Variance components due to direct and maternal effects for growth traits of Australian beef cattle

Variance components for birth, weaning, yearling and final weight in Australian Hereford, Angus and Zebu Cross cattle were estimated by Restricted Maximum Likelihood. Six different animal models were fitted for each trait and breed, ranging from a simple model with animals as the only random effect to the most comprehensive model allowing for both genetic and environmental maternal effects and a genetic covariance between direct and maternal effects. The most detailed model generally provided the best fit to the data, though differences between models with at least one maternal effect (genetic or environmental) were often not significant. Ignoring maternal effects, direct heritability (h²) estimates were inflated substantially, in particular for growth till weaning. Significant maternal effects were found in all analyses except for final weight in Angus. There were marked differences between breeds in the relative magnitude of h² and the maternal heritability, and the direct-maternal genetic correlation (rAM). For Angus, rAM was low, positive and not significantly different from zero for all traits. For Hereford and Zebu Cross cattle, rAM was negative, moderate to large for weaning weight (-0.59 and -0.78) and somewhat smaller for yearling weight (-0.48 and -0.39). For Herefords, maternal environmental effects were consistently more important than maternal genetic effects.