The TMR provided to supplemented and non-supplemented lambs was the same and, apparently, not challenging to the ruminal environment and its microbiome, since the rumen pH (Table 3) remained within the normal range of 6.0–7.0 (Grünberg and Constable, 2009). In fact, ruminal pH seems to present some stability in sheep fed TMR containing up to 700 g/kg of concentrate. Some studies demonstrate that when they are fed TMR containing 600–700 g/kg of concentrate, or subjected to abrupt changes by alternating the roughage:concentrate ratio from 400 to 600 g/kg every 5 days, the rumen pH remains within the range of 6.0–6.5 (Gerude Neto et al., 2016; Herzog et al., 2021; Tavares et al., 2021).
In the current study, the maintenance of a normal rumen pH is related to the NDF content of TMR (346.3 g/kg DM; Table 1), despite being lower the range (375.4–433.2 g/kg DM) in which the best performance results were achieved for Dorper × Santa Ines lambs from the same commercial flock in other studies (Sponchiado et al., 2019; Herzog et al., 2021; Teixeira et al., 2021). The ground Tifton 85 hay added to TMR (Table 1) also contributed to the stability of rumen pH, since it is a source of physically effective neutral detergent fiber (peNDF) that has the important role of maintain the rumen pH within the normal range in ruminants fed with high-concentrate diets (Yang and Beauchemin, 2007; Zebeli et al., 2012). The NDFI (mainly of peNDF) stimulates the rumination activity and saliva secretion, which helps to buffering the rumen pH (Van Soest, 1994; Grünberg and Constable, 2009).
The stability of rumen pH likely led to the similar fecal pH between supplemented and non-supplemented lambs (Table 3). Studies have demonstrated that the addition of live yeast culture at 15–45 g/kg DM of TMR does not affects the fecal pH of steers (Adams et al., 1981) and fattening goat kids (Özsoy et al., 2013) when rumen pH ranges from 6.0–6.3 up to 4 hours after feeding. In the current study, the rumen pH of lambs remained within this range and the ratio between autolyzed yeast supply (5 g/animal/day) and DMI of supplemented lambs (1.057 kg/day; Table 7) was 5.3 g/kg DM of TMR, which is lower than the lowest dose of live yeast culture tested in the aforementioned studies. It is expected that additives containing live and autolyzed yeast cultures promote the same effect on rumen pH, maintaining its stability with low variation (Kröger et al., 2017; Tavares et al., 2021). With a stable ruminal condition determined by both additives, a balanced metabolism is established on rumen and the fecal parameters (color, consistency or pH) are not affected (Bagheri et al., 2009; Özsoy et al., 2013). Our findings suggest that this condition was achieved with and without autolyzed yeast supplementation, which justifies the similar results for rumen pH and fecal pH between both groups. Probably, the effect of autolyzed yeast would be evidenced if the TMR contained higher concentrate feed, in which the starch content would be challenging to the ruminal environment.
A similar DMD between supplemented and non-supplemented lambs (Table 3) was not expected, since the autolyzed yeast supplementation at 4–7 g/day increases the diet digestibility by 1.2–2.0% in beef steers (Neumann et al., 2020; Pontarolo et al., 2021). As previously stated, the organic acids, B vitamins, and amino acids contained in the autolyzed yeast may stimulate the growth of cellulolytic bacteria (Callaway and Martin, 1997), and the bypass nucleotides (that scape from ruminal fermentation) may also stimulate the small intestine and liver functions (Sauer et al., 2011), improving the diet digestibility. This suggests that the lack of effect of supplementation on DMD in our study is likely related with the low daily dose of autolyzed yeast provided to the lambs. Thus, higher supplementation doses of this additive should be tested seeking to increase the DMD in feedlot lambs.
