Direct and Maternal Genetic Effects and Weaning Weight Trends in Indigenous Tuli Cattle of Zimbabwe

Abstract

Variance components for weaning weight (WWT) in local Tuli cattle were estimated by Restricted Maximum Likelihood. A single trait animal model was fitted, allowing for genetic maternal effects and a genetic covariance between direct and maternal effects. Estimates of heritability for direct genetic effects (h²_A), maternal genetic effects (h²_M) and the genetic covariance between direct and maternal effects as a proportion of the total variance (c_AM) for weaning weight were, 0.30, 0.22, -0.20, respectively. Direct additive genetic variance was larger than maternal variance. The correlation between direct and maternal effects was negative implying selection be directed at both components considering the maternal heritability estimate which was moderate. Estimates of genetic trends for direct and maternal effects were obtained by averaging corresponding breeding values for animals born in a given year, and regressing these averages on year of birth (Henderson, 1973). The regression of average direct breeding values on year and average maternal breeding values on year was 0.02 kg/yr and -0.01 kg/yr, respectively. The maternal genetic trend indicates an increase in maternal ability for weaning weight in this population. The nature of environmental values suggests that the environmental conditions had negative effects on weaning weight.

Keywords: Variance component, Weaning, Trends, Tuli