with numerically lower RFI in low line bulls but no line differences in heifers. Neither weaning nor initial serum IGF-I concentration was correlated with feed conversion ratio or RFI in either study, although regression relationships differed by sex. These results provided limited evidence that serum IGF-I concentration differences altered feed efficiency, with only a tendency for reduced RFI in low-line calves under a roughage-based diet.

Although both studies addressed feed efficiency, they evaluated different biological aspects of the trait. The positive associations reported by Stick et al. [12] involved average daily gain and gain-to-feed ratio and may have reflected, at least in part, variation in growth rate across nutritional environments. In contrast, RFI measures feed intake after accounting for expected requirements for body size and growth. The Lancaster et al. [19] results provide little evidence that selection for lower postweaning serum IGF-I concentration consistently improved RFI. Any apparent advantage of the low line was modest, diet- or sex-dependent, and based on relatively small experimental groups. The results indicate that serum IGF-I is responsive to nutritional status and may accompany faster or more efficient gain under some conditions, but it was not a consistently reliable predictor of genetically superior feed efficiency across diets and sexes.

Interest in circulating IGF-I as an inexpensive indicator of feed efficiency led to practical implementation in Australia during the early 2000s. Moore et al. [17] measured plasma IGF-I in 6,520 seedstock Angus cattle during 2002–2004 and estimated a heritability of 0.35 and a genetic correlation with net feed intake (NFI) of 0.41 ± 0.21. NFI, also termed RFI, represents feed intake adjusted for body size and growth rather than the conventional feed conversion ratio. The positive correlation suggested that selection for lower IGF-I could reduce (i.e., improve) NFI. Wood et al. [20] compared predicted genetic and economic responses from direct RFI recording and indirect selection using IGF-I. This modeling study evaluated potential selection strategies rather than demonstrating realized response. A 2004 Meat & Livestock Australia report documented adoption of IGF-I testing and incorporation of industry IGF-I records into BREEDPLAN NFI estimated breeding values (EBVs) [21]. Thus, the proposed indicator progressed beyond research into genetic evaluation.

Later results in Australian Angus cattle substantially qualified the early expectations. Jeyaruban et al. [22] analyzed 9,216 IGF-I records and evaluated postweaning NFI (NFI-P) separately from feedlot-finishing NFI (NFI-F). Genetic correlations of IGF-I with NFI-P and NFI-F were 0.18 ± 0.11 and −0.14 ± 0.18, respectively. The genetic correlation between NFI-P and NFI-F was 0.65 ± 0.14, indicating that efficiency at these stages was related but not genetically identical. The expanded dataset included records used by Moore et al. [17], whose NFI analysis had pooled postweaning and feedlot records. Jeyaruban et al. [22] concluded that low correlations and differing directions limited IGF-I’s usefulness as a genetic indicator of NFI.

An Australian study of 2,216 Brahman and Tropical Composite steers provided further evidence of population and stage dependence [23]. Barwick et al. [2009] estimated genetic correlations of IGF-I with feedlot RFI of −0.38, −0.28, and −0.56 for IGF-I measured postweaning, at feedlot entry, and at feedlot exit, respectively. These negative estimates contrasted with the positive relationship in the earlier Australian Angus study. The authors suggested that differences in age and feedlot testing conditions could contribute to the discrepancy. Together with the limited RFI response in the Ohio State selection lines [19], these results do not support circulating IGF-I as a generally reliable substitute for individual feed intake. Its potential contribution to genetic evaluation must be validated for the population, sampling age, and production stage of interest.

The Australian comparisons also illustrate the importance of considering correlated changes in body composition. Moore et al. [17] reported positive genetic correlations between IGF-I and subcutaneous and intramuscular fat, whereas cattle from the low IGF-I selection line in the Ohio State experiment generally showed greater marbling and, in some comparisons, greater external fat [28,29]. Therefore, associations between circulating IGF-I and feed efficiency and body composition should not be interpreted as universal causal relationships.

Texas A&M studies provide a complementary comparison of early promise and subsequent limitations. In a conference abstract, Brown et al. [24] reported positive phenotypic correlations between initial serum IGF-I and RFI in 112 Braunvieh-crossbred steers (r = 0.22) and 62 Bonsmara bulls (r = 0.38) fed roughage-based diets. Lower IGF-I was therefore associated with greater efficiency in these groups. In contrast, Gomez et al. [25] reported that initial serum IGF-I concentration was unrelated to RFI or feed:gain ratio in a four-year study of Brangus heifers, with 114 to 119 heifers tested annually on a high-roughage diet. Both reports were conference abstracts, and their phenotypic associations do not establish genetic correlations or predict response to selection. Nevertheless, the contrast supports IGF-I’s population-dependent usefulness and aligns with the limited RFI response observed in the Ohio State selection lines [19].

Carstens et al. (2007) raised concerns about using IGF-I as an indicator trait in genetic evaluation for feed efficiency [26]. The authors noted inconsistent IGF-I–RFI relationships across breed types, weaker correlations in newer Australian results, and possible age-related differences, and at that time recommended against using circulating IGF-I as an indicator trait for RFI in genetic evaluation programs. They also emphasized the need for direct feed-intake phenotypes and further evaluation of relationships with growth, product quality, and reproduction. This recommendation documents contemporary caution about implementation rather than evidence that IGF-I lacks biological relevance.

Further work using the Ohio State cattle extended the investigation from endocrine indicators to cellular mechanisms of feed efficiency. Lancaster et al. [27] evaluated hepatic mitochondrial function in low- and high-RFI animals selected from 29 Angus heifers from the divergent IGF-I lines and, in a separate trial, 119 Santa Gertrudis steers. Low-RFI cattle had greater mitochondrial acceptor control ratios in both trials, suggesting greater control of oxidative phosphorylation by ADP. Other respiration measures were less consistent, and proton-leak kinetics did not differ between RFI groups. These findings identify mitochondrial regulation as a potential contributor to variation in feed efficiency. However, the mitochondrial comparisons were based on RFI values rather than a demonstrated response to IGF-I selection and do not establish that circulating IGF-I caused the mitochondrial differences.

6.3. Ultrasound Measures and Carcass Composition

Carcass evaluation provided an additional test of whether divergent selection for serum IGF-I concentration altered the partitioning of growth between muscle and fat. Davis and Simmen [28] analyzed IGF-I records from 1,283 bulls and heifers and carcass records from 452 nonbreeding bulls. Because conclusions from carcass studies can depend on the endpoint used for comparison, carcass traits were evaluated at age-, fat-, and weight-constant endpoints. At the age-constant endpoint, additive genetic correlations of IGF-I with backfat thickness, longissimus muscle area, hot carcass weight, marbling score, quality grade, and yield grade averaged −0.26, 0.19, −0.04, −0.53, −0.45, and −0.27, respectively. These relationships suggested that genetic factors associated with lower serum IGF-I favored overall fat deposition rather than the selective accumulation of intramuscular fat. Although marbling and quality grade were improved, backfat thickness also increased, resulting in less favorable yield grades.

Real-time ultrasound was used to evaluate fat deposition and muscling in live bulls and heifers. Davis et al. [29] analyzed 636 calves with ultrasound measurements of backfat thickness and longissimus muscle area collected on day 56 and 140 of the postweaning performance test. Heritability estimates were low to moderate, ranging from 0.17 to 0.28 for backfat thickness and from 0.20 to 0.36 for longissimus muscle area, depending on measurement time and whether age or weight was used as a covariate. When both IGF-I and ultrasound traits were adjusted for age, genetic correlations of IGF-I with backfat thickness were 0.02 on day 56 and 0.20 on day 140, whereas correlations with longissimus muscle area were −0.08 and 0.23, respectively. After adjusting ultrasound measurements for weight, the corresponding correlations were 0.05 and −0.07 for backfat thickness and −0.22 and −0.04 for longissimus muscle area. The disappearance, and in some cases reversal, of the positive day 140 associations after weight adjustment suggested that IGF-I was associated primarily with overall growth rate and body size rather than with independent genetic differences in fat deposition or muscling.

