Genetic variances for growth traits in crossbred beef cattle

In one study three variants of an animal model were used to estimate variance components for birth (BW) and weaning (WW) weights, and birth (BH) and weaning (WH) hip heights of three synthetic lines of beef cattle differing in mature size. Data (n = 6869) came from two research farms in Iowa. Model 1 included fixed effects of year, sex, and age of dam. Random effects were additive direct and additive maternal genetic, maternal permanent environmental, and residual. Model 2 also included dominance and Model 3 included dominance plus additive x additive genetic effects. Only slight changes occurred in estimates of other variance components when dominance effects were included in Model 2. Average estimates of direct(maternal) heritabilities were.53(.11),.42(.04),.27(.12) and.35(.04) for BW, BH, WW and WH, respectively. Corresponding fractions of variation explained by dominance genetic effects were.18,.26,.28 and.11. Most estimates of additive x additive genetic variances were negligible. Another study investigated heterogeneity of variance for 200- (WW) and 365-d (YW) weights for sex, sire and dam breed subclasses. Data consisted of records of progeny (n = 7829) from 673 sires of 23 breeds evaluated in a topcross experiment. Sires were mated to Hereford and Angus cows. Sire and dam models were fitted for each trait. For both WW and YW, variances differed (P $<$.02) in scale, but not as fractions of phenotypic variance (P $>$.10), for sire breed and sex subclasses. Variances were not different (P $>$.10) for dam breed subclasses, either in scale or as fractions. Estimates of correlations among genetic effects on weights of calves expressed in different sex-dam breed subclasses were at least.85. Effects of accounting for different phenotypic variances according to sire breed and sex subclasses on estimation of sire breed effects and prediction of genetic values of sires were investigated in a third study. Two models were compared: Model I assumed homogeneous variances; Model II accounted for differences in phenotypic variances for sire breed and sex subclasses. Differences between breed effects estimated with the two models were not significant for WW and YW. Product-moment and rank correlations between genetic values predicted with Models I and II were greater than.93.