ESTIMATION OF GENETIC VARIANCE AND COVARIANCE COMPONENTS FOR WEANING WEIGHT IN SIMMENTAL CATTLE

The objectives of this study were to estimate genetic parameters for w eaning weight of Simmental cattle from data without selective reportin g and to examine heterogeneity of parameters with a multiple trait app roach; Heterogeneity of (co)variance; components (VC) by sex is accoun ted for in national genetic evaluations for Simmental cattle. Complete ly reported data were split into bull, heifer, and steer populations t o obtain VC estimates. Estimates of direct-maternal genetic correlatio n were negative, which suggests that selective reporting was not a cau se of a negative correlation in Simmental data. However, analyzing onl y data for males does not account for selection on females and vice ve rsa. Heterogeneous VC for sex were evaluated by analyzing Simmental da ta using a multiple trait model where male and female data were treate d as two traits. Estimates of heritability of direct (maternal) effect s were .19 (.07) and .25 (.12) and estimates of the direct-maternal ge netic correlation were -.05 and -.20 for males and females, respective ly. The multiple trait model fit the data better (P < .01) than the mo del under the assumption of homogeneous VC.