The later line-comparison experiments were broadly consistent with this interpretation [19]. Ultrasound estimates of longissimus muscle area and its change during the feeding period were similar in the two selection lines. Responses in backfat thickness were less consistent. In the grain-based study, the high-IGF-I line showed greater final backfat thickness and backfat gain, whereas in the roughage-based study the difference between lines was a nonsignificant numerical trend. These findings provide little evidence that divergent selection for serum IGF-I concentration consistently changed body composition. Instead, differences in fat deposition appeared to depend on dietary and environmental conditions.

6.4. Overall Interpretation

The growth, efficiency, ultrasound, and carcass studies demonstrate that circulating IGF-I was biologically informative but was not a simple indicator of overall performance. Selection produced substantial divergence in serum IGF-I concentration, while correlated responses in body weight and gain varied by age and analytical period. Associations with conventional measures of gain efficiency were generally stronger than those with RFI, which were small and inconsistent across diets and sexes. Carcass results suggested that lower IGF-I was associated with increased marbling and quality grade, but also with increased external fat and less favorable yield grades. Ultrasound analyses further demonstrated that relationships with fatness and muscling depended in part on the comparison endpoint, differing when traits were evaluated at constant age rather than constant weight. Consequently, the value of serum IGF-I as a selection criterion depends on the breeding objective and on simultaneous consideration of growth rate, feed intake, mature size, fat distribution, and carcass value. The experiment’s most important contribution in this area may therefore be the demonstration that selection on serum IGF-I concentration can generate multiple correlated responses across growth, efficiency, and body composition traits, some favorable and others unfavorable. As a result, IGF-I’s biological significance cannot be reduced to a simple favorable-versus-unfavorable characterization.

7. Reproductive Consequences of Divergent Selection for IGF-I

Reproduction was an important component of the correlated response research because circulating IGF-I participates in the endocrine and metabolic pathways that support sexual development, gametogenesis, conception, and pregnancy. Management and other environmental factors strongly influence most reproductive traits, which typically are lowly heritable. Consequently, our reproductive studies focused on two complementary sources of evidence: direct comparisons of reproductive performance between the high- and low-IGF-I selection lines and estimates of the genetic relationships between circulating IGF-I and specific reproductive traits. These approaches did not always yield the same conclusions. Therefore, interpreting the long-term results requires distinguishing observed correlated responses from underlying genetic associations.

7.1. Male Reproductive Performance

Yilmaz et al. [30] provided the first direct comparison of male reproductive traits in the divergent selection lines. Scrotal circumference, percentage of motile sperm cells, and percentage of morphologically normal sperm cells did not differ significantly between high- and low-IGF-I line yearling bulls. However, regressions on the individual postweaning IGF-I measurements indicated several linear or curvilinear relationships. In particular, scrotal circumference and the percentage of normal sperm cells tended to increase with IGF-I to an intermediate range and then decline at higher concentrations. Thus, the initial line comparison did not demonstrate a simple correlated response in bull reproductive performance, but it suggested that circulating IGF-I was related to specific components of breeding soundness.

A subsequent multiple-trait analysis placed these observations in a quantitative genetics context [31]. Estimated direct heritabilities were 0.51 ± 0.13 for scrotal circumference, 0.08 ± 0.12 for percentage of motile sperm cells, and 0.47 ± 0.07 for percentage of normal sperm cells. The additive-genetic correlations of mean IGF-I with these traits were 0.35 ± 0.11, 0.43 ± 0.32, and 0.00 ± 0.03, respectively. The estimates therefore suggested that selection for increased IGF-I could produce favorable correlated changes in scrotal circumference and sperm motility, although the correlation with motility was imprecise and no corresponding genetic relationship was evident for sperm morphology. The apparent contrast between these genetic correlations and the nonsignificant line differences reported earlier is not contradictory. Line means reflect the realized response in the animals measured at that stage of the experiment, whereas genetic correlations estimate shared additive genetic variation using the pedigree and all available records.

7.2. Female Reproductive Performance

Our first detailed analysis of female reproductive traits found little evidence of line differences [32]. Pregnancy rate was nearly identical in the high and low selection lines, and age at puberty did not differ significantly between the lines. High-line heifers reached puberty an estimated 5.3 ± 6.4 d earlier and first calved 4.02 ± 2.18 d earlier than low-line heifers. Neither difference was statistically significant, although the contrast in age at first calving approached significance. Within the combined population, residual correlations between age at puberty and postweaning IGF-I measurements were negative, with relationships for IGF28, IGF56, and mean IGF-I reaching significance. Thus, heifers with higher IGF-I concentrations generally reached puberty earlier, even though the selection lines had not yet diverged enough to produce a significant difference in mean age at puberty.

Although the earlier study detected only modest phenotypic differences, the larger analysis by Zhang et al. [33] provided stronger evidence of favorable genetic and phenotypic relationships between IGF-I and female reproductive performance. Direct heritability estimates were 0.11 ± 0.06 for conception rate, 0.13 ± 0.06 for calving rate, and 0.35 ± 0.20 for age at first calving. Additive genetic correlations between mean IGF-I and these traits were 0.32, 0.43, and −0.40, respectively, where all correlations were significantly different from zero. High-line heifers had a higher calving rate than low-line heifers (64.24 vs. 56.76%; P = 0.05). In addition, more high line females calved during the first 21 d of the calving season (37.26 vs. 33.10%; P < 0.001), increasing both rebreeding opportunity and calf age at weaning. These findings suggest that higher IGF-I concentrations are favorably associated with reproductive success and earlier attainment of key reproductive milestones.

Studies in other cattle populations also support a relationship between circulating IGF-I and reproductive function, although the associations are not uniformly predictive. In postpartum Holstein cows, Gobikrushanth et al. [34] found that cows with higher serum IGF-I concentrations early in lactation had a greater probability of conceiving at the first insemination. However, serum IGF-I concentration was not consistently associated with pregnancy probability throughout the breeding period. These findings support the somatotropic axis’ biological involvement in reproductive recovery and fertility but, like the results from the Ohio State selection lines, do not indicate that reproductive performance consistently improves with increasing circulating IGF-I concentrations. Comparisons should also be made cautiously because postpartum dairy cows differ substantially from growing beef cattle and mature beef cows in nutritional status and physiological demands.

7.3. Genetic Relationships and Their Interpretation

Interpreting our reproductive studies requires careful attention to how we defined and analyzed traits. In the earlier variance-component study, we coded calving outcome as 1 for females that calved and 100 for those that did not. Consequently, the genetic association between mean IGF-I and calving outcome was estimated as −0.41 ± 0.16; the negative sign reflected a favorable relationship between higher IGF-I and calving success [31]. In a later analysis, conception and calving rates were defined so that positive genetic correlations directly represented favorable reproductive outcomes [33]. Once these differences in trait definition are recognized, both studies support the same biological conclusion that increased serum IGF-I concentration was associated with improved reproductive performance.

Heritability estimates varied considerably among reproductive traits. Scrotal circumference and sperm morphology were moderately to highly heritable, whereas sperm motility, conception rate, and calving rate exhibited relatively low heritability. Age at first calving was more heritable, although its estimate had a relatively large standard error. These results suggest that direct selection for female fertility traits would likely produce relatively slow genetic progress. In contrast, a correlated endocrine phenotype such as IGF-I may provide additional information for selection if favorable genetic associations are maintained. However, several parameter estimates, particularly those for sperm motility, age at first calving, and traits measured in relatively small subsets of animals, were imprecise and should be interpreted cautiously.

