The effect of selection and sex on genetic parameters of body weight at different ages in a commercial broiler chicken population

Abstract

Abstract Data from 54 selection rounds (SRs) for growth rate in a selection line from Cobb breeding company was used for analysis with the objective to investigate the extent of sex by genotype interaction for body weight (BW) in a commercial broiler chicken population, estimate genetic parameters of BW at three different ages, and study the development in genetic variance for BW over several generations of selection. BW was measured at three different ages (t, t-4 and t-7 days) in different birds of both sexes. For the first 39 SRs, BW was recorded at t days, however, as selection continued the birds reached desired BW earlier and the weighing age was changed to t-4 days for 7 SRs and then to t-7 days for the last 8 SRs. A bivariate random regression model regarding BW in each sex as different traits using segmented linear splines and heterogeneous residual variance was used to estimate genetic parameters of BW across the selection trajectory via Gibbs sampling. The genetic variance and the heritability estimates of BW in males and females were found to be different at all three ages. The genetic correlation between BW measured in males and females was less than unity and decreased further as age at weighing was increased. This illustrated that BW in the two sexes should be considered as different but correlated traits and models for BW should account for a decreasing genetic correlation as the distance between the weighing ages increased. An increased genetic variance of BW along the selection trajectory in the first period was observed and genetic correlations between BW measured in different SRs gradually decreased as the distance between SRs increased. Genetic correlations lower than unity between BW at early and late SRs in the first period indicated that the genetic background of BW has gradually changed along the selection trajectory. This change probably contributes significantly to maintaining large levels of genetic variance in highly selected broiler populations.