Bos taurus_feces samples_16S rRNA sequencing

Milk urea nitrogen (MUN), the primary form of non-protein nitrogen in milk, is an indirect indicator of nitrogen metabolism in dairy cows. MUN concentrations are modulated by various factors, including dietary composition, physiological status, and environmental conditions. However, the roles of host gut microbiota and metabolome in the development of distinct MUN phenotypes remain insufficiently elucidated. In this study, fecal microbiota and fecal and serum metabolomes were compared between high-MUN (HMUN) and low-MUN (LMUN) cows (n = 7 per group) under uniform feeding and management conditions using 16S rRNA gene sequencing and LC-MS-based metabolomics. Compared with LMUN cows, HMUN cows exhibited increased abundances of UCG-009, UCG-002, and Christensenellaceae_R-7_group, and decreased abundances of Succinivibrio, Lachnospiraceae_NK3A20_group, Acetitomaculum, Prevotellaceae_UCG-001, and norank_f__Bifidobacteriaceae in fecal samples. Metabolomic analysis revealed that HMUN cows had lower fecal levels of hydroxypropionic acid, N-myristoyl arginine, and N-eicosapentaenoyl tryptophan, as well as reduced serum concentrations of L-serine, linoleic acid, and butyrate. KEGG pathway analysis indicated that metabolites enriched in LMUN cows were primarily involved in the beta-alanine metabolism pathway. Correlation analysis showed that UCG-009 and Christensenellaceae_R-7_group were positively associated with MUN, whereas Succinivibrio, Lachnospiraceae_NK3A20_group, Anaeroplasma, and Acetitomaculum were negatively associated with MUN. These microbial taxa were also significantly correlated with several fatty acid and amino acid metabolites, including adipic acid, undecenoic acid, dodecanedioic acid, DL-tryptophan, and L-leucine. Collectively, these results demonstrate distinct differences in gut microbiota composition and metabolic profiles between cows with high and low MUN levels, suggesting that gut microbial and metabolic alterations may contribute to the regulation of MUN phenotypes and nitrogen metabolism in dairy cows.