Adding the probiotic Bacillus coagulans to yak feed helped the animals grow faster, improved their feed efficiency, and raised blood levels of growth‑related hormones like IGF‑1 and GH, as well as immune markers. It also shifted the gut microbes toward more beneficial species and cut down harmful ones, which seemed to support better metabolism and immune function.

The role of probiotics in improving animal growth performance and immune function is increasingly recognized. However, their effects on the growth performance, immune function, rumen microbiota and metabolites of yak in China's high-altitude regions remain unclear. In this study, 20 two-year-old yaks of similar body weight were randomly divided into two groups, with 10 animals in each group. One group served as the control group (CG) and was fed a basal diet, while the other group was fed a basal diet supplemented with 1 g/kg of Bacillus coagulans (live bacteria count ≥ 5 × 10⁹ CFU/kg) as the experimental group (CP) for 60 days. The supplementation of probiotics resulted in a 20.6% increase in body weight in the CP group, which was 5.5% higher than that in the CG group, and the feed-to-gain ratio in the CP group was 2.42 lower than that in the CG group. Additionally, compared with the CG group, the levels of IgM, IgA, IgG, IL-10, IGF-1, GH, and NOS were significantly increased in the CP group (p < 0.05). Notably, the addition of Bacillus coagulans led to a significant increase in beneficial bacteria such as Prevotella and Roseburia in the rumen of the CP group, while pathogenic bacteria such as Treponema were significantly reduced. This may have regulated metabolic pathways involving amino acids and promoted the production of immune and growth-related metabolites such as Ile-Leu-Thr and Glu-Gly-Ser, ultimately enhancing the immune function and growth performance of yak. Overall, these results are encouraging, suggesting that probiotics may serve as an effective alternative to antibiotic growth promoters for improving yak growth performance and intestinal health.