Effects of live Saccharomyces cerevisiae supplementation on rumen pH dynamics, volatile fatty acid profiles, and subacute ruminal acidosis mitigation in feedlot steers fed high-concentrate diets

Introduction

The practice of feeding live yeast to ruminants is far older than the scientific understanding of why it works. Argentine gauchos in the 19th century reportedly added beer dregs-rich in residual Saccharomyces cerevisiae-to the grain rations of working horses and cattle, observing that the animals seemed to digest their feed more efficiently and suffer fewer digestive upsets. It was not until the 1920s that microbiologists began to characterise the rumen ecosystem, and not until the 1980s that controlled experiments demonstrated that live yeast cultures could measurably influence rumen fermentation parameters. The modern rationale for yeast supplementation in high-concentrate feedlot diets rests on three mechanisms. First, S. cerevisiae scavenges dissolved oxygen that enters the rumen during feeding and rumination, creating a more strictly anaerobic environment that favours the growth of cellulolytic bacteria (particularly Fibrobacter succinogenes and Ruminococcus flavefaciens) and lactate-utilising bacteria (Megasphaera elsdenii and Selenomonas ruminantium). Second, by stimulating these lactate utilisers, yeast reduces the accumulation of lactic acid-the organic acid most responsible for the precipitous pH declines that characterise SARA. Third, yeast cell wall components (beta-glucans, mannanoligosaccharides) may modulate rumen microbial community structure independently of the live organism's metabolic activity.

SARA is defined by prolonged periods of rumen pH below 5.8, and it affects an estimated 19-26% of feedlot cattle in North America and an unknown but likely similar proportion in Argentina, where feedlot finishing has expanded rapidly over the past two decades. The disorder reduces feed efficiency, causes rumenitis and liver abscesses, and predisposes cattle to laminitis and secondary infections. Preventing SARA through dietary management is therefore a priority for both animal welfare and economic performance. While numerous trials have evaluated S. cerevisiae supplementation in North American and European feedlots, data from South American production systems are remarkably limited. Argentine feedlots differ from their North American counterparts in several respects-predominance of British breeds (Angus, Hereford), heavier reliance on corn grain and sunflower meal rather than corn silage, warmer ambient temperatures, and different management intensity-that may influence the magnitude and consistency of yeast effects. This research was designed to address that gap by evaluating two dose levels of a commercial live S. cerevisiae product in cannulated Argentine feedlot steers, with continuous rumen pH monitoring as the primary outcome.

Materials and Methods

Materials

Thirty-six ruminally cannulated Angus steers (initial body weight 387 + /-18 kg; age 14-16 months) were used in a completely randomised design at the Instituto Pampeano de Ciencias Animales experimental feedlot facility in Bahia Blanca, Buenos Aires province, Argentina, from April to July 2023 (autumn-winter). Steers had been fitted with 10 cm rumen cannulae (Bar Diamond, Parma, ID, USA) at least 60 days prior to the start of the trial. Animals were housed in individual pens (12 m2) with ad libitum access to water and were fed twice daily (07:00 and 17:00) at 2.5% of body weight (dry matter basis). The basal diet consisted of 78% dry-rolled corn grain, 10% sunflower meal, 6% chopped wheat straw, 4% molasses, and 2% mineral-vitamin premix (dry matter basis). The live yeast product (Saccharomyces cerevisiae strain CNCM I-1077, Lallemand Animal Nutrition, Montreal, Canada) was provided at a guaranteed concentration of 2 x 10^10 colony-forming units per gram. Steers were randomly assigned to three treatments (n = 12 per group): Control (0 g/d yeast), Low SC (5 g/d yeast, equivalent to 1 x 10^11 CFU/d), and High SC (10 g/d yeast, equivalent to 2 x 10^11 CFU/d). The yeast was top-dressed onto the morning ration. The experimental period lasted 84 days, preceded by a 14-day adaptation period during which all steers received the basal diet without yeast. All procedures were approved by the Instituto Pampeano de Ciencias Animales Institutional Animal Care and Use Committee (Protocol IPCA-IACUC-2023-019).

