Estimation of Additive and Nonadditive Direct and Maternal Genetic Effects for Reproductive Traits in Beef Cattle.

Abstract

A total of 4,301 cow exposure records from a four generation rotational crossbreeding study were used to calculate least squares means and heterosis estimates and partition breed and heterotic genetic effects for calving rate (CR), calf survival (CS), weaning rate (WR), calf birth date (CBD) and percent assisted births (PA). Angus (A), Brahman (B), Charolais (C) and Hereford (H) breeds were involved in two-, three- and four-breed rotation systems with the restriction that each breed combination include B. Straightbreds were maintained as controls. Generation, year within generation, breed group of dam within generation and the linear and quadratic effects of cow age were significant sources of variation for all reproductive traits. Sex of calf influenced (P $<$.01) CBD, PA and CS. Overall means for CR, CS, WR, CBD and PA were 80.2%, 92.7%, 73.5%, 50.2 days and 3.9%, respectively. Male calves had lower survival to weaning, were born later in the calving season and required more assistance at birth. Crossbred combinations were superior to straightbreds for CR, CS, WR and PA. Heterosis estimates for CR, CS and WR were significant for most breed combinations, but generally nonsignificant for CBD and PA. Means and heterosis estimates for CR and WR were higher in B-sired crossbred cows. A regression model was used to partition breed direct (Ig) and maternal (Mg) additive and heterotic (Ih and Mh) genetic effects. The cow was the direct observation for CR and WR whereas the calf was the direct observation for CBD, PA and CS. Breed direct and maternal additive genetic effects were expressed as a deviation from $\mu$ and summed to zero. Breed direct and maternal additive effects for the B significantly decreased CR and WR and increased CBD. Breed additive effects for CS were nonsignificant. Heterotic genetic effects were significant for all traits except PA. The IhAB, IhBC and IhBH effects improved CR and WR. Increased CS and earlier CBD's were associated with MhAB, MhBC and MhBH effects. These results confirm the importance of B crossbred cows for increased reproductive performance in the Gulf Coast region.