Our reproductive studies should be viewed as successive analyses of long-term selection lines rather than independent experiments. Later investigations incorporated additional years of data, larger numbers of animals, and in some cases refined trait definitions and statistical models. Consequently, the consistency of the observed relationships across analyses, together with results from larger, more comprehensive studies, provides a stronger basis for interpretation than the number of publications reporting a particular association.

7.4. Overall Interpretation

Our reproductive analyses provide no evidence that long-term selection for increased serum IGF-I concentration impaired fertility in the Angus population. The earliest line comparisons were largely neutral, but subsequent genetic analyses and expanded reproductive data sets revealed generally favorable associations between IGF-I and reproductive performance. Higher IGF-I was associated with larger scrotal circumference, improved sperm motility, greater conception and calving success, earlier age at first calving, and a greater likelihood of calving early in the breeding season. The responses were not uniform. Sperm morphology showed little evidence of a genetic association with IGF-I, age at puberty did not differ significantly between selection lines, and several parameter estimates had large standard errors. Serum IGF-I appears to be a biologically relevant indicator with generally neutral-to-favorable reproductive consequences in this experiment, but it should not be viewed as a stand-alone measure of fertility or as evidence of a direct causal effect on every reproductive outcome.

8. Molecular Genetics of the GH–IGF Axis

In addition to the quantitative-genetic studies, we investigated DNA variation in genes associated with the growth hormone–IGF axis. We wanted to know if individual genetic markers could help explain variation in circulating IGF-I or its relationships with economically important traits. The work progressed from gene mapping and polymorphism discovery to association analyses in the divergent IGF-I selection lines. Because the studies targeted biologically plausible candidate genes, they should be interpreted as tests of particular loci rather than as a comprehensive description of the genetic architecture underlying the selection response.

8.1. Gene Mapping and Identification of Polymorphisms

Bishop et al. [35] provided an early molecular foundation by using somatic-cell hybrid analysis to assign bovine IGF1 to chromosome 5 and investigating restriction fragment length variation at the locus. This work established a genomic location for the gene encoding the selection phenotype. Subsequent screening identified two single-strand conformation polymorphism (SSCP) alleles in the 5′-flanking region of bovine IGF1 [36]. Identifying variation near a candidate regulatory region provided a marker for further study but did not, by itself, establish an effect on gene expression, circulating IGF-I, or animal performance.

We also examined variation in the growth hormone receptor gene, GHR. Reports by Ge et al. [37,38] described polymorphisms detected by denaturing gradient gel electrophoresis in exon 10 and in the promoter region. The subsequent rapid communication characterized single-nucleotide polymorphisms in exon 10, which encodes the receptor’s intracellular domain [39]. These discoveries supplied sequence-level markers for association testing. At this stage, their importance was in identifying and characterizing segregating variation, rather than demonstrating that the variants accounted for divergence between the selection lines.

8.2. Associations of the IGF1 Marker with Growth and Serum IGF-I

In our 6th World Congress on Genetics Applied to Livestock Production proceedings paper, we reported the effects of an IGF-I marker on serum IGF-I concentration, growth, carcass, and male reproductive traits [40]. The subsequent journal paper characterized the IGF1 promoter-region polymorphism as a T (allele A)-to-C (allele B) transition and developed a PCR–restriction fragment length polymorphism assay for genotyping [41]. In 760 Angus calves, animal-model analyses associated the BB genotype with greater weight gain during the first 20 d after weaning. Across selection lines, BB calves gained 3.88 ± 1.53 kg more than AA calves during that interval. Within the low line, BB calves also had greater early postweaning gain and on-test weight; no significant marker associations were detected within the high line.

The marker was not significantly associated with the postweaning serum IGF-I measurements in the across-line analysis [41]. Thus, the marker did not show an association with growth through a detectable change in circulating IGF-I. Its regulatory location made a functional effect plausible, but the findings remained marker associations requiring validation in other populations.

8.3. Growth Hormone and Growth Hormone Receptor Associations

Ge et al. [42] extended association testing to polymorphisms in the GH and GHR genes, using animal models that accounted for pedigree relationships and selection line effects. The GHR promoter marker designated RP(1), an A-to-G transition in the liver-specific promoter region, was associated with IGF-I concentration on d 42 and with mean postweaning IGF-I. Significance testing included a Bonferroni adjustment. In contrast, the GH polymorphisms examined did not show significant associations with serum IGF-I or growth traits.

The GHR result provided a potential molecular link between variation in the growth hormone receptor and the circulating endocrine phenotype under selection. A regulatory effect on receptor expression was biologically plausible, but the association analysis did not identify a causal variant or the underlying mechanism. Ge et al. [42] therefore emphasized the need for independent validation before drawing broader conclusions regarding line divergence or application in other cattle populations.

8.4. Related Candidate Genes and Supporting Ovine Research

The candidate-gene approach was subsequently extended beyond IGF1 and GHR to additional components of the somatotropic axis and related growth-regulatory pathways. These studies examined both upstream regulators of growth hormone secretion and signaling genes with potential effects on growth and circulating IGF-I concentration.

The candidate-gene approach was broadened to PIT1, the pituitary transcription-factor gene also known as POU1F1. Zhao et al. [43] examined polymorphisms spanning intronic and exonic regions in 416 Angus calves from the divergent lines. Despite the biological rationale for considering this regulator of pituitary function, the tested polymorphisms were not significantly associated with growth or carcass traits. These null findings are an important part of the review: a gene’s physiological role does not ensure that the variants segregating in a particular population will provide useful predictors of performance. They do not, however, exclude effects of other variants or effects in other populations.

A second extension of the molecular research examined IGF2, a growth-factor gene closely related to, but distinct from, the endocrine IGF-I pathway that served as the selection criterion. Zhao et al. [44] evaluated an AciI polymorphism in exon 9 of IGF2 in 52 Angus calves from the divergent IGF-I selection lines. Sequencing identified a T-to-G substitution. AA, AB, and BB frequencies were 0.10, 0.33, and 0.57, respectively. The genotypic frequencies did not differ significantly between the selection lines (P = 0.54). In analyses combining the lines, no significant genotype associations for growth traits or serum IGF-I concentrations were identified. The association with gain during the 20-d interval between weaning and the beginning of the postweaning test was suggestive (P = 0.09), with least-squares means of 18.81 ± 3.68 kg for AA and 12.68 ± 1.61 kg for BB. Within the high line, BB animals had higher serum IGF-I on d 56 than AA animals (P = 0.01).

This study directly extended molecular investigations within the selection lines to IGF2, although IGF-II itself was not the selection criterion. The results should be regarded as preliminary because of the small sample size, the low frequency of AA animals, and the number of traits and within-line analyses examined. The within-high-line finding alone did not establish that marker effects differed between lines or explain the genetic basis of serum IGF-I divergence. Although the authors proposed possible applications in marker-assisted selection, the nonsignificant growth results and suggestive early-gain association did not establish predictive utility, and the report called for additional genotyping.

Beyond the Angus selection lines, a related study in sheep identified two nucleotide substitutions and putative promoter elements in the 5′-flanking region of the ovine IGF1 gene [45]. Although not a direct test of the cattle selection experiment, the work reflected our broader interest in regulatory variation of IGF1 across livestock species and supported continued investigation of noncoding variation as a potential source of phenotypic differences. However, it did not involve the Angus herd or demonstrate effects on bovine serum IGF-I concentration or performance.

8.5. Overall Interpretation

These studies added molecular detail to the quantitative genetics selection experiment without reducing its outcome to a single gene or marker. Their contributions fall into three distinct categories: mapping and discovering variation, estimating marker–trait associations, and proposing biological explanations for those associations. The first two were addressed directly; functional causation generally remained a hypothesis. The IGF1 growth association, GHR association with circulating IGF-I, and null PIT1 findings illustrate why these categories should remain separate.

