ASSOCIATION BETWEEN GENETIC-POLYMORPHISM OF MILK-PROTEINS AND PRODUCTION TRAITS DURING 3 LACTATIONS

Polymorphic forms of alpha-s1-casein, beta-casein, kappa-casein, and beta-lactoglobulin were used as genetic markers for milk yield, percentages of fat, and percentages protein during three lactations in 546 Quebec Holstein herds. beta-Casein A2A2 cows produced more milk of lower fat content than beta-casein A1A1 cows for the first, second, and third lactation. alpha-s1-Casein types affected milk yield in the second lacation only (BB > AB > BC). Protein content of milk was influenced by phenotypes of kappa-casein and beta-lactoglobulin for all three lactations. The replacement of A by B allele at the kappa-casein locus would increase protein levels in milk by .08, .06, and .04%, respectively, for the first, second, and third lactation. Similarly, replacement of B by A allele at the beta-lactoglobulin locus is followed by an increase of .05, .07, and .08% protein for the 3 lactation periods. Because of the relatively low frequency of kappa-casein B gene (24%) and moderately low frequency of beta-lactoglobulin A gene (35%) in the Holstein population, it is possible to increase the protein in milk by proper selection of genetic variants.