Impact of Monensin Sodium and Essential Limonene Oil on the Fermentation and Chemical Composition of TMR Silages With Moisture Variations

1. Introduction

The use of total mixed rations (TMR) in animal feed is widely adopted, which allows a nutritionally balanced diet to be provided and reduces feed selection by the animals [1]. However, the need to mix the ingredients daily can add cost to the productive process [2]. Moreover, the use of humid agro-industry coproducts is limited due to their rapid spoilage and unviable extended storage.

TMR ensilage can cut down on costs to prepare the diet daily and allows including humid ingredients [2,3]. The production of ensiled TMR provides economic advantages and enables providing a consistent diet throughout the year, especially during periods of scarcity [3]. Several studies have explored the effects of different ingredients and microbial additives on the fermentative and nutritional quality of ensiled TMR [2,4,5]. Nevertheless, few have assessed the influence of ruminal fermentation modulators such as ionophores and essential oils on the fermentation process of ensiled TMR.

Satisfactory fermentation patterns have been observed in TMR silages containing 33 mg/kg DM monensin (MON) [6]. However, it should be pointed out that all treatments tested by [6] had the same MON dose and the lack of a control treatment prevented concluding whether MON impacts the fermentation process. Another noteworthy aspect of the research by [6] was the high dry matter (DM) contents in the TMRs at an average of 60% since it is known that water activity is directly linked to the intensity of fermentation [7]. Since MON [8] and essential oils [9] often act by unbalancing ion concentrations in microbial cells, it is supposed that the higher the water activity in the silage (more humid silages), the greater will be the action of those additives on the microorganisms of the medium.

Moreover, under higher moisture conditions, essential oils can penetrate the forage more uniformly, which leads to a more consistent antimicrobial effect. Appropriate moisture levels also ensure that the essential oils are adequately mixed throughout the silage, thus preventing areas where spoilage microorganisms can thrive [10].

In this context, the present research was based on the following hypotheses: 1) The use of MON and essential limonene oil (ELO) in TMR formulation inhibits microbial growth, thus negatively impacting lactic acid production and improving aerobic stability (AS) without significantly changing the nutritional value of the silage; 2) The action of MON and ELO on fermentation will be more intense in more humid silages. This study aimed to assess the influence of adding MON and ELO on the quality of the TMR ensilage process and to identify the main changes in TMR fermentation at different moisture contents.

2. Materials and Methods

2.1. Experimental Assays

The research was split into two assays, with the first focusing only on identifying the possible changes in the fermentation parameters of TMR silages with the addition of MON and ELO and the second testing the effect of adding MON and ELO under two moisture conditions.

The first assay was carried out in April 2022 at Embrapa (Agropecuária Oeste) (22°16′ 44” S, 54°49′10” W), located in the municipality of Dourados, MS, Brazil. Since that research institution focuses on lamb rearing, the TMR was formulated to meet the nutritional requirements of lambs (20 kg mean body weight) in the growth phase (mean weight gain of 300 g/day) with the intake of 0.7 kg DM/day according to the recommendations by the [11] (Table 1). The first assay was conducted using a completely randomized design with four repetitions per treatment for a total of 20 experimental units. The treatments tested in that first assay were control (TMR with no additive), MON35 (TMR with 35 mg monensin/kg DM), MON45 (TMR with 45 mg monensin/kg DM), ELO300 (TMR with 300 mg ELO/kg DM), and ELO600 (TMR with 600 mg ELO/kg DM).

The second assay was developed in March 2023 at the Experimental Farm of the Federal University of Grande Dourados (UFGD) (22° 13′ 52.44′ 95” S, 54° 59′ 10.53′ 72” W) in the municipality of Dourados, MS, Brazil. In the second assay, the TMR was formulated to meet the nutritional requirements of dairy cows (500 kg body weight) in lactation (15 kg milk/day) and mean intake of 14 kg DM/day according to the recommendations by [12] (Table 1). That assay followed a completely randomized design in a 5 x 2 factorial arrangement with four repetitions per treatment combination for a total of 40 experimental units. The main factors corresponded to the same treatments of the first assay at two TMR moisture levels (30% and 40% DM). In order to provide a higher moisture level and maintain the proportion of ingredients in TMR formulation, distilled water had to be added when mixing the ingredients in part of the treatments of the second assay.