Another conclusion is that no single candidate gene provided a compelling explanation for the divergence that accumulated between the high- and low-IGF-I selection lines. Several markers were associated with specific phenotypes, but the evidence generally did not show major differences in marker frequencies between lines, consistent effects across analyses, or functional variants that accounted for a substantial proportion of the line divergence. The long-term response to selection therefore was consistent with a polygenic model rather than with large effects attributable to a single candidate locus.

Interpretation of results also depends on the population and analysis. In selected lines, an association may reflect a functional variant, linkage disequilibrium with another locus, or aspects of genetic background and sampling. Pedigree-based models and adjustment for selection line strengthen inference, but do not replace replication or functional testing. Likewise, a significant result in one line and a nonsignificant result in the other does not by itself demonstrate that marker effects differ between lines; that claim requires a direct test of the difference. Multiple publications arising from the same selection population should not be counted as independent validation.

The principal value of our molecular research was therefore to identify testable candidates and connect the long-term phenotypic resource with investigations of specific genes. Practical application requires confirmation of effect size, direction, and predictive value beyond the discovery population. This interpretation also leaves room for regulation beyond DNA sequence at the assayed loci, including the IGF-binding-protein relationships discussed below.

9. IGF-Binding Proteins and Biological Regulation of the Circulating IGF System

The pronounced response of serum IGF-I to divergent selection raised an important mechanistic question: did selection also alter the circulating proteins that bind IGF-I and regulate its transport and biological availability? Most circulating IGF-I is associated with high-affinity IGF-binding proteins (IGFBPs), which extend its half-life and modulate its delivery to tissues and interaction with cellular receptors. Consequently, differences in serum IGF-I concentrations do not necessarily translate directly into proportional differences in IGF-I action. Evaluation of circulating IGFBPs therefore provided an opportunity to determine whether selection-induced changes in serum IGF-I were accompanied by broader alterations in the circulating IGF system.

9.1. Study Design and Identification of Circulating IGFBPs

Pagan et al. [46] evaluated serum samples from 68 fall-born calves in the 1992 calf crop and 84 fall-born calves in the 1999 calf crop. The two cohorts represented contrasting stages of the experiment: the 1992 calves were sampled near its beginning, whereas the 1999 calves had undergone approximately 3.5 generations of selection. Relative serum IGFBP abundance was measured by [125I] IGF-I Western ligand blotting. Samples collected on d 28, 42, and 56 of the postweaning test were examined in the 1992 cohort. Because IGFBP levels did not differ among these sampling times, the 1999 study used the d-56 samples.

Four IGF-binding species were detected, with estimated molecular masses of 38 to 42, 34, 30, and 24 kDa. Immunoblotting identified the 34-kDa species as IGFBP-2, whereas previous bovine studies supported assigning the 38- to 42-kDa bands to IGFBP-3 variants. We could not definitively identify the 30- and 24-kDa species and therefore reported them by molecular mass rather than attributing them to specific IGFBPs.

9.2. Selection-Line Comparisons

Significant high- versus low-line differences were not detected for any IGFBP species in either cohort. The absence of line differences in 1992 was consistent with the lack of divergence in serum IGF-I and growth traits at that stage of the experiment. In 1999, however, the biological context differed substantially. High-line calves averaged approximately 123 ng/mL more serum IGF-I than low-line calves across d 28, 42, and 56 (P < 0.0001). They were also heavier at several postweaning time points and had greater off-test weight and hip height. Nevertheless, relative levels of the four circulating IGFBP species remained similar between lines, and we detected no line-by-sex interaction for any IGFBP.

Thus, total circulating IGF-I diverged substantially without a detectable parallel change in the relative abundance of the serum IGFBPs measured by ligand blotting. This negative result is mechanistically informative. It indicates that serum IGF-I and associated growth traits can diverge markedly without detectable changes in the relative abundance of the major circulating IGFBP species identified by ligand blotting. It does not, however, exclude changes in absolute IGFBP concentration that the semiquantitative method could not resolve, post-translational modification or proteolysis, IGFBP activity within tissues, or other components of the somatotropic axis. These observations suggest that the pronounced response of serum IGF-I to divergent selection was driven primarily by factors influencing IGF-I production or clearance rather than by major alterations in the relative abundance of circulating IGFBPs.

Genome-wide evidence from other cattle populations supports the conclusion that circulating IGF-I concentration is controlled by a distributed genetic architecture rather than variation at a single major locus. In Holstein cows, Gobikrushanth et al. [34] identified associations between serum IGF-I concentration and 37 single-nucleotide polymorphisms distributed across 10 autosomes. Although the breed, physiological stage, and timing of measurement differed from those in the Ohio State experiment, the distribution of associations across multiple chromosomes supports the interpretation that the candidate-gene and IGF-binding-protein associations identified in the selection lines represent components of a broader polygenic regulatory network.

9.3. Sex Differences and Relationships with Growth

Sex was consistently associated with IGFBP-2. Heifers had higher relative serum IGFBP-2 levels than bulls in both 1992 (P < 0.0001) and 1999 (P < 0.0005). On the other hand, bulls had substantially higher serum IGF-I concentrations, were heavier, and grew faster. In 1992, bulls also had higher levels of the 38–42- and 24-kDa species, but these differences were not reproduced in 1999. Interpreting the sex effects requires caution because bulls and heifers were fed at different locations and under different feeding programs. The recurrence of the IGFBP-2 difference in both cohorts, together with comparable observations reported elsewhere, nevertheless suggests a biologically meaningful sex-related pattern. Beyond these sex-related differences, relationships between IGFBPs and growth-related traits were not entirely consistent across cohorts.

Relationships between IGFBPs and the traits measured in our study also differed between cohorts. In 1992, the 38–42- and 24-kDa species exhibited weak but statistically significant residual correlations with serum IGF-I (r = 0.11 and -0.11, respectively), whereas neither IGFBP-2 nor the 30-kDa species was significantly related to IGF-I. However, these associations were not detected in the 1999 cohort, providing little evidence for a consistent relationship between circulating IGFBPs and serum IGF-I concentration. IGFBP-2 was, however, negatively correlated with birth weight, weaning weight, all postweaning weights, off-test weight, and off-test hip height. It was not significantly correlated with postweaning gain. None of the four IGFBP species was associated with ultrasound backfat thickness or longissimus muscle area at d 56 or 140 of the postweaning test.

The negative relationships between IGFBP-2 and body weight are consistent with a potential role for this binding protein in regulating bovine growth, although they do not establish causation. Notably, IGFBP-2 was associated with body weights measured throughout development but not with postweaning gain, suggesting that it may reflect cumulative differences in body size or earlier growth processes rather than average daily gain during the test period. The biological basis for these associations remains uncertain because sex, nutritional management, endocrine status, and other correlated factors may also have contributed to the observed relationships.

9.4. Mechanistic Interpretation

The IGFBP study helps explain why circulating IGF-I should not be interpreted as a direct measure of IGF-I bioactivity at target tissues. Selection clearly altered circulating IGF-I concentrations yet did not produce corresponding line differences in the IGFBP profile. This finding suggests that the physiological mechanisms responsible for the divergent phenotype were not mediated primarily through the major circulating IGFBPs detected by ligand blotting. Instead, responses may have involved changes in IGF-I production or clearance, growth hormone signaling, hypothalamic regulation of the somatotropic axis, or tissue-level features of the IGF system that the study did not assess directly. These possibilities are biologically plausible, but were not tested in our study.

