Impacts of Fermented Distillers Grains Inclusion on Growth Performance, Nutrient Digestibility, Rumen Fermentation, and Gut Microbiota of Crossbred Simmental Steers

This study evaluated the effects of dietary incorporating with fermented distillers grains (FDGs) on growth performance, nutrient digestibility, rumen fermentation, and ruminal/fecal microbiota in crossbred Simmental steers. During a 28-day feeding trial, 36 steers were randomly assigned to control (basal diet only), 4%, and 8% FDG groups, with FDG supplemented on a dry matter basis. Dietary treatments did not alter growth performance or total tract apparent nutrient digestibility. However, 8% FDG improved ruminal neutral detergent fiber (NDF) digestibility and elevated ruminal concentrations of microbial crude protein (MCP), propionic acid, valeric acid, and total volatile fatty acids, shifting fermentation toward a propionate-oriented profile. High-throughput sequencing demonstrated that 8% FDG increased ruminal alpha diversity and altered beta-diversity community structure. Spirochaetota abundance decreased, whereas Cyanobacteriota and Rikenellaceae_RC9_gut_group abundance increased. Redundancy and Spearman correlation analyses identified six upregulated genera, notably Saccharofermentans and an unclassified genus within Victivallaceae, which strongly drove propionate, valerate, and MCP levels. PICRUSt2 predictions revealed enrichment of interconnected pathways involved in nucleotide, carbohydrate, and amino acid metabolism. Fecal microbiota had stable alpha diversity, with only selective enrichment of the phylum Bacillota. In conclusion, 8% FDG restructured ruminal microbial, enhancing fiber degradation and propionate-oriented fermentation efficiency while maintaining hindgut homeostasis.