Both assays used sorghum (Sorghum bicolor L. Moench) as a source of roughage and concentrates based on ground corn, soybean meal, dicalcium phosphate, calcitic lime, and table salt. The proportion of ingredients used and the chemical composition of the diet in each of the experimental assays are presented in Table 1.

The additives and their respective doses were previously added to the concentrate to facilitate mixing and ensure homogenous distribution in the silage mass. In the first assay, the mix of roughage with concentrate resulted in TMR with mean contents of 62.45% DM due to the higher proportion of concentrate needed to meet the requirements of lambs. In the second assay, roughage had a higher participation and resulted in TMR with mean contents of 41% DM, very close to the desired DM content of 40%, thus that TMR was readily used in the experimental silos. To obtain TMR with content of 30% DM, 171 mL distilled water were added to the material used in each experimental silo (average of 4.25 kg material per silo). To prevent effluent formation, the material was homogenized while adding the distilled water.

Both assays employed experimental silos built using PVC pipes (10 cm diameter and 50 cm height) with useful volume of 3.8 L. The material was manually compacted using wood rods. At the bottom of each silo, a layer of approximately 4.5 cm of sand (300 g) was placed for effluent drainage. A fine cotton fabric mesh was used to keep the forage from touching the sand. After filling, the experimental silos were sealed with double-faced (black and white) plastic film and adhesive tape and stored in the laboratory at room temperature (average of 25.15 °C) for 90 days.

During silo filling, TMR samples were collected from each treatment. The first sample, of approximately 300 g, was used to determine the chemical composition and the second, of approximately 70 g, was frozen for later processing and to determine pH values and buffer capacity.

2.2. Methodologies Employed in the Assays

To calculate fermentation loses, all components in the silos (silo, sand, and fabric) and the TMR mass ensiled were weighed both before and after ensiling. Dry matter recovery, gas losses, and effluent production were calculated according to the equations by [13].

The silos were then opened and the material inside them was removed and homogenized for sample collection. A sample of approximately 300 g from each experimental silo was sent to the laboratory for chemical composition analysis. Another sample was frozen for later analysis of pH and of the profile of short-chain organic acids. The chemical composition of the TMRs was determined in a spectrophotometer model Foss 5000 (Eden Prairie, MN, USA) after calibration (WinISI version 4.6.11, FOSS Analytical A/S, Denmark) to determine the concentrations of dry matter (DM), mineral matter (MM), crude protein (CP), soluble protein (SP), neutral detergent insoluble protein (NDIP), acid detergent insoluble protein (ADIP), neutral detergent fiber (NDF), acid detergent fiber (ADF), lignin, ether extract (EE), non-fiber carbohydrates (NFC), and starch.

A 25 g sample of TMR was diluted in 225 mL distilled water and manually homogenized for approximately 20 min for the production of an aqueous extract. The extract was used to measure pH both before and after silage, buffer capacity before silage, and the profile of short-chain organic acids. pH was determined using a digital potentiometer (mPA210 MS Tecnopon) and buffer capacity was determined according to [14]. A portion of that extract was filtered using a paper filter, centrifuged for 15 min at 10,000 rpm, and the supernatant was frozen at -20 °C for later analysis of volatile organic acids. The organic acids were determined by gas chromatography with mass detector (GCMS QP 2010 Plus, Shimadzu, Kyoto, Japan) using a capillary column (Stabilwax, Restek, Bellefonte, USA, 60 m, 0.25 mm Ø, 0.25 µm Crossbond Carbowax polyethylene glycol). Lactic acid concentration was determined using the colorimetric method proposed by [15]. The concentration of ammoniacal nitrogen (NH₃-N) was determined by the Kjeldahl method [16]. In vitro DM digestibility (IVDMD) was determined according to [17] with samples incubated for 48 h.