Comparing the 1992 and 1999 cohorts cautions against assuming that relationships observed at one stage of a selection experiment reflect stable regulatory mechanisms. The earlier cohort detected associations between specific IGFBPs and serum IGF-I, but these associations were not reproduced after continued selection. This inconsistency could reflect evolving genetic backgrounds, cohort or management differences, sampling variation, or changes in the relative importance of other regulatory pathways. The findings suggest that correlations among circulating IGF system components may change during long-term selection and should be interpreted cautiously when inferring biological mechanisms.

9.5. Overall Interpretation

Pagan et al. [46] extended the selection experiment from quantitative genetics and candidate-gene associations to the circulating protein environment in which IGF-I acts. The principal conclusion was that divergence in serum IGF-I did not require detectable divergence in the relative levels of circulating IGFBPs. IGFBP-2 nevertheless emerged as a potentially informative indicator because it differed consistently between sexes and was negatively associated with body weight measures in the later cohort. These findings suggest that the biological consequences of selection depend on a regulatory network rather than on serum IGF-I concentration alone. More broadly, they reinforce a central conclusion of this review: selection can substantially alter a heritable endocrine phenotype, while its relationships with growth, body composition, and other physiological traits remain trait-specific, context-dependent, and mechanistically complex.

10. Integrated Biological and Genetic Interpretation

The long-term divergent-selection experiment showed that circulating serum IGF-I is an endocrine phenotype that selection can change in beef cattle, but it also showed that the biological significance of selection on a physiological indicator cannot be assessed from the direct response alone. Over nearly three decades, the experiment linked quantitative-genetic change in IGF-I to correlated responses in growth, feed utilization, body composition, reproduction, candidate-gene variation, and the circulating IGF-binding-protein system. These findings support a broader view of IGF-I as an intermediate phenotype that reflects heritable variation in the somatotropic axis while integrating genetic and physiological influences on performance. However, its phenotypic consequences depend on age, sex, nutrition, genetic background, and the specific trait under consideration.

10.1. What the Selection Experiment Established

The strongest conclusion from the divergent-selection experiment is that postweaning serum IGF-I concentration contains substantial additive genetic variation and can respond to artificial selection over many years. Moderate heritability estimates were obtained at successive stages of the experiment despite changes in sample size, generations represented, and analytical models [3,4,5,18]. Sustained divergence in estimated breeding values through the 2005 calf crop provided direct evidence of continued response to selection in both lines. Subsequent analyses of records through 2017 indicated that considerable additive genetic variation in serum IGF-I concentration remained available for selection. The high genetic correlations among IGF28, IGF42, IGF56, and their mean further indicate that the repeated postweaning measurements represented a largely common phenotype. Averaging the three measurements was therefore a defensible selection criterion because it reduced short-term sampling and assay variation without altering the biological trait under selection.

Beyond demonstrating heritable variation and long-term selection response, the experiment also established that circulating IGF-I is not simply a surrogate for body weight or growth rate. Genetic relationships differed among birth weight, preweaning performance, postweaning gain, and skeletal growth, and their estimated magnitudes changed as the dataset expanded. Genetic trends also differed among traits in the selected lines. These findings are not contradictory; rather, they demonstrate that a circulating hormone can share some genetic determinants with performance traits while retaining a distinct genetic basis. The divergence achieved in IGF-I therefore reflects selection on a biologically meaningful endocrine phenotype rather than merely an indirect consequence of selection on body size or growth.

10.2. Correlated Responses Across Economically Important Traits

Correlated responses in growth and efficiency traits were generally modest and often depended on the analytical approach and population studied. Selection for lower serum IGF-I concentration was accompanied by increased birth weight and postweaning gain in the genetic trend analysis through 2005, whereas the final covariance analysis showed negative genetic relationships with birth weight but modest positive relationships of postweaning IGF-I with postweaning gain and hip height [4,5]. Feed-efficiency results were similarly conditional. Under controlled feed intake, serum IGF-I was positively associated with gain and conventional feed efficiency in crossbred cattle, but comparisons of the Angus selection lines provided little evidence of a consistent advantage in residual feed intake [12,19]. Therefore, neither selection direction produced an unequivocal improvement in growth and efficiency across ages, sexes, and diets.

Body-composition responses also depended on how traits and endpoints were defined. Carcass analyses suggested that lower serum IGF-I concentration was genetically associated with greater fat deposition, including greater marbling, but also with greater external fat and less favorable yield grade when animals were compared at a common age [28]. Ultrasound analyses provided limited evidence for an independent relationship between IGF-I and fatness or muscling after adjusting for body weight [29], and selection-line comparisons were not fully consistent across diets [19]. These findings illustrate an important principle of selection on physiological traits: an apparently favorable response in one component of carcass value may be accompanied by an unfavorable response in another.

The reproductive evidence was more reassuring. Divergent selection for IGF-I did not impair male or female fertility. Instead, later analyses identified generally neutral-to-favorable relationships between IGF-I and scrotal circumference, sperm motility, conception and calving success, age at first calving, and calving timing, although not every trait or line comparison was significant. The lack of a uniform response is biologically plausible because fertility integrates endocrine function, growth, energy balance, management, and reproductive opportunity. The evidence supports IGF-I as one informative component of reproductive biology rather than a stand-alone predictor of fertility.

10.3. Mechanistic Insights from Molecular and Protein Studies

The molecular studies reinforced the conclusion that genetic variation in serum IGF-I concentration is biologically complex and unlikely to be explained by a single major locus. We identified or evaluated variants in IGF1, GH, GHR, IGF2, and PIT1, and some were associated with serum IGF-I measurements or growth. However, these associations were not consistently replicated across traits, populations, or selection lines, and most studies could not distinguish causal variants from markers in linkage disequilibrium with other loci. The absence of a single marker explaining line divergence is consistent with the moderate heritability, sustained response, and complex correlated responses observed in the quantitative-genetic analyses. The evidence supports a polygenic architecture involving multiple regulatory and structural genes, each contributing a relatively small proportion of the phenotypic variation.

The IGF-binding-protein study provided a complementary physiological perspective. By 1999, serum IGF-I concentration differed markedly between the selection lines, but the relative abundance of circulating IGFBP species measured by ligand blotting did not [46]. Thus, the direct response in IGF-I did not require a detectable, parallel divergence in the circulating IGFBPs. This finding helps constrain, but does not identify, the underlying mechanisms. Differences in IGF-I synthesis, secretion, or clearance; growth hormone signaling; receptor abundance or sensitivity; tissue-specific binding-protein activity; and other unmeasured regulatory processes remain plausible explanations for the divergence in IGF-I between the selection lines. The consistent sex difference and negative associations of IGFBP-2 with body weight further indicate that biologically important variation within the IGF system can occur independently of selection-line divergence and may reflect regulatory mechanisms not captured by circulating IGF-I concentrations alone.

10.4. Value and Limitations of Serum IGF-I as a Selection Criterion

Serum IGF-I has several attributes desirable in an indicator trait. It can be measured in young animals of both sexes, is moderately heritable, and is related biologically and genetically to growth, body composition, and reproduction. Selection based on IGF-I could therefore alter aspects of performance before all economically important outcomes are expressed. The experiment nevertheless provides little support for using serum IGF-I concentration as an isolated, universally favorable selection criterion. The direction and magnitude of correlated responses varied across traits, ages, diets, sexes, and analytical periods. Moreover, total circulating IGF-I concentration does not fully capture tissue responsiveness, local IGF signaling, or the amount of free, biologically available IGF-I. A breeding objective focused on IGF-I alone could consequently produce unintended responses or fail to improve the specific production trait of interest.

