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

Department of Animal Science, Macdonald College of McGill University, 21,111 Lakeshore Road, Ste Anne de Bellevue, Quebec, Canada H9X 1CO

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ABSTRACT

Test-day samples were collected from individual Holstein cows in 62 herds enrolled in the Quebec Dairy Herd Analysis Service. Samples were analyzed for protein, fat, casein, and serum protein content, somatic cell count, and relative percentages of -, ß-, and -casein and a-lactalbumin. Cows included in the study were phenotyped for the genetic variants of _s1-, ß-, and -casein. Unadjusted means for relative percentages of _s-, ß-, and -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 ß-casein contributed to variations in the relative percentages of _s- and ß-casein. Month of test, somatic cell count, and phenotype of the cow for -casein also had a significant effect on the relative percentage of -casein. When test-day milk yield; percentages of fat, protein, casein, and serum protein; casein to protein ratio; and relative percentage of -lactalbumin were included in the model as covariates, only casein percentage did not have a significant effect on the relative percentages of _s- and ß- casein. For -casein, only fat percent was significant.