Adjustment for heterogeneity of variance for milk and fat yield among herds of Brown Swiss in Brazil

In order to verify the effect of including the interactions of sire x herd and the sire x herd-year, as a adjust factor of the variance heterogeneity, registers of milk and fat yields were classified into two classes of standard deviation: low ( <= 1.280kg) and high (> 1.280kg), based on phenotypic standard deviation of the milk production adjusted. Three models, without and with interaction of sire x herd and sire x herdyear, were used in the general univariate analyses and in each standard deviation class. Averages and variance components were higher in the high standard deviation. In the class of low standard deviation, heritability didn't alter with the inclusion of the interaction effects in the model, being of 0.34 for milk yield and 0.32 for fat yield. In the class of high standard deviation, heritabilities were: 0.37, 0.35 and 0.36, and of 0.35, 0.32 and 0.35, for milk and fat yield, in the models without and with interaction of sire x herd and with interaction of sire x herd-year, respectively. The inclusion of the interaction of sire x herd was significant (P< 0.01) for fat yield, in general analyses and in the high standard deviation class, in the likelihood ratio test.