Selection for Egg Mass in Different Social Environments: 4. Selection Response in Phase I

Abstract

Selection for part-year egg mass was carried out in replicated strains of White Leghorns within each of two social environments. Responses were measured as deviations from replicated, unselected control strains. Realized heritabilities averaged .17 over the seven generations studied. Selection appeared to be equally effective when based on sire-family means whether sire families were separately housed or intermingled in floor-flock environments. Egg mass, measured as daily grams of egg produced per hen housed, increased as much for the full-year period as for the selection period, which began when pullets were 30 weeks old. Correlated responses occurred in terms of decreased age at sexual maturity, increased egg weight, and increased hen-housed rate of lay. Random genetic drift appeared to influence all traits studied.Relatively spacious floor pens and individual-hen cages were associated with essentially equal overall productivity, but 18-hen colony cages were stressful as indicated by large reductions in part and full-year egg mass, hen-housed rate of lay, percentage survival, and by increased age at sexual maturity. Adaptability to specific laying-house environments was not differentially influenced by selection on the basis of sire-family means when families were separately housed or intermingled in floor pens.