Estimation of Direct and Maternal Additive and Heterotic Genetic Effects for Preweaning Traits in Beef Cattle.

Progency records from 4 generations of a rotational crossbreeding experiment were fitted to a regression equation to obtain estimates of direct and maternal additive (Ig, Mg) and heterotic (Ih, Mh) genetic effects of preweaning traits for Angus (A), Brahman (B), Charolais (C), Hereford (H) breeds and their combinations. These estimates were fitted to prediction equations to compare preweaning performance of breed groups under different mating systems. Heterosis estimates were used to compute expected heterosis for each rotational cross. The C had the largest Ig estimate for BWT (5.2 kg), ADG (.074 kg/d) and WWT (22.1 kg) while H had the largest Ig estimate for SCORE (.73 units). Angus and C had similar and larger Mg estimates for BWT than H and B. Angus and B had the lowest Ig and Mg estimates, respectively, of $-4.2$ kg for BWT. The B had the largest Mg estimate (.070 kg/d) for ADG. Brahman and C had similar and larger Mg estimates for WWT and SCORE than A and H. The H had the lowest Mg estimate for ADG ($-.107$ kg/d), WWT ($-23.0$ kg) and SCORE ($-.95$ units). Brahman crosses (AB, BC and BH) had the largest Ih estimates for all preweaning traits. Brahman crosses also had the lowest Mh estimates for BWT. Except for BH cross, Brahman crosses had the lowest Mh estimates for ADG, WWT and SCORE. Hereford crosses (AH, BH and CH) had the largest Mh estimates for ADG, WWT and SCORE. Brahman- and Charolais-sired crossbred calves generally had larger predicted BWT, ADG and WWT than those sired by A and H bulls. Brahman-Charolais and CB F1 cross calves had the largest predicted ADG and WWT. Angus- and Hereford-sired backcross calves generally had larger predicted SCORE than backcross calves sired by B and C bulls. Most stabilized rotational crossbred calves with more B and C breeding had the largest predicted ADG and WWT while those with more A and H breeding had the largest predicted SCORE. Comparisons of predicted vs realized heterosis suggested the presence of unaccounted variation in the expression of heterosis. The sign of the differences suggested that recombination loss effects may be positive or negative. Comparisons of breed group and genetic effects models suggested no evidence of epistasis and linkage contributions to differences among breed groups