Aerobic stability (AS) was determined in all silos after they were opened. Samples (2 ± 0.005 kg) of each replicate of each treatment were freely placed in the clean experimental silos. Temperature sensors were placed in the geometric center of the silages and a double layer of gauze was placed on top of each experimental silo to prevent drying and contamination while allowing for air penetration. Ambient temperature, as well as the temperature of each silage, was recorded every minute and the average was calculated every 20 min using a datalogger (RC-4, Elitech^®). Aerobic stability was defined as the number of hours required for the silage temperature to reach 2 °C above ambient temperature (Kung Jr et al., 2018).

2.3. Statistical Data Analysis

The data of the first assay were analyzed using the statistical program Sisvar 5.8 (build 92, 2018). In case of significant difference (α ≤ 0.05), the averages of the treatments were compared by Scott-Knott test at 5% significance. The data of the first assay were analyzed according to the following model:

Yik = μ + Ai + εik

(1)

Where: Yik = dependent variable, μ = overall average, Ai = effect of different additives (fixed effect; i = Control, MON35, MON45, ELO300, and ELO600), and εijk = random error associated with each observation (k = 4).

The data of the second assay were analyzed using the statistical program Sisvar 5.8 (Build 92, 2018). When the interaction of factors was significant (α ≤ 0.05), the factors were read separately for analysis. The means were compared by Scott-Knott test at 5% level of significance. Assay data were analyzed according to the following model:

Yijk = μ + Ai + Mj + A * Mij + εijk

(2)

Where: Yijk = dependent variable, μ = overall average, Ai = effect of different additives (fixed effect; i = Control, MON35, MON45, ELO300, and ELO600), Mj = moisture effect (fixed effect; j = A and B), A * Mij = effect of the interaction between additives and moisture, and εijk = random error associated with each observation (k = 4).

3. Results

3.1. First Assay

No difference (P > 0.05) was observed between the treatments tested for gas losses, effluent production, and DM recovery (Table 2). In the first assay, the gas losses varied from 2.03 to 2.88% ensiled DM while mean effluent productions were 5.1 kg/ton DM and DM recovery losses were above 97%.

The lowest (P < 0.05) final pH values were observed for treatments ELO600 and ELO300, with no differences (P > 0.05) in pH values between the control and the treatments added with monensin (Table 2). The silages that received additives ELO300 and ELO600 had the highest concentrations (P < 0.05) of lactic acid (average of 4.95% of DM). No significant differences were observed for lactic acid content in the silages from treatments MON35, MON45, or control (Table 2). Acetic acid concentrations were higher (P < 0.05) for the silages with additives when compared with the control silages (Table 2). On average, the silages added with MON or ELO had 38.8% higher acetic acid contents when compared with the control treatment. Propionic acid concentrations increased with the use of additives, with the lowest concentration observed for the control treatment followed by MON35, MON45, ELO300, and ELO600, respectively (Table 2).

During the first assay, no difference (P > 0.05) was found between the treatments regarding contents of butyric acid, isobutyric acid, and valeric acid, which were, on average, 8.2 mg/kg DM, 2.41 mg/kg DM, and 1.23 mg/kg DM, respectively. Isovaleric acid concentrations were higher (P < 0.05) for silages ELO300 and ELO600 at 5.71 mg/kg DM on average.

Aerobic stability was impacted by the additives tested (Table 2). The silage with treatment ELO600 had the highest (P < 0.05) AS value (average of 297.88 h), followed by treatments ELO300, MON35, and MON45. The control treatment had the lowest AS at 231.65 h.

The NH₃-N contents were the lowest for the treatments with some type of additive (Table 2). The control treatment had the highest NH₃-N value with an average of 10.48% of TN. Treatment ELO600 had the lowest NH₃-N value. The highest ethanol production was observed for treatment ELO300 (0.21% DM), whereas the other treatments had no significant differences between each other (Table 2).