With no changes in ruminal environment and DMD, a lack of effect of autolyzed yeast supplementation was naturally expected in the feeding behavior of lambs (Table 4). The range for the time spent in behavioral activities by feedlot Dorper × Santa Ines lambs fed with TMR containing 500–700 g/kg of concentrate and 300–500 g/kg of Tifton 85 hay as roughage is 164–287 min/day for feeding, 377–618 min/day for rumination, and 589–855 min/day for idleness (Grandis et al., 2015; Silva et al., 2016; Santos et al., 2018; Sponchiado et al., 2019; Alba et al., 2021; Herzog et al., 2021). Thus, the times spent on feeding, rumination, and idleness recorded in our study are within the expected ranges for these activities. For water intake, the time spent by lambs was slightly higher than the 20–22 min/day recorded by Sponchiado et al. (2019) and Herzog et al. (2021). Although have been statistically similar (P = 0.130) for supplemented and non-supplemented lambs, it is highlighted that the time spent on water intake was 61% higher (29 vs. 18 min/day) for the supplemented ones.
There was no statistical difference for the fractions of different particle size and APS of orts between supplemented and non-supplemented lambs (Table 4), however, the proportion of particles between 7.8–19.0 mm was 10.1% points lower (49.85 vs. 59.92% for L2), and the proportion of particles smaller than 1.7 mm was 3.1% points higher (8.53 vs. 5.47% for L4) for the supplemented ones. This indicates that the autolyzed yeast supplementation promoted a slightly increase on peNDF intake by lambs. Indeed, feedlot lambs regulate the peNDF intake by changing the proportion of 7.8–19.0 mm particles selected from TMR (Herzog et al., 2021; Teixeira et al., 2021). Although has not been evaluated, a shift on lambs microbiome profile driven to the increase of cellulolytic bacteria population (Callaway and Martin, 1997) may have induced the increase of peNDF intake. This may have also stimulated the water intake by supplemented lambs as an attempt to promote the peNDF hydration and increases its specific gravity (Welch, 1986), seeking to maximize the fermentation and passage rates of the fibrous fractions.
The levels of all blood metabolites (Table 5) remained within the normal range for lambs reared in the tropics (Varanis et al., 2021). Also, the metabolites remained within the expected ranges for Dorper × Santa Ines lambs from the same commercial flock when fed ad libitum in feedlot in the finishing phase, which are 27.4–38.3 g/L for serum albumin, 0.91–1.23 mg/dL for serum creatinine, 45.7–61.2 mg/dL for serum urea, and 76.7–95.0 mg/dL for plasma glucose (Vivian et al., 2017; Herzog et al., 2021; Teixeira et al., 2021). Changes in these metabolites are related with changes in protein and energy metabolism, that are commonly induced by different diet treatments. However, yeast supplementation did not affect the DMI and nutrient intake (Table 7), neither the DMD (Table 3), which explains the similar blood metabolite levels between groups (Table 4).
The effect of sampling day on blood metabolites (Fig. 1; Table 6) is related with the increase on CPI and NFC intake by lambs throughout the feedlot period. Both fractions are contained at higher levels in the concentrate feed and, thus, their intakes were driven by the increase on TMR intake during the trial.
The increased CPI for a longer period leads to an increase on serum albumin (Teixeira et al., 2021). In addition, the increased intake of rumen-degradable protein (RDP) from concentrate feed increases the release of ammonia in the rumen, which is absorbed by rumen epithelium and converted to urea in the liver – urea cycle (Russell and Roussel, 2007; Varanis et al., 2021). Due to this metabolic pathway, the serum urea is increased. Indeed, studies have demonstrated that an increase on RDP to rumen-undegradable protein (RUP) ratio in the TMR leads to the increase of ammonia concentration in the rumen and serum urea of growing lambs (Dorri et al., 2021; Valizadeh et al., 2021).
