Genetic parameter estimates of early growth traits in the tygerhoek Merino flock
Genetic parameters were estimated for birth weight (BWT), weaning weight (WWT) and pre-weaning average daily gain (ADG) using Restricted Maximum Likelihood (REML) procedures. Four different animal models were fitted, differentiated by including or excluding maternal effects. The direct heritability estimates (h2) ranged from 0.19 to 0.38, 0.25 to 0.36 and 0.26 to 0.34 for BWT, WWT and ADG, respectively. The estimates were substantially higher when maternal effects, either genetic or environmental, were ignored from the model. The maternal heritability (m2) for BWT was 0.38 when only maternal genetic effects were fitted in the model but decreased to 0.25 when the maternal permanent environmental effect (c2) was fitted. The m2 for WWT and ADG ranged from 0.05 to 0.11 and from 0.04 to 0.10, respectively. The respective c2 estimates ranged from 0.06 to 0.10 and from 0.06 to 0.09. Moderate negative genetic correlations (ram)between direct and maternal genetic effects were observed in BWT, while close to zero estimates were obtained for WWT and ADG. The direct genetic correlation estimates between BWT and WWT and BWT and ADG were 0.16 and 0.04, respectively. The corresponding maternal genetic correlation estimates were 0.93 and 0.60. The direct and maternal genetic correlation estimates between WWT and ADG were 0.99 and 0.85, respectively. It is suggested that selection should be applied to WWT.