Methods

Rumen pH was measured continuously throughout the 84-day experimental period using indwelling pH data loggers (Dascor pH Logger, Dascor Inc., Escondido, CA, USA) programmed to record pH at 60-second intervals. Loggers were calibrated weekly in pH 4.0 and 7.0 buffer solutions. Daily pH summary statistics (mean, minimum, maximum, and time below pH 5.8, 5.5, and 5.2) were calculated from the raw 1-minute data. Rumen fluid samples (50 mL) were collected via the cannula at 0, 3, 6, 9, and 12 hours post-morning feeding on days 28, 56, and 84 for VFA analysis. Samples were strained through four layers of cheesecloth, acidified with 1 mL 25% metaphosphoric acid per 5 mL sample, and frozen at-20 degrees C until analysis. VFA concentrations were determined by gas chromatography (Shimadzu GC-2010 Plus, Kyoto, Japan) using a Nukol capillary column with crotonic acid as internal standard. Dry matter intake (DMI) was recorded daily by weighing offered and refused feed. Body weight was measured on days 0, 28, 56, and 84 before morning feeding. Average daily gain (ADG) was calculated by linear regression of body weight on time. Feed efficiency was expressed as gain-to-feed ratio (G:F). Blood samples were collected from the jugular vein on days 0 and 84 for serum glucose, beta-hydroxybutyrate (BHB), and lactate determination. Statistical analyses were performed using the MIXED procedure of SAS 9.4, with treatment as a fixed effect and steer as the experimental unit. Repeated measures (day, hour) were modelled using an autoregressive covariance structure. Orthogonal polynomial contrasts were used to test linear and quadratic dose effects. Significance was declared at p<0.05 and tendencies at p<0.10. Both mean and minimum daily rumen pH increased linearly with yeast dose (linear contrast P = 0.001 and P = 0.003, respectively). The time spent below the SARA threshold of pH 5.8 was reduced by 20% in the low-dose group and 41% in the high-dose group relative to control. SARA incidencedefined as days on which rumen pH remained below 5.8 for more than 180 minutes-was 38.2% of days in control steers, 24.7% in low-dose, and 14.3% in high-dose (linear P = 0.002). The pH nadir occurred approximately 6 hours after the morning feeding in all groups, but the depth of the nadir was attenuated by yeast supplementation.

Results

Comprehensive Interpretation

The consistent linear dose-response across pH parameters and VFA profiles supports a mechanistic interpretation. The 41% reduction in time below pH 5.8 in the high-dose group, combined with the shift from acetate toward propionate fermentation and the unchanged total VFA pool, indicates that yeast supplementation redirected fermentation pathways rather than suppressing overall microbial activity. The most parsimonious explanation is that S. cerevisiae stimulated lactate-utilising bacteria, converting lactic acid to propionic acid before it could accumulate to levels that depress pH. The tendency toward improved ADG (1.67 versus 1.54 kg/d, P = 0.087) in the high-dose group, while not statistically significant, is consistent with a more stable rumen environment supporting more efficient fibre and starch digestion.

Discussion

The magnitude of the pH-stabilising effect observed in this research-a 0.23-unit increase in mean daily pH and 41% reduction in SARA risk at the 10 g/d dose-places these results squarely within the range of responses reported in North American and European feedlot trials, where yeast supplementation typically raises mean rumen pH by 0.1-0.3 units. This consistency across geographic and management contexts strengthens the case that the yeast effect is robust and not merely an artefact of specific dietary or environmental conditions.

The ). Given the relatively low cost of the yeast product, the higher dose appears justified in high-risk situations such as the transition from backgrounding to finishing diets, periods of heat stress that depress DMI and increase meal-to-meal variation, and herds with a history of SARA-related liver abscesses.

Conclusion

Three things stand out from this 84-day trial. First, live Saccharomyces cerevisiae at 10 g/head/day produces a clear, dose-dependent improvement in rumen pH stability in Argentine feedlot steers fed high-concentrate diets-raising mean pH by 0.23 units and cutting the time spent in the SARA danger zone by 41%. Second, the fermentation shift from acetate toward propionate, without any change in total VFA production, points to enhanced lactate channelling as the operative mechanism-a conclusion that aligns with the bacterial ecology work done in French and North American laboratories over the past decade. Third, the economics are encouraging: the numerical ADG advantage, if confirmed in larger-scale trials, would yield a three-to-four-fold return on supplement cost under current Argentine market conditions. What remains uncertain is whether these benefits persist beyond 84 days, whether they hold up under the more variable conditions of commercial feedlots (where individual animal supplementation is impractical and yeast must be mixed into the total ration), and whether the CNCM I-1077 strain performs differently from other commercial S. cerevisiae strains under Argentine conditions. These questions deserve attention in future research, but the present data provide a sound basis for recommending live yeast supplementation as part of a SARA prevention strategy in intensive beef finishing operations across the Argentine Pampas and comparable production environments.

Acknowledgement

Acknowledgements Funding BodyThis research was partially funded by the Argentine National Institute of Agricultural Technology (INTA) through Research Grant INTA-PIT-2023-042. Facility AccessThe authors thank the management of Establecimiento La Esperanza feedlot for providing access to their facilities and technical support during the feeding trial. Analytical SupportVFA analyses were performed at the INTA Balcarce Animal Nutrition Laboratory under the supervision of Ing. Agr. Gabriela Peralta.

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