Reciprocal crossbreeding of Angus and Hereford cattle 2. Steer growth and carcass traits

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Abstract
Breed and heterosis effects for growth and some carcass characteristics were estimated from purebred Angus (A .times. A), purebred Hereford (H .times. H), and crossbred (H .times. A and A .times. H) steers bred at the Waikite Land Development Block near Rotorua over a 4-year period. Data from 477 steers alive at 5-month weaning and 445 steers slaughtered at 20 months-of-age were analysed. Age of dam and age of calf were significant non-genetic effects for all liveweights, 20-month carcass weight, and eye muscle area. The age of dam effect was not significant for average fat depth over the 12th rib of the carcass, but age of calf showed a significant effect. At a constant slaughter age (618 days) steers by Hereford sires had a higher slaughter liveweight, carcass weight, and dressing-out percent than those by Angus sires. Steers out of Hereford cows relative to those from Angus cows were heavier at slaughter and had heavier carcasses but there was no significant effect of cow breed on other carcass traits. At a constant hot carcass weight (169 kg), eye muscle area was larger for steers sired by Angus bulls or out of Angus dams, with little effect of sire breed or dam breed on the other carcass traits. Comparison of purebred calves with the reciprocal crosses showed significant (P < 0.001) heterosis for empty liveweight before slaughter (8.4%), hot carcass weight (10.4%), dressing-out percent (1.9%), fat depth (26.5%), eye muscle area (6.1%), and export carcass grade (-10.2%), when evaluated at a constant slaughter age. At a constant hot carcass weight, heterosis was significant (P < 0.05) for dressing-out percent (0.7%) and fat depth (13.7%). Paternal half-sib heritability estimates increased from 0.19 for 5-month weaning weight to 0.62 for 20-month empty liveweight and 0.60 for hot carcass weight. Heritabilities for carcass traits at a constant slaughter age were moderate to low: fat depth (0.26), dressing-out percent (0.23), export carcass grade(0.39), and eye muscle area (0). Genetic and phenotypic correlations among steer liveweights recorded from weaning to 20 months-of-age and hot carcass weight were high and positive (0.64-0.97). Phenotypic correlation of final liveweight with dressing-out percent, fat depth, and eye muscle area were all moderate to low and positive. Genetic correlations of fat depth with final liveweight or hot carcass weight were negative.