Balancing High Yield and Metabolic Health in Dairy Ruminants: The Central Hub Role of the Rumen Microbiota

Modern dairy production has greatly increased milk yield, but high productivity is often accompanied by greater metabolic pressure, particularly during the transition period. Ketosis, fatty liver, and subacute ruminal acidosis are major disorders that limit health, efficiency, and sustainability in high-yielding dairy ruminants. This review examines the rumen microbiota as a central biological interface linking diet, ruminal fermentation, epithelial function, hepatic metabolism, and inflammation. Under homeostatic conditions, the rumen microbiota supports lactation by converting dietary fibre, starch, and nitrogen into volatile fatty acids, microbial protein, and other metabolites required for gluconeogenesis, milk component synthesis, and epithelial maintenance. However, under excessive nutritional or physiological stress, especially high-concentrate feeding and periparturient negative energy balance, this system may shift toward dysbiosis, acid accumulation, lipopolysaccharide release, epithelial barrier impairment, and activation of gut-liver inflammatory pathways. These changes can contribute to the occurrence and interaction of subacute ruminal acidosis, ketosis, and fatty liver. We further summarize key factors affecting rumen microbial stability, including diet structure, host variation, physiological stage, environmental stress, feeding management, and ruminal epithelial volatile fatty acid absorption. Finally, microbiome-oriented strategies, such as gradual dietary transition, nutritional preconditioning, probiotics, postbiotics, functional metabolites, host metabolic support, and epithelial-targeted interventions, are discussed. Maintaining rumen microbial homeostasis should be regarded as a core principle for balancing high milk yield with long-term metabolic health. Future research should move beyond descriptive profiling toward causal validation of host-microbe interactions and the development of microbiome-based early-warning and individualized nutritional management systems.