Abstract
Urea has been used in diets of cattle as a non-protein nitrogen source. It is hydrolyzed to ammonia, which can be used for microbial protein synthesis. Use of metabolomics methodologies to study the rumen microbial and host blood metabolic profiles induced by urea nitrogen has not been previously characterized. The objective of this study was to identify changes in rumen microbial and plasma metabolite profiles in dairy cows after urea supplementation using a nuclear magnetic resonance (NMR)-based untargeted quantitative metabolomic approach. Six dairy cows with rumen fistulas were randomly assigned to two groups used in a two period cross-over trial and each experimental period lasted 21 days. All the cows were fed the same total mixed rations, but were intraruminally supplemented with 180 g urea per cow daily or not during the experimental period. Rumen fluid and blood samples were collected and analyzed using nuclear magnetic resonance spectroscopy and multivariate analysis of variance. Differences in rumen and plasma metabolite concentrations in cows from the two groups were assessed using orthogonal partial least-squares discriminant analysis and identified by searching against related databases. Concentrations of valine, aspartate, glutamate, and uracil in the rumen, and urea and pyroglutamate in the plasma, were higher (P < 0.05) in the urea-supplemented group than in the control group. Metabolic pathway analysis of the affected metabolites revealed that pantothenate and CoA biosynthesis, beta-alanine metabolism, valine, leucine, and isoleucine metabolism in the rumen, and urea and glutathione metabolism in the plasma were significantly increased by urea nitrogen. The levels of aspartate and glutamate in the rumen correlated strongly (P < 0.01) with the level of urea in plasma. These findings provided novel information to aid understanding of the metabolic pathways affected by urea nitrogen in dairy cows, and could potentially help to guide efforts directed at improving the efficiency of urea utilization in the rumen.
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