Effect of the Level of a Glucogenic Substrate on Performance, Carcass Traits and Fatty Acid Profile in Feedlot Steers

1. Introduction

Consumers and their increasing interest in nutritional factors affecting their health and well-being promote the enhancement in quality of edible animal products [1,2]. Special emphasis has been placed on reducing intake of saturated fatty acids (SFA) by increasing intake of unsaturated fatty acids (UFA) instead. The latter has been promoted since the evidence of cardiovascular diseases and the intake of SFA have been positively correlated [3,4]. Several strategies have been encouraged influencing the fat deposition and fatty acids composition of beef [5]; however, most of them are focused on reducing SFA by increasing the content of Ω3 UFA, including conjugated linoleic acid (CLA; C18:2 c9, t11) which biological effects on human health have been extensively studied in the last decades [6,7].

It has been stated that beef cattle which is fed with grass produce higher ratios of unsaturated fatty acids, especially Ω3 fatty acids [8,9]. In addition, the use of grass and other alternative ingredients as animal feedstuffs offers minimal environmental impact; consequently there is an increasing interest in production of these types of forages [10]. Moreover, some studies have revealed that the use of additives as supplementation for beef cattle fed with forage increases production efficiency and improves meat quality [11,12]. The main additives used in intensive beef animal production are yeast cultures, fibrolytic enzymes and ionophores [13,14,15]. Nevertheless, glucogenic precursors (i.e., propionate salts) are commonly used to increase production of ruminal propionate by incorporating propionate as a substrate in the gluconeogenesis pathway that takes place in the liver [16,17]. Similarly, propane-1,2-diol (PG) has been utilized as a glucogenic substrate increasing milk yield and energetic balance in cattle [18,19]. Thus, both glucogenic substrates, propionate salts and PG, are often used to improve productive characteristics of cattle. However, there are few published reports about supplementation of glucogenic precursors and their effect on body condition, beef lipids profile and feed efficiency. Therefore, our hypothesis is that supplementation with a glucogenic substrate may improve productive performance, carcass traits and fat deposition and composition in meat. For that reason, this study was designed to assess productive performance, carcass characteristics, and fatty acid composition in steers that received varying levels of a glucogenic substrate supplement.

2. Materials and Methods

The study received approval of the institutional Animal Care Committee of the Agriculture and Livestock of Durango, Mexico, and was conducted according to the Official Mexicans Norms [20].

2.1. Animals and Experimental Treatments

Forty crossbred yearling steers (260 ± 5.9 kg LW) were used in a feeding trail to investigate the effects of a glucogenic substrate (3.3% of propane-1,2-diol; 6.9% of calcium propionate) on growth performance, carcass quality and fatty acids profile. Steers were vaccinated and wormed upon arrival at experimental center prior to start the study. The animals were randomly housed into individual pens (3 × 6 m) supplied with feed bunks and water and were fed with 1 of 4 experimental treatments. The chemical composition and experimental treatments are presented in Table 1. Diets were provided twice daily (8:00 and 15:00 h). Intake was restricted to 2.8% LW daily to each animal. Minerals and vitamins were supplied by meeting nutritional requirements for beef cattle gaining 1.5 kg/d [21].

2.2. Growth Performance

Animals were weighed every 28 days to record average daily gain (ADG) during the entire experiment by dividing the shrunk BW gained (final BW minus initial BW) by the number of days on feed. Initial and final BW were determined by taking the BW means of the steers before feeding during 2 consecutive days at the beginning and at the end of the experiment. Feed delivery to each pen was recorded daily, and refusals were weekly collected to determine dry matter intake (DMI) as the difference between dietary offered and refusals. Feed efficiency (FE) was calculated by dividing ADG by DMI.

