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

Relative concentrations of αS -casein (αS -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 αS -CN phosphorylation isoforms and the phosphorylation degree of αS -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 αS -CN phosphorylation isoforms and with the phosphorylation degree of αS -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 αS -CN phosphorylation isoforms and in the phosphorylation degree of αS -CN. As lactation progressed, we observed a significant increase for relative concentrations of αS -CN isoforms with higher degrees of phosphorylation (αS1 -CN-9P, αS2 -CN-13P, and αS2 -CN-14P) as well as for the phosphorylation degree of both αS1 -CN and αS2 -CN. Furthermore, the β-CN I variant was associated with a greater proportion of isoforms with lower degrees of phosphorylation (αS1 -CN-8P, αS2 -CN-10P, and αS2 -CN-11P); the β-CN B variant was associated with a greater proportion of isoforms with higher degrees of phosphorylation (αS1 -CN-9P, αS2 -CN-12P to αS2 -CN-14P). The heritability estimates were low to moderate for relative concentrations of αS2 -CN phosphorylation isoforms (0.07 to 0.32), high for relative concentrations of αS1 -CN-8P (0.84) and αS1 -CN-9P (0.56), and moderate for phosphorylation degrees of αS1 -CN (0.37) and αS2 -CN (0.23). Future studies investigating relations between the phosphorylation degree of αS -CN and technological properties of milk will be beneficial for the dairy industry.