EFFECTS OF MILK PROTEIN GENOTYPES ON THE VARIATION FOR MILK-PRODUCTION TRAITS OF HOLSTEIN AND JERSEY COWS IN CALIFORNIA

The objectives of this study were to form appropriate composite kappa- beta-alpha(s1)-CN genotypes in order to assess which genotypes were fa vorably associated With first lactation milk production, fat and prote in percentages, and fat and protein production for data of 916 Holstei n and 116 Jersey cows. Multiple-trait animal models were used with ass umed fixed effects for herd, year and season of calving, age at calvin g, days open, composite kappa-beta-alpha(s1)-CN genotypes, and beta-LG genotypes. The differences between the beta-LG genotypes for producti on traits were not statistically significant for either breed. The pro portion of phenotypic variance that was due to the composite kappa-bet a-alpha(s1)-CN genotypes was 5% for milk production, 4% for protein pr oduction, and 3% for fat percentage. The kappa-beta-alpha(s1)-CN genot ype ABA(1)A(2)BB was superior to the comparable AAA(1)A(2)BB and ABA(1 )A(1)BB genotypes by 252 sind 338 kg for first lactation milk yield, r espectively, and 8.7 and 11.5 kg for protein yield, respectively. Thus , neither the beta-CN A(2) allele nor the kappa-CN B allele alone had a positive effect on milk and protein production, but the joint; effec t was strongly positive. These results may be explained by epistatic e ffects between the kappa-CN and beta-CN loci and possibly by closely L inked quantitative trait loci with favorable alleles on the kappa-beta -alpha(s1)-CN BA(2)B haplotype.