Serum metabolome profiling revealed potential biomarkers for milk protein yield in dairy cows

Milk yield (MY) and milk protein (MP) content are crucial milk performance traits of dairy cows that directly affect the dairy profits. This study first proposed milk protein yield (MPY) by considering MY and MP content together. Forty multiparous cows were selected from the 348 Holstein dairy cows, which fed the same diet under the same management condition, to investigate the serum metabolome profiles and to identify key metabolites associated with MPY. Among them, 20 cows with a higher MPY (MY > 34.5 kg/d and MP > 3.2%. i.e., MPY > 1.11 kg/d) were defined as the HH group, and 20 cows with a lower MPY (MY < 31 kg/d and MP < 2.9%, i.e., MPY < 0.87 kg/d) as the LL group. The GC-TOF/MS and the ultra HPLC-MS/MS platforms were used to identify metabolites and quantify biomarkers, respectively. Orthogonal partial least squares discriminant analysis of serum metabolomes revealed a clear separation between the 2 groups. Thirty-six significantly different metabolites were identified, which mainly involved in valine, leucine and isoleucine biosynthesis and glycine, serine and threonine metabolism. With biomarker analysis and validation, hippuric acid, nicotinamide and pelargonic acid may serve as key metabolites associated with MPY. Biological significance: This study reports the application of serum metabolomics to identify biomarkers related to MPY and to reveal the biological pathways affecting milk protein synthesis. Three novel serum biomarkers were discovered to reflect the MPY variation of dairy cows, which may be useful in quality control in dairy cow production and for optimizing industrial production of dairy products. This study confirms that individual physiological and metabolic differences contribute to the variations in MPY and provides directions for further improving the MPY of dairy cows.