Components of Genetic Variance in Milk Yield

Abstract

The purpose of this research was to determine the importance of non-additive genetic effects and genetic maternal effects in the inheritance of milk production in dairy cattle. Expressing milk yield as a deviation from herdmate average eliminated most of the variation due to herds, years, seasons, and their interactions without affecting the expression of additive genetic differences between sires for milk yield. It is clearly demonstrated that expressing records as deviations does not eliminate effects of dam and herd-year-season confounding. Genetic maternal effects were found to be of little or no importance. The magnitude of the sire by maternal grandsire component estimate implies that non-additive genetic variance is near zero. An example is used to show that even should genetic maternal effects and non-additive genetic effects be sizeable, the selection index weights for records on relatives and the accuracy of the index are only slightly different from those obtained assuming strictly additive inheritance.