Effects of yeast β-glucan on fermentation parameters, microbial community structure, and rumen epithelial cell function in high-concentrate-induced yak rumen acidosis in vitro.

This study employed a completely randomized design (CRD) to evaluate the effects of graded levels of yeast β-glucan supplementation on subacute ruminal acidosis (SARA) induced by high-concentrate diets in yaks. Seven treatment groups were established: a low-concentrate control (LC; 50 % concentrate + 50 % forage), a high-concentrate control (HC; 80 % concentrate + 20 % forage), and HC diets supplemented with 0.5 %, 1 %, 2 %, 4 %, or 8 % yeast β-glucan. Rumen pH, gas production, and volatile fatty acid (VFA) profiles were measured at 3, 6, 9, 12, and 24 h of in vitro fermentation. At 9, 12, and 24 h, concentrations of deleterious metabolites-lactic acid, endotoxin (LPS), and histamine-were quantified. 16S rRNA gene sequencing was performed to compare microbial community structures among the LC, HC, and HC + 1 % β-glucan (1 % HCG) groups. The effects of fermentation supernatants on inflammatory gene expression and epithelial barrier integrity were evaluated using a yak rumen epithelial cell (YREC) model. Results demonstrated that the LC group maintained ruminal pH > 5.5 at all time points, whereas the HC group exhibited pH < 5.5 after 9, 12, and 24 h. Supplementation with 1-8 % yeast β-glucan significantly elevated pH (P < 0.05) to values comparable with the LC group. Relative to LC, the HC group showed significant reductions in pH, microbial crude protein (MCP), acetate concentration, and acetate:propionate ratio (P < 0.05), concomitant with significant increases in NH₃-N, propionate, butyrate, total VFA, and harmful metabolites (lactic acid, LPS, and histamine) (P < 0.05). Microbial community analysis revealed that HC feeding decreased the relative abundances of specific genera within the Firmicutes and Bacteroidetes phyla. Moreover, the HC diet upregulated pro-inflammatory gene expression and downregulated genes related to oxidative phosphorylation and cytoskeletal integrity in YRECs. Yeast β-glucan supplementation (1-8 %) mitigated SARA-associated perturbations in fermentation parameters-elevating pH, acetate concentration, acetate:propionate ratio, and MCP content; reducing NH₃-N accumulation; and lowering lactic acid, LPS, and histamine concentrations-while modulating key bacterial taxa and attenuating inflammatory responses and epithelial damage. In conclusion, yeast β-glucan effectively alleviates high-concentrate diet-induced ruminal acidosis and promotes rumen health.