Genetic and nongenetic factors contributing to differences in α-casein phosphorylation isoforms and other major milk proteins.

Relative concentrations of α-casein (α-CN) phosphorylation isoforms vary considerably among milk of individual cows. We aimed to explore to what extent genetic and other factors contribute to the variation in relative concentrations of α-CN phosphorylation isoforms and the phosphorylation degree of α-CN defined as the proportion of isoforms with higher degrees of phosphorylation. We also investigated the associations of genetic variants of milk proteins and casein haplotypes with relative concentrations of α-CN phosphorylation isoforms and with the phosphorylation degree of α-CN in French Montbéliarde cattle from the cheese production area of Franche-Comté. Detailed milk protein composition was determined by liquid chromatography coupled with electrospray ionization mass spectrometry from 531 test-day morning milk samples. Parity, lactation stage, and genetic variation of cows contributed to the phenotypic variation in relative concentrations of individual α-CN phosphorylation isoforms and in the phosphorylation degree of α-CN. As lactation progressed, we observed a significant increase for relative concentrations of α-CN isoforms with higher degrees of phosphorylation (α-CN-9P, α-CN-13P, and α-CN-14P) as well as for the phosphorylation degree of both α-CN and α-CN. Furthermore, the β-CN I variant was associated with a greater proportion of isoforms with lower degrees of phosphorylation (α-CN-8P, α-CN-10P, and α-CN-11P); the β-CN B variant was associated with a greater proportion of isoforms with higher degrees of phosphorylation (α-CN-9P, α-CN-12P to α-CN-14P). The heritability estimates were low to moderate for relative concentrations of α-CN phosphorylation isoforms (0.07 to 0.32), high for relative concentrations of α-CN-8P (0.84) and α-CN-9P (0.56), and moderate for phosphorylation degrees of α-CN (0.37) and α-CN (0.23). Future studies investigating relations between the phosphorylation degree of α-CN and technological properties of milk will be beneficial for the dairy industry.