During the first assay, differences (P < 0.05) were observed only for contents of DM, starch, and NFC between the treatments tested (Table 3).

The use of additives (MON and ELO) led to silages with lower DM contents when compared with the control treatment. Starch contents were lower (P < 0.05) for treatments ELO300 and ELO600 (average of 24.88% of DM) when compared with MON35, MON45, and control (average of 28.52% of DM), which did not differ (P > 0.05) (Table 2). Contents of NFC were lower (P < 0.01) in the silages with some type of additive (MON or ELO) regardless of the dose employed.

3.2. Second Assay

In the second assay, differences (P < 0.05) were observed between the additives for gas losses (Table 4). The highest values were found for treatments MON35 and ELO600, which were 77.28% higher than in the other treatments tested. The more humid silages had gas loss values of 134.57% higher when compared with those with lower moisture. No differences (P > 0.05) were observed between the additives for effluent production, which were the highest in the silages with high moisture contents (average of 59.2 kg/t DM) (Table 4).

Significant interaction was found between TMR silage humidity and the additives tested for DMR (Figure 1). Overall, the silages with high moisture content had the lowest (P < 0.05) DM recovery values, especially ELO600 at only 88% DM recovery. Among the silages produced with high moisture content, the treatments using MON had the highest DM recovery values. No differences (P > 0.05) were observed in DM recovery for the silages with low moisture content, which were all above 97%.

An interaction (P < 0.05) was found between the additives tested and the moisture contents of the TMR silages, where ELO600 in the TMR with the highest moisture resulted in the highest pH values (average of 4.31) (Figure 1). The other treatments tested, irrespective of moisture (including ELO600 in the silages with lower moisture) did not differ (P > 0.05), with mean pH of 4.02.

The silage with the highest (P < 0.05) lactic acid content (average of 8.1% of DM) was ELO300 with higher moisture (Figure 1). Only treatment ELO600 had higher (P < 0.05) lactic acid production in the drier silage when compared with the more humid one. For the other additives, the silages with higher moisture content always had higher lactic acid productions.

Lactic acid concentration was higher (P < 0.05) for silage ELO600 with higher moisture (2.11% of DM) (Figure 1). The opposite behavior was observed for the control, MON35, and MON45 treatments, which had the lowest acetic acid values for the silages with higher moisture content. On average, treatment ELO600 in silages with higher moisture produced 2.45 times more acetic acid compared with the average of the other treatments.

The silages with the highest (P < 0.05) lactic acid/acetic acid ratios were the ones added with MON with higher moisture (Figure 1). Only ELO600 had higher (P < 0.05) lactic acid:acetic acid ratio in the silage with lower moisture when compared with the silage with higher moisture. For the other additives tested, the lactic acid:acetic acid ratios were always higher or similar in the silages with higher moisture when compared with those with lower moisture. The silages with higher moisture added with ELO600 had the lowest lactic acid:acetic acid ratios (average of 2.3).

The additives tested did not impact (P > 0.05) ethanol production, while higher ethanol values were found for silages with higher moisture compared with those with lower moisture (Table 4). A difference (P < 0.05) was found between the silages with higher and lower moisture for propionic and butyric acid contents (Table 4). The highest butyric acid concentrations (75.66 mg/kg DM) were observed for the silages with higher moisture, whereas the opposite comparison was observed for the concentrations of propionic and isobutyric acids. Overall, the control treatment had the highest concentrations of propionic, butyric, isovaleric, and valeric acids, with a significant reduction in these acids observed when MON and ELO were added.

The NH₃-N contents in the silages tested were impacted (P < 0.05) by the additives and moisture contents of the TMRs. Overall, the silages with higher moisture had the highest NH₃-N values (9.09% of TN) compared with those with lower moisture (8.48% of TN). The control, MON35, MON45, and ELO300 treatments has the lowest NH₃-N contents, while ELO600 was the treatment with the highest NH₃-N value.