For these reasons, IGF-I is best used as an intermediate phenotype within a multi-trait breeding objective. If breeders used IGF-I in genetic evaluation, they would need to define its economic and biological value relative to direct measures of growth, mature size, feed intake, carcass merit, and reproductive performance. Contemporary genomic prediction could also evaluate whether IGF-I records add accuracy beyond routinely measured phenotypes and dense genomic information. Such use would require validation in populations independent of The Ohio State University Angus selection lines and careful standardization of animal age, nutritional status, sampling procedures, and assay methodology. The evidence suggests that IGF-I is most valuable not as a direct breeding objective, but as a biologically informative indicator that may contribute to a broader understanding and prediction of genetic merit. The Australian research reinforces this requirement. Initial commercial use was followed by estimates indicating weak or inconsistent genetic relationships with NFI [17,21,22,23]. Moderate heritability of an endocrine indicator is insufficient to establish its value for indirect selection. A useful indicator trait must also have a sufficiently strong and reproducible genetic relationship with the target trait.

10.5. Experimental Strengths and Limitations

The principal strength of this research project is its duration and breadth. The project maintained the original selection-line structure and collected related records over nearly three decades, even as the selection criterion changed. Few livestock experiments have combined a prolonged phase of divergent selection on a circulating endocrine phenotype with nearly three decades of related genetic, performance, reproductive, molecular, and physiological data collection. The common pedigree and management framework allowed successive studies to address related questions in the same biological resource. Repeated IGF-I sampling, bidirectional selection, and pedigree-based analyses further strengthened inferences regarding heritability and response.

Several limitations should temper interpretation of our divergent selection results. First, the experiment lacked an unselected control line. Consequently, divergence between the high- and low-selection lines provides strong evidence of differential response, but changes in either line cannot be fully separated from environmental trends, genetic drift, or changes that might have occurred in the foundation population without selection. Second, the published studies frequently used overlapping animals and generations. They represent successive analyses and complementary trait evaluations, not independent replications of the selection response. Third, some specialized experiments and molecular studies had modest sample sizes, limiting power and increasing the possibility that nominal associations were sample-specific. Multiple trait and marker tests also require caution, especially where adjustment for multiple comparisons or external validation was limited. Fourth, later observations, especially birth weight, weaning weight, and IGF-I concentration at weaning, reflect both the original line history and subsequent differences in sire breed and ME EPD, as selection criteria shifted from IGF-I to ME EPD in the final years of the research project.

The experiment’s long duration created additional sources of heterogeneity. Selection criteria, management practices, locations, analytical methods, and statistical approaches evolved over time, and some sex-specific cohorts were managed separately. These unavoidable features of a multidecade livestock experiment were addressed through contemporary groups, fixed effects, covariates, and related model adjustments where possible, but they complicate direct comparisons across study periods. In addition, long-term selection is expected to alter allele frequencies and genetic covariance structures, meaning that parameter estimates from early and late phases describe related but not identical populations. Consequently, differences among estimates may reflect biological and population changes in addition to sampling variation. The magnitude of correlated responses in our study may not be generalizable to other breeds or production systems.

10.6. Broader Implications

The broader lesson is that selection on an endocrine phenotype can generate sustained genetic change without producing a simple, proportional suite of responses in downstream traits. Physiological indicators occupy an intermediate position between genes, the environment, and economically important phenotypes. Their heritability may make them attractive selection targets, but regulatory networks and environmental context filter their biological effects. The Ohio State selection experiment illustrates both the promise and difficulty of using physiological indicator traits. Serum IGF-I captured heritable biological variation, yet no single gene, circulating binding protein, or performance outcome fully explained its response.

These findings remain relevant in the genomic era. High-throughput genotyping can improve prediction, but it does not eliminate the need for carefully measured intermediate phenotypes or long-term experimental populations. Archived samples, detailed pedigrees, and repeated trait records from selection experiments can help connect genomic predictions with biological mechanisms. At the same time, the results caution against assuming that an endocrine biomarker associated with performance in one population or environment will have the same predictive value elsewhere. Validation across breeds, management systems, and developmental stages is essential before practical implementation.

In summary, divergent selection substantially and persistently changed serum IGF-I concentration, while correlated changes in growth, feed efficiency, carcass composition, and reproduction were generally modest, heterogeneous, and biologically nuanced. Molecular and IGFBP studies supported a complex, polygenic, and context-dependent interpretation rather than a single-pathway explanation. The experiment’s greatest contribution is therefore not identifying IGF-I as a universally favorable selection criterion but demonstrating how long-term selection on a physiological phenotype can reveal the trait’s genetic architecture, biological integration, and practical limitations.

11. Conclusions and Future Research Directions

The Ohio State University divergent-selection experiment showed that serum IGF-I concentration is a moderately heritable endocrine phenotype that responds substantially to artificial selection in beef cattle. Divergence between the high- and low-IGF-I lines through the 2005 calf crop provided the clearest evidence of a direct response, while analyses of the more extensive dataset collected through 2017 confirmed that important additive genetic variation remained. Repeated postweaning measurements indicated that circulating IGF-I was largely consistent throughout the testing period, and weaning IGF-I was genetically related to subsequent postweaning concentrations. These findings establish circulating IGF-I as a biologically meaningful quantitative trait rather than merely a transient indicator of nutritional or physiological status.

The correlated responses were more complex than the direct IGF-I response. Relationships with body weight, growth, skeletal development, feed efficiency, carcass composition, and reproduction varied with age, sex, diet, trait definition, and analytical approach. Selection for either higher or lower IGF-I did not produce a uniformly favorable combination of economically important traits. Reproductive performance was generally maintained, but evidence for consistent improvements in growth, efficiency, or carcass value was limited. Consequently, serum IGF-I should not be viewed as an independent breeding objective. Instead, its greatest potential lies as an intermediate phenotype incorporated within a broader multi-trait breeding framework that also includes direct measures of growth, feed intake and efficiency, carcass merit, mature size, and reproductive performance.

Molecular and physiological investigations further demonstrated that variation in circulating IGF-I arises from a complex regulatory system. Candidate variants in IGF1, GH, GHR, IGF2, and PIT1 showed genetic associations with IGF-I and related traits but did not identify a single major locus responsible for the divergent selection response. Similarly, serum IGF-I concentration diverged without corresponding divergence in the circulating IGFBPs. These findings support a model in which IGF-I concentration reflects the integrated effects of numerous genetic and physiological processes rather than a single regulatory mechanism. The sustained selection response, moderate heritability, and variable correlated responses best fit a polygenic architecture involving multiple components of the somatotropic axis and related biological pathways.

Several questions remain unresolved, offering opportunities for future research. Modern genomic approaches could characterize the genetic architecture underlying long-term selection response more comprehensively than the candidate-gene era allowed. Genome-wide analyses, genomic prediction, and functional genomic studies may help identify the many loci and regulatory pathways contributing to variation in circulating IGF-I. High-density SNP genotyping and whole-genome sequence data could evaluate patterns of selection and linkage disequilibrium in chromosomal regions surrounding the IGF1 locus and other components of the somatotropic axis. Haplotype-based analyses may prove especially informative because long-term divergent selection could have altered the frequencies of multi-marker chromosomal segments even when individual polymorphisms exhibit only modest effects. Integration of genomic information with epigenomic approaches, including DNA methylation profiling, may further identify regulatory mechanisms that influence IGF1 expression and circulating IGF-I concentration without requiring changes in coding sequence. Additional work is also needed to distinguish the biological consequences of circulating IGF-I concentration from those of tissue-specific IGF signaling, receptor sensitivity, and IGF-binding-protein activity.

Equally important is evaluating the broader applicability of these findings. The Ohio State University Angus selection lines provide a uniquely informative experimental population, but independent confirmation in other cattle populations is needed before generalizing the conclusions. Future studies should also determine whether IGF-I measurements contribute predictive value beyond routinely collected phenotypes and dense genomic information. Such evaluations will require careful attention to animal age, physiological status, nutritional management, and assay standardization.

