Breed effects and heterosis in advanced generations of composite populations for preweaning traits of beef cattle.

The effects of heterosis for gestation length, dystocia, calf survival, birth weight, 200-d weight, and ADG from birth to weaning were evaluated in F1, F2, and combined F3 and F4 generations in three composite populations. Breed effects were evaluated for the nine parental breeds (Red Poll, Hereford, Angus, Limousin, Braunvieh, Pinzgauer, Gelbvieh, Simmental, and Charolais) that contributed to the three composite populations. Breed effects were significant for all traits evaluated except survival at birth. The large differences among breeds in additive direct and additive maternal genetic effects offer a great opportunity to use the genetic differences among breeds to achieve and maintain optimum additive genetic (breed) composition to match genetic resources to a wide range of production-marketing ecosystems. There was no heterosis for gestation length. Mean heterosis for dystocia was significant estimated in F1 but not in F2 or in the combined F3 and F4 generations. Mean heterosis was not significant in any generation for survival at birth, to 72 h, and to weaning for the F1 generation; mean heterosis was significant for survival to weaning for the F2 generation and approached significance (P = .06) for the combined F3 and F4 generations. Mean heterosis over all composite populations and heterosis for each composite population were significant in all generations for weight at birth and at 200 d and for ADG from birth to weaning. Retained heterosis was not less than expected from retained heterozygosity in composite populations for the traits evaluated. These results suggest that heterosis for these traits likely is due to dominance effects and, thus, can be attributed to the recovery of accumulated inbreeding depression in the parental breeds.