Predicted response in food conversion ratio for growth by selection on the ratio or on linear component traits, in a (sequential) selection programme

Abstract

1. Direct versus indirect selection for food conversion ratio of growth (FCR) after selection for live body weight (LWT), a sequential scheme often applied in broilers, was considered. In the present study loss of response in either FCR or aggregate genotype (H) when LWT was included was investigated under selection on a linear index of ratio component traits (and LWT) or FCR (and LWT) by selection index methodology. 2. Relative responses in FCR and H were generally very similar under single‐step and sequential selection. Without LWT in H, selection for linear index or ratio gave similar responses in FCR when heritabilities of components were equal. With large differences in heritabilities (0.2 compared with 0.5) or genetic and environmental correlation (>0.6) significant differences in response (5 to 12%) in FCR emerged. Therefore, whether additional costs are justified for parameter and breeding value estimation when using a linear index in place of selection for the ratio depends on the difference in heritabilities and correlations of the ratio component traits. With LWT in H, loss of response in FCR was partially or entirely offset by response in LWT. 3. The non‐normality of FCR and consequences for (co)variance component estimation were studied in terms of the coefficients of variation of the component traits of FCR. Restricted Maximum Likelihood (REML) estimation of (co)variance components for both FCR and logarithm transformed FCR (closer to normality) showed the robustness of REML to such deviations from normality.