EFFECT OF ACCOUNTING FOR DIFFERENT PHENOTYPIC VARIANCES BY SIRE BREEDAND SEX ON SELECTION OF SIRES BASED ON EXPECTED PROGENY DIFFERENCES FOR 200-DAY 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 Heref ord and Angus cows were investigated. Data consisted of 6,977 and 6,53 0 records of (weaning) and 365-d (yearling) weights, respectively, of F-1 calves sired by bulls (662 and 661, respectively) of 23 breeds tha t have been evaluated in the Germ Plasm Evaluation Program at the U.S. Meat Animal Research Center, Clay Center, NE. Models compared include d fixed effects of genetic group of sire (samples of sires evaluated a t 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 eff ects. Variance structures were different: Model I assumed homogeneous variances across sire breed-sex subclasses; Model II accounted for dif ferences in phenotypic variance by sire breed and sex subclasses. Diff erences 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 a nd .99 or larger when computed across breeds. There were slight change s in the numbers of sires contributed by different breeds to the propo rtions selected across breeds under different selection intensities wh en 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 predic tion for expected progeny differences and standard errors for estimate s of breed effects obtained when adjusting for differences of phenotyp ic variances, compared to not adjusting, were proportional to the rati os of the phenotypic standard deviations of the sire breeds to the com mon phenotypic standard deviation.