Aerobic stability was affected by the interaction (P < 0.05) between the additives and moisture contents of the TMRs tested (Figure 1). Regardless of the moisture tested, the lowest AS values were observed in the control treatment, with an average of 83.5 h for AS failure. Both ELO600 and MON35 had the best AS values (> 92 h) irrespective of silage moisture. The silages with higher moisture added with ELO300 had lower AS (average of 85.33 h) compared with those with higher moisture (average of 91.84 h).

The silages with lower moisture had the highest (P < 0.05) DM, NDF, and ADF contents, whereas those with higher moisture had the highest (P < 0.05) EE values (Table 5). All additives tested resulted in lower fiber contents and higher starch contents. Ether extract contents were higher in the treatments added with MON (Table 5). The higher IVDMD coefficient (77.56% of DM) was observed in the silages added with MON35, followed by MON45, ELO300, and ELO600 with intermediate values and the control treatment with the lowest IVDMD value (71.09% of DM) (Table 5).

4. Discussion

4.1. First Assay

The gas loss values observed were below 3% of the ensiled DM, characteristic of silages with good fermentation quality (prevalence of lactic fermentation), according to [7]. Effluent production in this assay was minimal, which is common in silages with high DM content [6,19]. According to [2], high DM contents (above 40%) contribute to the conservation of the ensiled material and increase DM recovery values. Similar data were reported by [6], who found DM recovery values above 96% when ensiling TMR with DM contents between 60 and 62%.

pH is an important parameter to assess the silage fermentation quality as it is directly related with the total organic acids produced [20,21]. Silages with higher DM values, i.e., low water activity, result in higher final pH values (lower fermentation intensity) [22]. That explains the pH values observed in this assay (average of 4.74). According to [4,5,23] pH values between 4.0 and 5.0 are common in silages with DM contents above 40%, with such pH range being sufficient to control the growth of undesirable microorganisms.

When assessing several works in the literature, [24] concluded in their meta-analysis that essential oils act by inhibiting the growth of undesirable microorganisms in the silages (particularly molds and yeasts), thus improving AS. That matches the results obtained in this research, especially with the highest ELO dose (600 mg/kg DM). To a lesser extent, MON also improved silage AS compared with the control treatment. [6]) observed that TMR silages with 33 mg MON/kg DM had AS values above 240 h, which corroborated the data obtained in this assay.

Another aspect that helps explain the higher AS of the treatments with MON and ELO was acetic acid production. According to [18], a major characteristic of acetic acid is to inhibit the growth of molds and yeasts and, at moderate amounts, it can contribute to improving silage stability. The mean normal concentration of acetic acid in the silage is 3 to 4% of DM, a value that is lower (0.5 to 2%) in silages with higher DM contents (45 to 55% DM) [18].

The treatments added with ELO, at either dose, had the highest lactic acid concentrations, which contradicts most data found in the literature on the use of essential oils in silages [24]. However, [25] observed that adding 300 mg/kg cumin EO had a selective antimicrobial effect, decreasing the number of harmful microorganisms (molds and yeasts) and increasing the number of beneficial microorganisms (lactic acid bacteria). Such situation led to higher lactic acid productions and lower pH values, which corroborated the data obtained in the present research.

The control treatment had the highest butyric acid and NH₃-N contents, suggesting the additives tested helped inhibit undesirable fermentations. In the research by [25] with cumin EO in oat silage, those authors also observed a reduction in NH₃-N contents when adding essential oil. According to those authors, cumin EO inhibited the growth of proteolytic bacteria in the silage, resulting in lower NH₃-N contents. Nevertheless, the butyric acid and NH₃-N values found in the present assay are close to the ideal range (1 g butyric acid/kg DMS and 10% of TN) for silage with good nutritional and sanitary quality [7,18,21].

According to [21], some heterofermentative bacteria can produce ethanol from the fermentation products or substrates present in the medium, which explains the higher ethanol contents in the treatments added with MON and ELO. Nonetheless, the ethanol values obtained in both assays can be considered low and did not negatively impact the quality of the fermentation process [26].

