HETEROGENEITY OF VARIANCE BY SIRE BREED, SEX, AND DAM BREED IN 200-DAY AND 365-DAY WEIGHTS OF BEEF-CATTLE FROM A TOP CROSS EXPERIMENT

The nature of the heterogeneity of variance for 200- and 365-d weights by sex, sire breed, and dam breed subclasses was studied. Data consis ted of records for weaning (n = 7,829) and yearling (n 7,367) weights of progeny from 673 and 672 sires, respectively, from 22 breeds that h ave been evaluated in the Germ Plasm Evaluation Program at the U.S. Me at Animal Research Center, Clay Center, NE. Sires were mated to Herefo rd and Angus cows. Each trait was analyzed separately. Three studies w ere undertaken separately to investigate heterogeneity due to the diff erent factors (i.e, sire breed, sex, or dam breed). Only data from sev en sire breeds were used to study the factor sire breed, but all data (22 sire breeds) were used to study the factors sex and dam breed. In each study, three sire and dam models with records of animals of the f our sex x dam breed combinations considered different traits and with the same model equation, but covariance structures for random effects (sires, dams, and residuals) of increasing generality were fitted. Fir st, (co)variances across subclasses were assumed equal. Second, correl ations and fractions of phenotypic variance were assumed equal but phe notypic variance differed by sire breed, sex, or dam breed as appropri ate. Third, variances and covariances were different for each subclass of the factor under study. Variance components were estimated by deri vative-free REML. Models for each trait and each factor were compared through likelihood ratio tests. For both traits, variances differed (P < .02) in scale, but not as fractions of phenotypic variance (P > .10 ), by sire breed and sex subclasses. Variances were not different (P > .10) by dam breed subclasses, either in scale or as fractions of phen otypic variance. Estimates of correlations among genetic effects on we ights of calves from different sex-dam breed subclasses were at least .85. Across all sex, sire breed, and dam breed subclasses, pooled esti mates of sire and dam variances as fractions of phenotypic variance we re, respectively, .06 and .39 for weaning weight and .11 and .24 for y earling weight. The conclusion is that the assumption of equal phenoty pic variances among sire breeds and between sexes may not be appropria te in genetic evaluations.