Data from 6,842 lactations of 3,274 Brown Swiss cows, daughters of 71 sires, from 100 herds, calving from 1980 to 1999 were used to evaluate the effects of heterogeneity of variance on genetic evaluation of bulls. The milk and fat yields adjusted for 305 days of lactation and mature equivalent age were used to classify the herds among high and low production level. An animal model used to estimate variance component, included the fixed effect of herds, age-season of calving and grade of cows, random effect of animal, permanent environment and temporary environment. Variance components were estimated to milk (fat) yield in booth levels, considering the milk or fat yield in each production level as different trait. Estimates of heritability were .38, .38, .39 and .32 for milk and fat yields in high and low production levels, respectively. Genetic correlations between high and low production levels were .85 and .79 to milk and fat yields, respectively. In genetic evaluation programs , it is important to consider differences in variability among herds, otherwise, under selection, the most variables herds would contribute with the majority of the animals, and the genetic evaluations of the animals could be more a function not only from its genetic potential, but also from its environment where the progenies expressed their traits.
dairy cattle; genetic evaluation; genetic parameters; milk yield; fat yield
