Early-life forage modulation is associated with ruminal microbiota and metabolic profiles in yak calves

Early-life dietary intervention plays an important role in rumen development and is associated with changes in ruminal microbial communities and metabolic profiles in young ruminants. However, the relationships among forage composition, ruminal microbial characteristics, and metabolic profiles during early yak development remain poorly understood. This study investigated the effects of replacing alfalfa hay with oat hay on rumen development and the associated changes in ruminal microbial communities and metabolic profiles in yak calves. Twenty-one male yak calves, approximately 45 days of age at the beginning of the experiment, were randomly assigned to three dietary treatments ( n = 7/group): milk replacer + starter feed + alfalfa hay (RSA); milk replacer + starter feed + a mixture of alfalfa and oat hay, with oat hay replacing 50% of alfalfa hay (RSAO; alfalfa: oat = 1:1); and milk replacer + starter feed + oat hay, with oat hay replacing 100% of alfalfa hay (RSO). Following a 21-day adaptation period, the calves underwent a 120-day feeding trial. Growth performance was evaluated using all seven calves per treatment. At the experimental endpoint, five calves from each treatment ( n = 5/group) were randomly selected for slaughter and collection of ruminal samples for fermentation, histomorphological, 16 S rRNA gene sequencing, and untargeted metabolomic analyses. Compared with single forage diets, the RSAO diet significantly increased body weight gain, ruminal weight, and development of ruminal tissue morphology, accompanied by improved fermentation characteristics. Microbial analysis revealed that forage composition reshaped ruminal microbial community structure, with RSAO calves exhibiting increased microbial richness and enrichment of carbohydrate-associated taxa, including Succiniclasticum , Rikenellaceae-RC9-gut-group , and members of Actinobacteriota and Verrucomicrobiota. These microbial alterations were associated with enhanced fermentation potential and improved nutrient utilization. Furthermore, ruminal metabolomic analysis demonstrated that forage intervention altered metabolic profiles, with differential metabolites mainly associated with microbial transformation processes and metabolic adaptation pathways. Microbiota–metabolite correlation analysis further identified potential associations between Succiniclasticum and ruminal metabolites, suggesting microbial contributions to metabolic remodeling of the rumen ecosystem. Collectively, these findings demonstrate that early-life forage composition regulates ruminal maturation through coordinated remodeling of microbial communities and metabolic networks. A moderate replacement of alfalfa hay with oat hay may provide an effective nutritional strategy for promoting beneficial microbiota establishment and improving developmental outcomes in yak calves. Clinical trial number not applicable.