INFLUENCE OF PARTITIONING DATA BY SEX ON GENETIC VARIANCE AND COVARIANCE COMPONENTS FOR WEANING WEIGHT IN BEEF-CATTLE

Heterogeneity of (co)variance components (VC) by sex is currently acco unted for in national genetic evaluations for Simmental cattle. Parame ters used in the national evaluation program are estimated from data s plit into male, female, and steer populations. Analyzing only male dat a does not account for selection on females, and vice versa. To determ ine the impact of selection, a Monte Carlo simulation was used, and es timates of VC for weaning weight were obtained when data were partitio ned by sex. Weaning weight data were simulated using homogeneous VC fo r males and females for random and selected populations. Restricted ma ximum likelihood estimates were obtained for direct and maternal genet ic and permanent and temporary environmental variances and genetic cov ariance between direct and maternal effects by analyzing complete or s plit data. Estimates differed (P <.01) from input values in data from selected populations split by sex, yielding a spurious heterogeneity o f VC for sex. The heterogeneity was reduced in models using genetic gr oups but not completely removed. Splitting data by sex also influenced VC estimates in data simulated with heterogeneous VC for males and fe males.