Our divergent selection experiment showed that circulating IGF-I is a heritable, biologically relevant endocrine phenotype that selection can substantially alter. However, the experiment showed that the biological and economic consequences of that variation are neither simple nor universal. The collective evidence supports using IGF-I as an informative component of a broader biological and genetic framework rather than as a stand-alone indicator of animal merit. The experiment provides a valuable model for understanding endocrine regulation of complex traits and reminds us that physiological biomarkers are most useful when interpreted within the larger network of traits and mechanisms that influence animal performance.

Table 1. Principal publications from The Ohio State University IGF-I research program and their major contributions.

Study	Primary focus	Animals or records	Principal contribution
Bishop et al. [1]	IGF-I and postweaning performance	Angus cattle in a structured performance test	Established the feasibility and biological rationale for evaluating circulating IGF-I as a selection criterion.
Davis et al. [2]	Nongenetic effects	Early generations of the divergent-selection herd	Identified age, sex, season, management, and other systematic effects requiring adjustment in genetic analyses.
Davis and Simmen [3,18]	Genetic parameters	731 animals initially; 1,761 in the expanded analysis	Demonstrated moderate heritability of postweaning IGF-I and documented changing estimates of its genetic relationships with growth as the dataset expanded.
Huang et al. [4]	Selection response	2,507 calves born 1989–2005	Documented sustained divergence in breeding values for mean IGF-I and correlated trends in growth traits.
Davis and Simmen [5]	Long-term genetic analysis	Records collected 1989–2017	Confirmed moderate IGF-I heritability, strong genetic consistency across sampling ages, and trait-specific relationships with weight, gain, and hip height.
Stick et al. [12]; Lancaster et al. [19]	Feed efficiency	Crossbred cattle at three intake levels; Angus selection-line bulls and heifers	Showed that IGF-I can accompany gain and conventional feed efficiency but did not consistently predict residual feed intake across diets and sexes.
Davis and Simmen [28]; Davis et al. [29]	Carcass and ultrasound traits	Carcass records and serial ultrasound measures	Found context-dependent associations with fatness and muscling; weight adjustment weakened many apparent relationships.
Yilmaz et al. [30,31,32]; Zhang et al. [33]	Male and female reproduction	Bulls, heifers, and cows from the divergent lines	Reproductive performance was generally maintained; some genetic and phenotypic associations favored higher IGF-I, especially for female fertility and timing.
Bishop et al. [35]; Ge et al. [36,37,38,39,40,41,42]; Zhao et al. [43,44]	Candidate genes and markers	Angus selection lines and supporting molecular studies	Identified variants in IGF1, GH, GHR, IGF2, and PIT1, but no single major locus explained the selection response.
Pagan et al. [46]	IGF-binding proteins	152 fall-born calves from the 1992 and 1999 crops	Serum IGF-I diverged without parallel line divergence in circulating IGFBP abundance; IGFBP-2 varied with sex and body weight.

Note: IGF-I is insulin-like growth factor I; IGFBP is insulin-like growth factor-binding protein. The table highlights the principal peer-reviewed outputs discussed in this review, not every abstract or proceedings contribution from the program.

Table 1. Principal publications from The Ohio State University IGF-I research program and their major contributions.

Study	Primary focus	Animals or records	Principal contribution
Bishop et al. [1]	IGF-I and postweaning performance	Angus cattle in a structured performance test	Established the feasibility and biological rationale for evaluating circulating IGF-I as a selection criterion.
Davis et al. [2]	Nongenetic effects	Early generations of the divergent-selection herd	Identified age, sex, season, management, and other systematic effects requiring adjustment in genetic analyses.
Davis and Simmen [3,18]	Genetic parameters	731 animals initially; 1,761 in the expanded analysis	Demonstrated moderate heritability of postweaning IGF-I and documented changing estimates of its genetic relationships with growth as the dataset expanded.
Huang et al. [4]	Selection response	2,507 calves born 1989–2005	Documented sustained divergence in breeding values for mean IGF-I and correlated trends in growth traits.
Davis and Simmen [5]	Long-term genetic analysis	Records collected 1989–2017	Confirmed moderate IGF-I heritability, strong genetic consistency across sampling ages, and trait-specific relationships with weight, gain, and hip height.
Stick et al. [12]; Lancaster et al. [19]	Feed efficiency	Crossbred cattle at three intake levels; Angus selection-line bulls and heifers	Showed that IGF-I can accompany gain and conventional feed efficiency but did not consistently predict residual feed intake across diets and sexes.
Davis and Simmen [28]; Davis et al. [29]	Carcass and ultrasound traits	Carcass records and serial ultrasound measures	Found context-dependent associations with fatness and muscling; weight adjustment weakened many apparent relationships.
Yilmaz et al. [30,31,32]; Zhang et al. [33]	Male and female reproduction	Bulls, heifers, and cows from the divergent lines	Reproductive performance was generally maintained; some genetic and phenotypic associations favored higher IGF-I, especially for female fertility and timing.
Bishop et al. [35]; Ge et al. [36,37,38,39,40,41,42]; Zhao et al. [43,44]	Candidate genes and markers	Angus selection lines and supporting molecular studies	Identified variants in IGF1, GH, GHR, IGF2, and PIT1, but no single major locus explained the selection response.
Pagan et al. [46]	IGF-binding proteins	152 fall-born calves from the 1992 and 1999 crops	Serum IGF-I diverged without parallel line divergence in circulating IGFBP abundance; IGFBP-2 varied with sex and body weight.

Note: IGF-I is insulin-like growth factor I; IGFBP is insulin-like growth factor-binding protein. The table highlights the principal peer-reviewed outputs discussed in this review, not every abstract or proceedings contribution from the program.

Table 2. Direct heritability estimates for postweaning serum IGF-I concentration in successive analyses of the Ohio State divergent selection experiment.

Study	Dataset	IGF28	IGF42	IGF56	Mean IGF-I
Davis and Simmen [3]	Initial generations; n = 731 with valid mean IGF-I	0.42 ± 0.13	0.53 ± 0.15	0.71 ± 0.16	0.48 ± 0.13
Davis and Simmen [18]	Expanded dataset; n = 1,761	0.44 ± 0.07	0.51 ± 0.08	0.42 ± 0.07	0.52 ± 0.08
Huang et al. [4]	2,507 calves born 1989–2005	0.44 ± 0.07	0.43 ± 0.07	0.35 ± 0.06	0.48 ± 0.07
Davis and Simmen [5]	Complete relevant records, 1989–2017	0.41 ± 0.07	0.42 ± 0.07	0.34 ± 0.07	0.46 ± 0.07

Note: IGF28, IGF42, and IGF56 are serum IGF-I concentrations measured on days 28, 42, and 56 of the postweaning performance test. Mean IGF-I is the average of these measurements. Estimates are direct heritabilities ± standard errors. Successive analyses contain overlapping records and should not be interpreted as independent replications.

Table 2. Direct heritability estimates for postweaning serum IGF-I concentration in successive analyses of the Ohio State divergent selection experiment.

Study	Dataset	IGF28	IGF42	IGF56	Mean IGF-I
Davis and Simmen [3]	Initial generations; n = 731 with valid mean IGF-I	0.42 ± 0.13	0.53 ± 0.15	0.71 ± 0.16	0.48 ± 0.13
Davis and Simmen [18]	Expanded dataset; n = 1,761	0.44 ± 0.07	0.51 ± 0.08	0.42 ± 0.07	0.52 ± 0.08
Huang et al. [4]	2,507 calves born 1989–2005	0.44 ± 0.07	0.43 ± 0.07	0.35 ± 0.06	0.48 ± 0.07
Davis and Simmen [5]	Complete relevant records, 1989–2017	0.41 ± 0.07	0.42 ± 0.07	0.34 ± 0.07	0.46 ± 0.07

Note: IGF28, IGF42, and IGF56 are serum IGF-I concentrations measured on days 28, 42, and 56 of the postweaning performance test. Mean IGF-I is the average of these measurements. Estimates are direct heritabilities ± standard errors. Successive analyses contain overlapping records and should not be interpreted as independent replications.

