Effects of Environmental Factors and Milk Protein Polymorphism on Composition of Casein Fraction in Bovine Milk

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Abstract
Test-day samples were collected from individual Holstein cows in 62 herds enrolled in the Quebec [Canada] Dairy Herd analysis Service. Samples were analyzed for protein, fat, casein and serum protein content, somatic cell count and relative percentages of .alpha.2-, .beta.- and .kappa.-casein and .alpha.-lactalbumin. Cows included in the study were phenotyped for the genetic variants of .alpha.s1-, .beta.- and .kappa.-casein. Unadjusted means for relative percentages of .alpha.s-, .beta. and .kappa.-casein were 59.85, 31.23 and 8.93%, respectively. Least-squares analyses showed that month of test, stage of lactation, age of the cow somatic cell count and phenotype of the cow for .beta.-casein contribution to variations in the relative percentages of .alpha.s- and .beta.-casein. Month of test, somatic cell count and phenotype of the cow for .kappa.-casein also had a significant effect on the relative percentage of .kappa.-casein. When test-day milk yield, percentage of fat, protein, casein and serum protein, casein to protein ratio and relative percentage of .alpha.-lactalbumin were included in the model as covariates, only casein percentage did not have a significant effect on the relative percentages of .alpha.s- and .beta.-casein. For .kappa.-casein, only fat percent was significant.