Effect of accounting for different phenotypic variances by sire breed and sex on selection of sires based on expected progeny differences for 200- and 365-day weights.

The effects of accounting for different phenotypic variances according to sire breed and sex subclasses on estimation of sire breed effects and prediction of expected progeny differences of sires mated to Hereford and Angus cows were investigated. Data consisted of 6,977 and 6,530 records of 200-d (weaning) and 365-d (yearling) weights, respectively, of F1 calves sired by bulls (662 and 661, respectively) of 23 breeds that have been evaluated in the Germ Plasm Evaluation Program at the U.S. Meat Animal Research Center, Clay Center, NE. Models compared included fixed effects of genetic group of sire (samples of sires evaluated at different times), dam breed, sex, birth year of calf and age of dam, plus sire within genetic group and dam within dam breed as random effects. Variance structures were different: Model I assumed homogeneous variances across sire breed-sex subclasses; Model II accounted for differences in phenotypic variance by sire breed and sex subclasses. Differences between estimates of sire group effects obtained with the two models were not significant for either trait. Product-moment and rank correlations between expected progeny differences obtained with Model I and Model II were greater than .93 when computed within each group and .99 or larger when computed across breeds. There were slight changes in the numbers of sires contributed by different breeds to the proportions selected across breeds under different selection intensities when sires were ranked with the two models. However, differences between means predicted under Model II were small when sires were ranked and selected based on the two models. Changes in standard errors of prediction for expected progeny differences and standard errors for estimates of breed effects obtained when adjusting for differences of phenotypic variances, compared to not adjusting, were proportional to the ratios of the phenotypic standard deviations of the sire breeds to the common phenotypic standard deviation.