The objectives of this study were to form appropriate composite kappa-beta-alpha(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 kappa-beta-alpha(s1)-CN genotypes, and beta-LG genotypes. The differences between the beta-LG genotypes for production traits were not statistically significant for either breed. The proportion of phenotypic variance that was due to the composite kappa-beta-alpha(s1)-Cn genotypes was 5% for milk production, 4% for protein production, and 3% for fat percentage. The kappa-beta-alpha(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 beta-CN A2 allele nor the kappa-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 kappa-CN and beta-CN loci and possibly by closely linked quantitative trait loci with favorable alleles on the kappa-beta-alpha(s1)-CN BA2B haplotype.