Using the ELO and MON additives led to a slight reduction (P < 0.05) in the DM contents of the silages. It is known that determining DM in a forced-air oven causes partial loss of the volatile fatty acids produced during ensiling, hence, lower DM values are commonly observed in silages with higher organic acid contents [27]. That may help explain why the silages added with ELO and MON (higher organic acid productions) had the lowest DM values.

In the literature, some studies also observed reductions in starch and NFC contents during TMR ensiling, similarly to the findings in the present study. [3] found reductions by 5.5% in starch contents in TMR based on corn kernels. According to those authors, some types of microorganisms present in the silages are able to use the starch as substrate for the production of lactic acid, acetic acid, and ethanol and the concentration of those compounds commonly increase in the final product. The silages added with ELO300 and ELO600 had the highest lactic acid concentrations (37.11% higher than in the control treatment) and the highest starch reductions, which corroborates the statements by [3].

4.2. Second Assay

Although higher, the gas losses in treatments MON35 and ELO600 were below 3% of the ensiled DM, which is considered adequate according to [7]. [4] observed that TMR with higher moisture (23% of DM) associated with heterofermentative microbial additives resulted in slightly higher gas losses (3.7% of DM), while drier TMR silages with no inoculants (32% of DM) had lower gas loss values (3.1% of DM), which corroborated the data obtained in this research.

Since silage moisture is directly correlated with effluent production, higher effluent production values were expected for the more humid TMRs. According to [7], excess effluent is avoided in most cases when the DM content of the material is above 35%. The higher effluent production in the more humid silages likely contributed to their lower DM recovery values.

According to [7], lactic fermentation results in silages with minimum DM and energy losses, whereas acetic, alcoholic, and butyric fermentations result in silages with higher DM and energy losses, i.e., lower DM recovery values. Treatment ELO600, with higher moisture, experienced more intense heterofermentation and used the sugars present in the medium for higher acetic acid production, which explains the lower DM recovery values.

The more humid silages had higher lactic acid concentrations in the control, MON35, MON45, and ELO300 treatments. However, treatments ELO600, with higher moisture, resulted in a significant reduction in lactic acid production and increased pH, which was not found when the same dose was employed in drier TMR silages. The higher EO dose (600 mg/kg DM), in association with the higher medium moisture, likely potentialized antimicrobial action and favored the growth of heterofermentative bacteria (more resistant to ELO) in detriment of homofermentative bacteria (less resistant to ELO).

Limonene, an important compound of several citric oils, has strong antimicrobial properties that may inhibit a series of microorganisms, including lactic acid bacteria (LAB). Studies have shown that limonene has significant antibacterial activity against several bacterial species, including food-related microorganisms such as Lactobacillus plantarum and Lactobacillus brevis, common in silage [28].

The meta-analysis by [24] concluded that not only do essential oils inhibit the growth of bacteria harmful to the silage, but they also inhibit the growth of bacteria that produce the main organic acids. However, most studies used by [24] were conducted on silages with moisture contents between 22 and 35%, i.e., silages with higher moisture that likely had more effective antimicrobial action by the EOs, particularly at higher doses, which corroborates the data observed in the present research.

The higher lactic acid:acetic acid ratios observed when adding MON allow concluding that the additive modulated fermentation in the TMRs and prioritized homofermentation. That is a positive result since it allows using MON in the formulation of TMR to be ensiled aiming at improving the fermentation process. Moreover, [6] found that MON does not undergo microbial breakdown during the ensiling process and remains active in the diets for later action in the rumen.

A synergistic effect likely took place between the low moisture of the silages and the antimicrobial action of limonene, thus reducing the activity of undesirable bacteria, which explains the lower butyric acid values in the silages with ELO300 and ELO600. Although the control treatment had the highest butyric acid and NH₃-N concentrations, they were within the acceptable range (1 g butyric acid/kg DM and 10% TN) for silages with good nutritional and sanitary quality [7,20,21].

