Review: Shifts of rumen microbiota by feeding non-fibrous carbohydrates to improve cattle performance.

Ruminants play an essential role in food production due to their ability to utilize forages through fermentation in the rumen. This fermentative chamber hosts a diverse microbial community capable of degrading fiber and non-fiber carbohydrates, producing short-chain fatty acids (SCFAs) and microbial protein, which are essential for the animal's metabolism. Throughout their evolution, ruminants developed a symbiotic relationship with microorganisms specialized in the degradation of plant fibers, enabling the use of forages as a dietary foundation. However, modern intensive production systems have introduced concentrate ingredients to their diets (such as grains and industrial by-products), which represent a significant departure from ancestral diets based exclusively on forages. Dietary composition is the primary factor driving changes in the ruminal microbiota and can significantly alter its composition. Variations in the forage-to-concentrate ratio can drastically alter microbial activity, affecting the stability of the ruminal ecosystem. Sequencing technologies and omics approaches have enhanced the understanding of this ecology, allowing for more effective nutritional interventions. The objective of this review is to assess how contemporary diets in intensive production systems differ from ancestral, forage-only diets and how these differences reshape the ruminal microbiota. To this end, we characterized the variations in the ruminal microbiota composition of animals fed high-concentrate and high-forage diets, describing the specific microbial profiles of each condition and identifying beneficial and potentially detrimental microorganisms. This review synthesizes current evidence on how dietary transitions reshape ruminal microbial cross-feeding networks and proposes an integrative framework linking microbial symbiotic balance, rumen health, and production efficiency. By emphasizing the dynamic regulation of microbial interactions rather than isolated taxa, this work highlights cross-feeding stability as a central target for nutritional, microbial, and genetic interventions in intensive ruminant production systems.