The increased NFC intake led to changes in the use of creatine and glucose as energy substrates by lambs during the feedlot period. Up to 30th day, the decrease of serum creatinine and increase of plasma glucose suggest that phosphocreatine catabolism in the muscle tissue was progressively reduced to the extent that blood glucose levels increased due to the higher gluconeogenesis activity in the liver. These metabolic processes are a consequence of the increased production of propionate from the fermentation of NFC in the rumen, resulting in its higher availability in the liver for glucose synthesis (Leng et al., 1967; Pereira et al., 2018). With the stability of plasma glucose from the 30th of the trial, serum creatinine turned to increase progressively due to muscle tissue accretion up to the end of the feedlot period. This metabolite is positively correlated with muscle mass in lambs (Meyer et al., 1996; Varanis et al., 2021).
The lack of effect of autolyzed yeast supplementation on lambs’ performance (Table 7) is related with its low tested dose in the current study. In steers, Pontarolo et al. (2021) and Neumann et al. (2020) reported 15.5% of increase on ADG during the first 14–28 days of feedlot in response to the autolyzed yeast supplementation at 4–7 g/animal/day, however, this effect did not remain up to the end of feedlot period (105–112 days). Both authors pointed out that the improvement of DMD and nutrient digestibility, as well as of immune response are the main benefits resulted from yeast supplementation, which helps to maintain a good performance under the stressful conditions at the start of feedlot period.
Mean values for DMI, CPI and ADG were 1.9, 22.3, and 14.4% higher than the expected (1.03 kg/day, 157 g/day, and 250 g/day, respectively) based on NRC (2007). However, the mean values for performance variables (Table 7) were close to the reported for Dorper × Santa Ines lambs from the same commercial flock that were finished in similar conditions on feedlot and were evaluated in other studies, with ranges of 36.5–43.4 kg for final BW, 1.12–1.36 kg/day for DMI, 0.225–0.310 kg/day for ADG, and 4.41–5.33 kg DM/kg gain for FCR (Vivian et al., 2017; Sponchiado et al., 2019; Herzog et al., 2021; Teixeira et al., 2021). The performance slightly superior achieved in the current study is likely related with the addition of pelleted hay (300 g/kg DM) in the TMR (Table 1), which improves the feed efficiency of feedlot lambs (Fontenot and Hopkins, 1965; Li et al., 2021).
The dose of autolyzed yeast tested in the current study is within the range (3.7–10 g/animal/day) that additives containing live S. cerevisiae are used to improve lamb performance (Malekkhahi et al., 2015; Elenin et al., 2016; Liu et al., 2019). However, the first did not promotes this result, which is probably related to their restrict function as a prebiotic. Also, the content of concentrate feed in the TMR was not challenging to the point that the autolyzed yeast promote benefits such as stimulates the growth of lactate-utilizing bacteria (Megasphaera elsdenii and Selenomonas ruminantium, e.g.) and maintaining the ruminal pH stable. This is a very important mechanism in high-concentrate fed ruminants, since the conversion of lactate into short-chain fatty acids by rumen microorganisms is the main pathway for lactate catabolism and ruminal pH regulation (He et al., 2022). Therefore, in future studies on autolyzed S. cerevisae yeast supplementation for lambs, two research lines are proposed: (1) a dose-response test of autolyzed yeast using the same diet provided to the lambs of the current study, to determines the dose that promotes any benefit in terms of diet digestibility, animal performance, and feed efficiency; (2) use the same dose of autolyzed yeast tested in the current study in TMR with increasing levels of concentrate (600, 700, and 800 g/kg DM, e.g.), to identify the diet in which the additive may help to maintain the rumen environment stable, avoiding acute or subacute acidosis, and improves animal performance and feed efficiency.
In conclusion, supplementation of 5 g/animal/day of autolyzed yeast from S. cerevisiae culture does not affect the diet digestibility, feeding behavior, levels of blood metabolites, and performance of Dorper × Santa Ines lambs finished in feedlot with a TMR composed of 400 g/kg of roughage and 600 g/kg of concentrate feed, on a DM basis. Regarding that metabolic and performance lamb responses were not improved, the supplementation of autolyzed yeast is not recommended under the feedlot condition (experimental or at faming) established in the current study.