Bacillus-based probiotic supplementation reshapes rumen bacterial and fungal communities and enhances carbohydrate-degrading functional capacity in weaned yaks.

This study evaluated the effects of dietary supplementation with Bacillus-based probiotics on growth performance, nutrient digestibility, rumen fermentation, and microbial functional capacity in weaned yaks. Twenty animals were randomly assigned to a basal diet (control group, CON) or the same diet supplemented with Bacillus subtilis and Bacillus licheniformis (probiotic group, PRO) for 90 days. Probiotic supplementation increased average daily gain (P < 0.05) and tended to increase dry matter intake (P = 0.059). In addition, neutral detergent fibre and acid detergent fibre digestibility were improved (P < 0.05), suggesting improved degradation of structural carbohydrates. Rumen fermentation was altered, with increased concentrations of butyrate and isovalerate and reduced ammonia nitrogen, suggesting improved fermentation efficiency and nitrogen metabolism. Microbial analysis showed that probiotics reshaped both bacterial and fungal community structures without affecting α-diversity, indicating selective modulation of key microbial taxa. Notably, the relative abundance of carbohydrate-degrading genera, including Xylanibacter, was increased. Metagenomic analysis further demonstrated changes in microbial functional capacity, as evidenced by increased abundance of carbohydrate-active enzymes and genes associated with cellulose, hemicellulose, chitin, lignin, and starch degradation. These results indicate that Bacillus-based probiotics were associated with improved growth performance and enhanced rumen microbial functional potential related to carbohydrate degradation.