Determining the Optimal Dietary Barley-to-Corn Ratio for Arbas White Cashmere Goats: Insights from Rumen Fermentation, Meat Metabolomics, and Gastrointestinal Microbiota

This study investigated the effects of graded dietary barley replacement for corn on growth performance, rumen fermentation, carcass traits, meat quality, fatty acid profiles, and the rumen microbiome in Arbas White Cashmere goats, and further applied untargeted muscle metabolomics to elucidate the metabolic mechanisms underlying meat quality alterations. Growth performance was monitored for all goats (n = 10 per group); rumen fermentation, carcass traits, serum biochemistry and meat quality were determined in six randomly selected goats per group (n = 6); and rumen microbiota profiling and muscle metabolomics were performed on three selected goats per group (n = 3). Partial barley substitution (33% and 67% of starch from barley) significantly improved final body weight and average daily gain (p < 0.05), whereas total replacement (100% barley starch) did not confer additional growth advantages. Moderate barley inclusion increased ruminal propionate concentration (p = 0.001) and enriched the fiber-degrading bacterium Prevotella, while total barley replacement markedly reduced rumen microbial diversity (Sobs, Chao1, ACE, and Shannon indices; p < 0.01) and depleted multiple fibrolytic and hydrogenotrophic genera, including Christensenellaceae_R-7_group, NK4A214_group, and Methanobrevibacter. The 33% barley group exhibited elevated serum total bile acids, FGF-19, and CYP27A1 levels (p < 0.05), suggestive of modulated bile acid metabolism, along with decreased glucose-6-phosphatase (p < 0.05), pointing to suppressed hepatic gluconeogenesis. Muscle metabolomics revealed that moderate barley substitution predominantly affected glycerophospholipid metabolism, with upregulation of PE(P-18:1/20:4) and PC(16:0/16:0), whereas total replacement induced extensive metabolic reprogramming characterized by downregulation of glycine and glutathione-related metabolites, indicative of compromised antioxidant defense. The 33% barley group also exhibited the highest muscular C18:3n-3 content (p < 0.01), while the 67% and total replacement groups showed increased C22:6n-3 deposition (p < 0.05). Correlation analysis confirmed that PE(P-18:1/20:4) and PC(16:0/16:0) were significantly positively correlated with ruminal propionate and ADG (r = 0.841–0.986; p < 0.05), and Prevotella abundance showed a strong negative trend with G-6-Pase (r = −0.771). Collectively, these findings demonstrate that replacing one-third of dietary corn starch with barley starch optimally balances productivity, rumen health, and meat quality through enrichment of Prevotella, enhanced propionate production, and modulation of bile acid metabolism, while excessive barley inclusion destabilizes the rumen ecosystem and triggers muscle oxidative stress, providing a theoretical basis for precision grain formulation in intensive cashmere goat production.