2.3. Slaughter and Carcass Data

After 120 days of feeding, all the animals were slaughtered at a commercial beef slaughterhouse. Animals handling was in accordance with good animal welfare practices whereas slaughtering was performed according to procedures stated in the Mexico’s official norm for slaughtering [22]. After slaughtering, carcasses were weighed and stored at 4 °C during 24 h. Samples of the 12th rib in the longissimus muscle were obtained and areas were measured with a planimeter. Measurements at 3/4 of the ventral length over the longissimus muscle were taken and rib fat thickness (RFT) was determined [23]. Dressing percentage (DP) was calculated using the hot carcass weight (HCW) divided by final BW and then multiplying the result by 100 [24].

2.4. Fatty Acid Composition

Approximately 1 kg of samples of the transverse section were collected from the longissimus muscle and frozen for further analysis. Samples were freeze-dried and the fatty material was extracted using a mixture of chloroform–methanol [25] and methylated according to the AOAC procedures [26] for fatty acid determination. Fatty acid methyl esters (FAME) were separated by injecting 5 µL in a GC 6890N (Agilent Technologies Inc., Wilmington, DE) equipped with a flame ionization detector and a HP-Innowax polyethylene glycol capillary column (30 m x 0.32 mm x 0.15 µm, J&W Scientifics). GC oven temperature was programmed from 200°C (held constant for 1 min) to 230°C at a rate of 5°C/ min and held for 5 min and to 250°C at a rate of 5°C/min and held for 5 min. Helium was the carrier gas at a split ratio of 40:1 and a constant flow rate of 40 mL/min. Injector temperature was set to 250°C. Identification and quantification of FAMEs were achieved by comparison of retention time and concentration of a FAME standard (Sigma-Aldrich, USA).

2.5. Statistical Analysis

All data were analyzed as a completely randomized design using GLM procedures (SAS Inst. Inc., Cary, NC, USA). HCW was used as a covariate and means were adjusted for RTF, RA and DP. Means comparison for all data was evaluated by Tukey’s multiple range test.

3. Results

3.1. Growth Performance and Carcass Characteristics

The growth performance parameters are presented in Table 2. Final weight (FW), ADG, and FE were different among treatments (p<0.05). Increases of 18 and 90% in the FW and ADG were observed in T4 with respect to T1, respectively (p<0.05). Otherwise, FE was reduced when supplementing T2 (p<0.05).

Table 3 presents the adjusted means for RTF, rib eye area (RA) and DP of the supplemented steers using HCW as a covariate. The HCW increased about 15% when T2 was fed compared with T1 (p<0.05). RTF increased 121% in animals supplemented with 60 g of glucogenic substrate (T4) compared to T1 (p<0.05). Otherwise, RA was not affected by supplementation (p<0.05). Nevertheless, DP showed no differences among experimental treatments (p<0.05).

3.2. Fatty Acids Composition

Table 4 presents the fatty acid content in the longissimus muscle. Significant differences were detected among C10:0, C14:0, C16:0, and C18:0 (p<0.05).

No changes were observed in cis 9,18:1 with supplementation of glucogenic precursor (p>0.05). However, CLA (cis 9, cis 12, 18:2) concentration increased 55% compared to T1 (p<0.05); whereas the concentration of rumenic acid increased 67.5% on T4 compared to T1 (p<0.05). Additionally, CLA increased on T2, T3 and T4. The addition of 20 g of glucogenic precursor decreased SFA by about 9% in T2 compared with T1 (p<0.05). Additionally, Ω6/Ω3 ratio was different between treatments (p<0.05). This ratio increased 20% in T2 compared to T1.

4. Discussion

4.1. Growth Performance and Carcass Characteristics

The observed differences in FW and ADG between treatments may be explained due to the energy density provided by the glucogenic substrate [27,28]. Moreover, Simioni et al. [29] and Lahart et al. [30] fed steers with high-grain diets during finishing phase and reported lower values in ADG, DMI and FE than those registered in this study. Additionally, a high-grain diet is more expensive than considering the possible addition or supplementation of glucogenic substrates as an energy source for finishing beef steers [31,32]. Likewise, Nusri-un et al. [33] and Fruet et al. [34] reported an increase of about 2% in HCW in animals fed with similar diets.

