Abstract
This study investigated the effects of isoacid supplementation on in vitro rumen fermentation characteristics, nutrient degradability, and bacterial community diversity in yaks using corn silage-highland barley straw-based substrates. An in vitro fermentation experiment was conducted with a substrate consisting of 80% whole-plant corn silage and 20% highland barley straw. Treatments included a control (without isoacids) and four isoacid supplemental levels (0.1%, 0.2%, 0.3%, and 0.4% of substrate dry matter, DM), each with six replicates. A 72 h in vitro gas production experiment was performed to measure cumulative gas production, fermentation parameters, nutrient degradability, and bacterial community diversity. Cumulative gas production increased by 12.96% with 0.2% isoacid supplementation compared to the control (p < 0.05). The contents of microbial protein (MCP), acetate, propionate, and total volatile fatty acids (TVFA) exhibited quadratic responses to the increasing isoacid dosage (p < 0.05). Specifically, MCP content reached a maximum of 0.76 mg/mL with 0.2% isoacids, representing a 31.03% increase compared to the control (p < 0.05). TVFA content was highest (146.85 mmoL/L) at 0.2% isoacid supplementation, with a 16.40% increase compared to the control (p < 0.05). Acetate content increased by 17.99% (p < 0.05), while propionate tended to increase with 0.2% isoacid supplementation (p = 0.08). Supplementation with 0.2% and 0.4% isoacids did not alter the bacterial composition and diversity (p > 0.05). However, at the genus level, g_Ruminococcus, g__Elusimicrobium, g_norank_f_Atopobiaceae, g_norank_o_Coriobacteriales, and g_Romboutsia were identified as differential biomarkers showing significant responses to isoacid supplementation (p < 0.05). Mantel-test analysis revealed positive correlation between g_Ruminococcus abundance and NH(3)-N content (r < 0.4, p < 0.05); g_Romboutsia abundance and acetate content (r < 0.40, p < 0.05); g_Defluviitaleaceae_UCG-011 abundance and both NH(3)-N content and the pH of rumen fluid (r < 0.40, p < 0.05); g_norank_o_Coriobacteriales abundance and rumen pH (r < 0.40, p < 0.01). Supplementation with 0.2% isoacids to corn silage-barley straw substrates improved in vitro rumen fermentation characteristics in yaks, which was associated with altered abundances of key bacterial genera including g_Ruminococcus, g__Elusimicrobium, g_norank_f_Atopobiaceae, g_norank_o_Coriobacteriales.