Supplementing late-fattening Angus steers diets with yeast culture remodels gastrointestinal microbiota and promotes physiological adaptation to heat stress

Heat stress (HS) poses a major threat to the beef cattle industry, causing significant economic losses. This study investigated the effects of yeast culture (YC) supplementation on HS mitigation, growth performance, and microbial modulation in late-fattening Angus steers. Twenty-seven steers (555.93 ± 22.39 kg BW, Mean ± SD) were randomly assigned to 3 groups: control (basal diet), low-dose YC (30 g/steer/d), and high-dose YC (60 g/steer/d) for 97 days. Two sampling time points were established: 30d (HS period) and 90d (recovery period). Results showed that 30 g/d YC significantly increased average daily gain (ADG) by 24.6% and reduced feed-to-gain ratio (F/G) by 19.7% compared with the control group during 0–90 d (P < 0.05). During HS, YC supplementation significantly elevated rectal temperature in steers within normal physiological ranges, but reduced respiratory rate during most periods (P < 0.05). YC supplementation elevated serum high-density lipoprotein cholesterol (P = 0.033), reduced low-density lipoprotein cholesterol (P = 0.046). A significant time effect was observed: the recovery period was accompanied by increased ruminal volatile fatty acids, elevated serum energy metabolites, and enhanced antioxidant enzyme activity (e.g., superoxide dismutase) (P < 0.05). Urinary creatinine (P = 0.024) and serum catalase levels (P = 0.096) varied with both YC dose and time. Microbiota analysis revealed that YC reshaped the ruminal and fecal communities by enriching fibrolytic bacteria, such as Ruminococcus, while simultaneously suppressing methanogens like Methanosphaera. We conclude that supplementing YC contributes to adaptation to HS by modulating glycolipid metabolism, antioxidant pathways, and microbial ecology. At the recommended dosage of 30 g/steer/d, YC supplementation enhances feed efficiency, improves farm profitability, and establishes a viable strategy for sustainable beef production.