Carbohydrates contained in diets contribute about 50% of metabolic energy [21]. Hence, compounds different to carbohydrates provide energy which is stored as glycogen and can be synthesized to non-essential amino acids and fatty acids, increasing the muscle mass in the animal [35]. In addition, Ferreira et al. [36] observed lower RA values of about 87.1 cm² when added de-oiled wet distiller grains (WDG) as feedstuffs to finishing beef steers. Van Cleef et al. [37] reported similar HCW and DP when supplemented finishing bulls with a glucogenic substrate. Otherwise, according to the quality standards proposed by the USDA, the DP obtained in the present study agrees to a Yield Grade 1 quality level, since the DP obtained is higher than 52%. The Yield Grade 1 is described as a carcass covered with a thin layer of external fat over the loin and rib, as well as slight deposits of fat in the flank, kidney, pelvis and heart [38,39].

4.2. Fatty Acids Composition

The fatty acid profile showed higher values than those reported by Castagnino et al. [40], especially myristic and palmitic acid, these authors evaluated the effect of a glucogenic precursor in Nellore breed bulls. Pilarczyk y Wójcik [41] stated that these results can be attributed to a low activity of the enzyme D9-desaturase. San Vito et al. [23] registered similarly values of palmitic acid in the muscle longissimus from Nellore young bulls fed pasture and supplemented with crude glycerin. The same authors mentioned that the glucogenic precursors increased intramuscular fat and higher marbling score.

Bai et al. and Maciel et al. [42,43] reported similar values in C18:1, C16:0, CLA and rumenic acid in steers fed with different sources of fatty acids, on grass and grains systems. Thus, increases in CLA and rumenic acid concentration would occur because of the availability of glucogenic compounds. These compounds can be used as precursors for fatty acids to be deposited intramuscularly [44]. Moreover, glycerol inhibits lipolysis in the rumen, a prerequisite for rumen fatty acid biohydrogenation [45]; thus, there is an improvement in the quality of meat by the incorporation of a higher proportion of unsaturated fatty acids due to a reduction in the accumulation of free fatty acids in rumen [46]. The optimal concentrations of conjugated linoleic acids (CLA) that promote beneficial effects on human health are still unknown [47]. According to Badawy et al. [47] and Dinh et al. [48] intramuscular fat consists of a variety of fatty acids. However, oleic, palmitic, stearic, linoleic and myristic acids represent more than 92% of all fatty acids. In addition, ruminant fat is the only one which contains fatty acids derived from ruminal biohydrogenation of dietary lipids. In this investigation the sum of these acids was on average 86%.

Saturated fatty acids are important for human health, because they are classified as hipercholestolemic. In this study, SFA decreased with the addition of 20 and 60 g/d of glucogenic precursor. In general, animals with lower intramuscular fat deposition rates present a more polyunsaturated profile, since intramuscular fat consists of SFA and MUFA; whereas, PUFA are almost exclusively found in phospholipids of muscle cell membrane [49].

Excessive amounts of Ω6 PUFA and a very high Ω6/Ω3 ratio, promote a pathogenesis, including cancer, and cardiovascular, inflammatory and autoimmune diseases. Thus, increased levels of Ω3 PUFA (a low Ω6/Ω3 ratio) present suppressive effects. In addition, a 4/1 ratio was associated with a reduction of 70% in total mortality [50].

5. Conclusions

Supplementation with glucogenic precursor used in this study promotes a suitable ADG, FW, and HCW in feedlot cattle. Likewise, supplementation increases RTF and fat deposition. Moreover, supplementation improves the fatty acid composition by decreasing SFA and increasing CLA and rumenic acid in intramuscular fat from the longissimus muscle. Supplementation of 40 g/d of the glucogenic substrate resulted in a profitable option to enhance quality parameters in meat and to increase productive performance in beef cattle. On the other hand, research on supplementation with the glucogenic substrate at higher levels and in different diets is highly recommended for further investigations.