Effects of Milk Protein Genotypes on the Variation for Milk Production Traits of Holstein and Jersey Cows in California

Abstract

The objectives of this study were to form appropriate composite κ-β-α_s1-CN genotypes in order to assess which genotypes were favorably associated with first lactation milk production, fat and protein percentages, and fat and protein production for data of 916 Holstein and 116 Jersey cows. Multiple-trait animal models were used with assumed fixed effects for herd, year and season of calving, age at calving, days open, composite κ-β-α_s1-CN genotypes, and β-LG genotypes. The differences between the β-LG genotypes for production traits were not statistically significant for either breed. The proportion of phenotypic variance that was due to the composite κ-β-α_s1-CN genotypes was 5% for milk production, 4% for protein production, and 3% for fat percentage. The κ-β-α_s1-CN genotype ABA₁A₂BB was superior to the comparable AAA₁A₂BB and ABA₁A₁BB genotypes by 252 and 338 kg for first lactation milk yield, respectively, and 8.7 and 11.5 kg for protein yield, respectively. Thus, neither the β-CN A₂ allele nor the κ-CN B allele alone had a positive effect on milk and protein production, but the joint effect was strongly positive. These results may be explained by epistatic effects between the κ-CN and β-CN loci and possibly by closely linked quantitative trait loci with favorable alleles on the κ-β-α_s1-CN BA₂B haplotype.