Determination of effects of milk protein genotype on production traits of Israeli Holsteins.

Genotypes of 112 Israeli Holstein sires were determined for beta-lactoglobulin and kappa-casein by restriction fragment length polymorphism analysis of amplified DNA. Frequencies of the A alleles were .89 and .52 for kappa-casein and beta-lactoglobulin, respectively. Genotype frequencies were in accordance with Hardy-Weinberg equilibrium. Two models were used to test the effect of these loci on production of milk, fat, and protein and on fat and protein percentages. The dependent variable in the first model was production of 84,593 daughters of 110 genotyped sires, and the independent variables were herd-year-season, sire genotype for beta-lactoglobulin and kappa-casein, the interaction of the two loci, and sire nested within genotype. In the second model, the dependent variable was individual animal model genetic evaluations of 111 sires for production traits, and milk protein genotypes were the independent variables. The only significant effect of genotype with both models was the interaction of the two loci on fat percentage. When sires, rather than cows, were genotyped, the contrasted groups differed by only one-half of the genotype effect, but the residual variance was reduced.