Associations between casein haplotypes and first lactation milk production traits in Finnish Ayrshire cows

The objective of this study was to estimate the effects of beta-kappa-casein (CN) haplotypes on first-lactation milk production traits. The beta-kappa-CN haplotypes were deduced using information on beta- and kappa-CN genotypes of cows and their sires for 16,973 Finnish Ayrshire cows that had at least nine paternal half sibs. Effects of CN haplotypes on milk production traits were estimated for one haplotype at a time using an animal model, which included the fixed effects for calving year and month, age at calving, days open, beta-lactoglobulin, and a beta-kappa-CN haplotype. Differences in milk production traits were also estimated between haplotype combinations A1A+A2B and A1B+A2A within beta-kappa-CN genotype A1A2AB and between combinations A1E+A2A and A1A+A2E within genotype A1A2AE. The beta-kappa-CN haplotypes A2A and A2B were associated with high milk and protein yields and low fat content, and those that included the beta-CN A1 allele were associated with low yields and high fat content. Protein content was affected by the kappa-CN locus; haplotype A1B was associated with high protein content and A1E was with low protein content. The haplotype combination A1A+A2B was associated with 140 kg more milk yield (P = 0.045) and 0.03 percentage units less protein content (P = 0.055) than combination A1B+A2A, and combination A1A+A2E showed 0.02 percentage units greater protein content (P = 0.098) than A1E+A2A. These results indicate that genes linked to the CN loci contribute to the variation in milk yield and protein content.