Direct and correlated responses to divergent selection for residual food intake in Rhode Island Red laying hens.

1. Divergent selection was undertaken in a Rhode Island Red population for residual food intake, measured in males and females, using mass selection. 2. In the absence of a control line, selection response during 14 generations was estimated by the within-year divergence between lines. 3. The direct response in residual food intake was found to be significant in both sexes, the divergence reaching almost three phenotypic standard deviations in each sex. 4. Significant correlated responses were obtained for food efficiency; it was improved in the low residual food intake line. Shank length, wattle length and rectal and comb temperature showed higher values in the high line, suggesting an increased heat production or dissipation. Inconsistent changes were observed for other egg production traits.