ADDITIVE AND NONADDITIVE GENETIC-EFFECTS ON MILK-PRODUCTION IN DAIRY-CATTLE - EVIDENCE FOR MAJOR INDIVIDUAL HETEROSIS

Coefficients for individual and maternal breed composition and the expected contributions of individual and maternal heterosis and breed source of cytoplasm were assigned to 42,554 primiparous Holstein-Friesian, Jersey, and crossbred cows. The individual additive genetic breed effect influenced all milk production traits. Highly significant maternal additive genetic breed effects equivalent to 3% of the mean were identified for milk yield and milk fat percentage. Individual heterosis was highly significant for milk yield and milk fat yield. A primiparous first cross cow produced 6.1% more milk and 7.2% more milk fat than the average of straightbred cows of both breeds. For milk fat yield, the individual heterosis effect was higher than the individual additive genetic breed difference between Jersey and Holstein-Friesian. A small negative maternal heterosis and a small effect of breed source of cytoplasm were estimated for milk fat percentage. Results suggest that individual heterosis is a major genetic effect for milk yield and milk fat yield. This heterosis could be utilized through a stratified breeding scheme in which high genetic merit nucleus herds maintain genetic progress in the two straightbred populations, and commercial dairy herds employ a rotational crossbreeding scheme to take advantage of both the additive genetic progress and nonadditive genetic effects.