Table 3. Evolution and interpretation of estimated genetic relationships between serum IGF-I concentration and growth traits.

Study or population	IGF-I and growth measures	Estimated relationship	Interpretation
Ohio State Angus, initial analysis [3]	Postweaning IGF-I with weaning and postweaning weights and gain	Generally negative; approximately −0.21 to −0.54 (average about −0.38)	Early results suggested an antagonistic relationship, but estimates were based on a comparatively small dataset.
Ohio State Angus, expanded analysis [18]	Postweaning IGF-I with weaning and postweaning weights and gain	Weights: 0.07 to 0.32; averages of 0.14 with preweaning gain and 0.29 with postweaning gain	With more data, relationships became weakly to moderately positive rather than consistently antagonistic.
Ohio State Angus, genetic trends [4]	Line breeding values through the 2005 calf crop	Birth weight and postweaning gain decreased in the high line and increased in the low line	Realized line trends reflect selection, drift, and covariance over generations and do not equal genetic correlations.
Ohio State Angus, complete analysis [5]	Weaning and postweaning IGF-I with weight, gain, and hip height	Birth weight: −0.08 to −0.29; weaning weight: −0.12 to 0.19; gain and hip height: 0.12 to 0.45	Relationships were age- and trait-specific; IGF-I was more consistently related to skeletal growth than to body weight.
Australian beef populations [14,15,16,17]	IGF-I with growth and feed-efficiency traits	Growth relationships were generally small or negative; higher IGF-I was genetically associated with higher net feed intake in one Angus analysis	Independent populations confirm heritable IGF-I but not a universal direction of association with performance.

Note: The Ohio State estimates arose from overlapping stages of one long-term experiment. Differences among analyses may reflect added generations, larger samples, altered trait definitions, and changes in the covariance structure of the selected population.

Table 3. Evolution and interpretation of estimated genetic relationships between serum IGF-I concentration and growth traits.

Study or population	IGF-I and growth measures	Estimated relationship	Interpretation
Ohio State Angus, initial analysis [3]	Postweaning IGF-I with weaning and postweaning weights and gain	Generally negative; approximately −0.21 to −0.54 (average about −0.38)	Early results suggested an antagonistic relationship, but estimates were based on a comparatively small dataset.
Ohio State Angus, expanded analysis [18]	Postweaning IGF-I with weaning and postweaning weights and gain	Weights: 0.07 to 0.32; averages of 0.14 with preweaning gain and 0.29 with postweaning gain	With more data, relationships became weakly to moderately positive rather than consistently antagonistic.
Ohio State Angus, genetic trends [4]	Line breeding values through the 2005 calf crop	Birth weight and postweaning gain decreased in the high line and increased in the low line	Realized line trends reflect selection, drift, and covariance over generations and do not equal genetic correlations.
Ohio State Angus, complete analysis [5]	Weaning and postweaning IGF-I with weight, gain, and hip height	Birth weight: −0.08 to −0.29; weaning weight: −0.12 to 0.19; gain and hip height: 0.12 to 0.45	Relationships were age- and trait-specific; IGF-I was more consistently related to skeletal growth than to body weight.
Australian beef populations [14,15,16,17]	IGF-I with growth and feed-efficiency traits	Growth relationships were generally small or negative; higher IGF-I was genetically associated with higher net feed intake in one Angus analysis	Independent populations confirm heritable IGF-I but not a universal direction of association with performance.

Note: The Ohio State estimates arose from overlapping stages of one long-term experiment. Differences among analyses may reflect added generations, larger samples, altered trait definitions, and changes in the covariance structure of the selected population.

Table 4. Major biological conclusions and implications for cattle breeding and future research.

Evidence domain	Synthesis	Implication or caution
Direct selection response	Postweaning serum IGF-I was moderately heritable and diverged substantially under selection [3,4,5,18].	Circulating IGF-I is a valid quantitative phenotype, but its measurement requires adjustment for age, nutrition, management, and assay effects.
Growth and skeletal development	Associations depended on age and trait; relationships with weight were weak or inconsistent, whereas some associations with gain and hip height were positive [3,4,5,15,16,17,18].	IGF-I should not be used as a simple substitute for growth at all developmental stages.
Feed efficiency	Gain-to-feed associations were favorable under controlled intake, but Ohio State RFI responses were limited. Australian IGF-I and NFI genetic correlations weakened or differed in direction across stages and populations [12,17,19,22,23].	IGF-I has not been validated as a generally reliable substitute for individual feed-intake records; indicator value requires population- and stage-specific validation.
Body composition	Associations with marbling, external fat, and muscling varied with endpoint and adjustment method [19,28,29].	Improvement in one carcass component may be accompanied by an unfavorable response in another.
Reproduction	Most line differences were modest, although higher IGF-I showed favorable associations with some male and female reproductive traits [30,31,32,33,34].	Reproduction was not broadly impaired, but IGF-I is not a stand-alone predictor of fertility.
Molecular architecture	Candidate-gene and genome-wide evidence supports a distributed, polygenic regulatory system [34,35,36,37,38,39,40,41,42,43,44,45].	Single markers require independent validation and do not explain the long-term selection response.
IGF-binding proteins	The selection lines differed in IGF-I but showed no parallel divergence in circulating IGFBP abundance [46].	Serum IGF-I integrates multiple regulatory processes and does not fully represent tissue-level IGF signaling.
Breeding application	No selection direction produced uniformly favorable responses across economically important traits.	If used, IGF-I should be incorporated into a balanced multi-trait index and validated across breeds, ages, diets, and management systems.

Table 4. Major biological conclusions and implications for cattle breeding and future research.

Evidence domain	Synthesis	Implication or caution
Direct selection response	Postweaning serum IGF-I was moderately heritable and diverged substantially under selection [3,4,5,18].	Circulating IGF-I is a valid quantitative phenotype, but its measurement requires adjustment for age, nutrition, management, and assay effects.
Growth and skeletal development	Associations depended on age and trait; relationships with weight were weak or inconsistent, whereas some associations with gain and hip height were positive [3,4,5,15,16,17,18].	IGF-I should not be used as a simple substitute for growth at all developmental stages.
Feed efficiency	Gain-to-feed associations were favorable under controlled intake, but Ohio State RFI responses were limited. Australian IGF-I and NFI genetic correlations weakened or differed in direction across stages and populations [12,17,19,22,23].	IGF-I has not been validated as a generally reliable substitute for individual feed-intake records; indicator value requires population- and stage-specific validation.
Body composition	Associations with marbling, external fat, and muscling varied with endpoint and adjustment method [19,28,29].	Improvement in one carcass component may be accompanied by an unfavorable response in another.
Reproduction	Most line differences were modest, although higher IGF-I showed favorable associations with some male and female reproductive traits [30,31,32,33,34].	Reproduction was not broadly impaired, but IGF-I is not a stand-alone predictor of fertility.
Molecular architecture	Candidate-gene and genome-wide evidence supports a distributed, polygenic regulatory system [34,35,36,37,38,39,40,41,42,43,44,45].	Single markers require independent validation and do not explain the long-term selection response.
IGF-binding proteins	The selection lines differed in IGF-I but showed no parallel divergence in circulating IGFBP abundance [46].	Serum IGF-I integrates multiple regulatory processes and does not fully represent tissue-level IGF signaling.
Breeding application	No selection direction produced uniformly favorable responses across economically important traits.	If used, IGF-I should be incorporated into a balanced multi-trait index and validated across breeds, ages, diets, and management systems.