Background <p>After weaning, calves undergo a critical transition from liquid milk to solid feed. During this period, the digestive system is not yet fully mature and the rumen microbial community is still being established, which can easily lead to stress responses, growth retardation and other problems. Nutritional management and microecological regulation at this stage are crucial for rumen development, maturation of immune function and subsequent growth performance. Probiotics are regarded as an important nutritional and microecological strategy to alleviate weaning stress and promote the maturation of rumen and intestinal function. Therefore, in the present study, sixteen 8-month-old weaned yaks were randomly allocated to two groups (<i>n</i> = 8 per group): a composite probiotic group (CP) and a control group (CK). In the CP group, a composite probiotic preparation (Bacillus subtilis, Lactobacillus acidophilus and Bacillus licheniformis; viable count ≥ 1 × 10¹⁰ CFU/g) was supplemented at 0.2% of dietary dry matter from the beginning of the formal trial, whereas the CK group received only the basal diet. This study investigated the effects of dietary supplementation with this composite probiotic on the growth performance, serum biochemical indices and rumen microbial community structure and metabolic function of weaned yaks.</p> Results <p>The results showed that dietary supplementation with 0.2% compound probiotics significantly improved growth performance in weaned yaks, reduced serum urea nitrogen, increased serum catalase (CAT) activity, and decreased the pro-inflammatory cytokine IL-1β, thereby providing immune and oxidative-stress protection during the weaning-stress period. Rumen fermentation parameters indicated that the CP group had significantly lower NH₃-N and significantly higher microbial crude protein (MCP), accompanied by significantly increased acetate, butyrate, and total volatile fatty acids (TVFA), suggesting enhanced ruminal energy yield and nitrogen assimilation. Compound probiotics also improved rumen microbial community structure by increasing the abundances of fiber degradation- and fermentation-related taxa, including Prevotellaceae_UCG_001, Ruminococcaceae_UCG_001, and Papillibacter, while decreasing Veillonellaceae_UCG_001, which may contribute to improved fermentation capacity and microecological balance. In addition, compound probiotics enhanced linoleic acid and unsaturated fatty acid–related metabolic pathways, increased the levels of multiple nutritional short peptides and beneficial lipid intermediates, and reduced lipid peroxidation and potentially pro-inflammatory metabolites. Collectively, these findings suggest that compound probiotics may remodel rumen microecology, promote fermentative energy production and efficient conversion of nitrogen into microbial protein, and coordinately regulate lipid metabolism and redox homeostasis, thereby providing a metabolic basis for greater weight gain and improved feed efficiency in weaned yaks under reduced feed intake.</p> Conclusion <p>Compound probiotics may promote the enrichment of fiber-degrading bacteria such as Prevotellaceae_UCG_001, thereby simultaneously enhancing short-peptide supply and VFA production, which in turn increases microbial protein synthesis and reduces losses of NH₃-N and urea nitrogen. Meanwhile, lipid metabolism appears to be reshaped toward a low-inflammation and low–lipid peroxidation profile, accompanied by enhanced antioxidant capacity and attenuated inflammatory responses. This coordinated optimization of ruminal carbon–nitrogen metabolism and host anti-inflammatory and antioxidant defenses may collectively contribute to higher average daily gain and improved feed conversion efficiency in weaned yaks, representing a key pathway through which compound probiotics exert beneficial effects. In addition, the robustness and reproducibility of these findings should be further validated in larger cohorts, through independent replication trials, or using in vitro fermentation systems. </p> Graphical Abstract <p></p>