Data consisting of 3,531 test day records of 620 first parity Jersey cows were used to estimate genetic parameters for milk yield. Covariance components were estimated by repeatability and random regression animal models. The Wilmink function was used in the random regression model to fit the fixed effect of season of calving and the additive genetic, permanent environment and residual random effects. The repeatability model included the fixed effects of herd-year-month of test, season of calving and age at calving as a covariable with linear and quadratic terms and animal, permanent environmental and residual random effects. The estimates of additive genetic, permanent environmental, residual and phenotypic variances, heritability and repeatability obtained by the repeatability model for milk yield were, respectively: 7.08; 1.88; 7.79 and 16.76 kg², 0.42 and 0.50. The estimates of heritability for milk yield obtained from the random regression model decreased from 0.81 in the beginning of the lactation to 0.20 in the end. Larger estimates of genetic variance were observed at the extremes of the lactation period, but they were practically constant in mid-lactation. The permanent environmental variances increased along lactation. Genetic correlations between milk yields were larger in close test days, particularly in mid-lactation and decreased for the most distant test days. Genetic parameters do not indicate the repeatability model as an alternative to use test day records in order to estimate breeding values for milk yield of Jersey cows in Brazil.
dairy cattle; genetic evaluation; heritability; random regression; repeatability; selection; variance components
