Estimation of genetic variance and covariance components for weaning weight in Simmental cattle.

The objectives of this study were to estimate genetic parameters for weaning weight of Simmental cattle from data without selective reporting and to examine heterogeneity of parameters with a multiple trait approach. Heterogeneity of (co)variance components (VC) by sex is accounted for in national genetic evaluations for Simmental cattle. Completely reported data were split into bull, heifer, and steer populations to obtain VC estimates. Estimates of direct-maternal genetic correlation were negative, which suggests that selective reporting was not a cause of a negative correlation in Simmental data. However, analyzing only data for males does not account for selection on females and vice versa. Heterogeneous VC for sex were evaluated by analyzing Simmental data using a multiple trait model where male and female data were treated as two traits. Estimates of heritability of direct (maternal) effects were .19 (.07) and .25 (.12) and estimates of the direct-maternal genetic correlation were -.05 and -.20 for males and females, respectively. The multiple trait model fit the data better (P < .01) than the model under the assumption of homogeneous VC.