Genetic Response to Selection for Body Weight at Nine Weeks of Age in New Hampshire Chickens

An experiment was conducted to measure the effect of individual selection for body weight at 9 weeks of age. After five generations of selection, expected and effective selection differentials as well as response to selection were calculated for male and female chickens, respectively.The effective selection differential for males which was weighted by the number of progeny, was smaller than the expected selection differential, while these two selection differentials for female chickens were almost identical.Genetic responses measured by regressing the generation mean divergences of selected line from unselected control line on the cumulative selection differentials, indicated a significant effectiveness of selection for body weight of male and female chickens. Realized heritabilities of male and female chickens which were defined as ratio of genetic respone to selection differential, were 0.224 and 0.363 respectively.Estimates of heritability for 9 week body weight were computed on within sex-generation basis by intra-class correlation for sib method and intra-sire regression of progeny on dam, and then were pooled over generations. Heritability estimates of female which were estimated from intra-class correlation (sire component) and intra-sire regression of female progeny on dam were in good agreement with realized heritability within their standard errors. However, the estimate of male from intra-class correlation was two times as large as realized heritability. Heritability estimated from intrasire regression of male progeny on dam was smaller than realized heritability of male and was the smallest value of all estimates in this study. The roles of genetic and non-genetic factors on body weight of male and female chickens at 9 weeks of age were discussed.