As in the first assay, adding ELO and MON likely helped reduce the growth of undesirable microorganisms after the silages were opened (especially molds and yeasts), thus improving their AS values, especially in treatments ELO600 and MON35. It is noteworthy that, in the second assay, the silages had higher moisture and, therefore, the AS values were much lower than those obtained in the first assay. Therefore, it can be said that using MON and ELO is a more interesting strategy to preserve the quality of TMR silage with higher moisture after opened.

The reductions in NDF and ADF contents when adding MON and ELO were likely due to higher activity of fibrolytic enzymes during the fermentation process. In an experiment with alfalfa and cumin EO at 300 and 500 mg/kg DM, [25] observed reductions in NDF and ADF contents in relation to the control treatment. According to those authors, the doses employed stimulated enzyme activities that promoted cell-wall breakage and released saccharose into the medium. Such saccharose release was likely used as substrate by the LAB to produce lactic acid [21]. The same behavior was observed by [29] when adding cumin EO at 200, 300, and 500 mg/kg to oat silages. Those authors observed that cumin EO significantly reduced the cellulolytic fraction of the silages.

[2] reported that the partial break of the cell wall is favored in silages with higher moisture and that the cell wall components can be used as substrate for silage fermentation. That helps explain why, in the present experiment, NDF and ADF contents were lower in the silages with higher moisture when compared with those with lower moisture.

The increase in starch content in the TMR silages added with MON and ELO is likely related to the reduction in prolamin, the layer that protects the starch in the seed. When reviewing TMR ensiling for ruminants, [2] argued that TMR silage fermentation promotes a positive effect on protease activity, favoring prolamin reduction and, consequently, increasing starch availability in the silage. The lower prolamin content may increase the efficiency of starch detection since the standard technique depends on the hydrolysis of starch granules [30]. Therefore, if prolamin impacts starch detection, that error will be intrinsic in the NIR calibration equations as they are derived from the extension of the conditions observed in the standard technique [31]. Another factor that may help explain the higher starch contents is the dilution effect since the decrease in fiber contents increases the proportion of other non-fiber nutrients in the silage, such as starch.

The higher EE contents observed in the treatments with ELO were also reported in other works in the literature [32,33,34]. When studying the origin of lipolysis in alfalfa silages, [35] found that the decrease in total fatty acid contents during ensiling took place mainly due to the breakdown of fatty acids C18:2n-6 and C18:3n-3. According to those authors, plant enzymes play a major role in lipolysis during alfalfa ensiling, however, several epiphyte microorganisms in alfalfa contributed much more to lipolysis. In that same research, the authors showed that homofermentative LAB purely inoculated in alfalfa silages did not impact the lipolysis of the material, but rather only the epiphytic flora contributed to it. The finding by [35] may help explain why treatments MON35 and MON45 preserved EE contents more since, during TMR ensiling with MON, homolactic fermentation (higher lactic acid:acetic acid ratios) prevailed, thus decreasing the lipolytic action of other bacterial groups.

The increase in IVDMD in the TMR silage added with MON35 may be related to the fiber contents in those silage. Since the activity of fibrolytic enzymes was higher in that treatment, the production of other more digestible compounds, among which lactic acid, may also have been higher. Following that assumption, the silages added with MON45, ELO300, and ELO600 may also have had the same stimulus for greater breakdown of the fibrous fraction, albeit at lower intensity.

The data obtained in this research are important as they show growth-promoting additives can be added to TMR formulations with no harm to the fermentation process or nutritional quality of the silage. However, the effect of ionophores and essential oils on the dynamics of microbial populations over the ensiling process must still be better studied. Moreover, further studies are needed with other types of ionophores and essential oils commonly used in animal diets.

5. Conclusions

The findings herein suggest that the strategic use of MON and ELO may effectively improve TMR silage quality and have potential benefits for ruminant nutrition and production profitability. However, ELO must be added at lower doses to silages with higher moisture so as to prevent harming fermentation quality. Therefore, adding ELO600 is recommended for TMR silages with DM content above 40% and adding ELO300 is recommended for silages with DM content below 30%.