Selection for Egg Mass in Different Social Environments

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 partand 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. (

Corticosteroid Levels in White Leghorn Hens as Affected by Handling, Laying-House Environment, and Genetic Stock

“…Egg mass produced per hen on a daily basis was depressed for colony-caged hens as compared with that of floor-pen hens throughout the 40-week egg laying period, and the difference tended to increase with each 10week period. Similar results have been obtained repeatedly under our conditions (Craig et al, 1982). Mortality data, pooled over the four strains and subjected to a chi-square test, indicated greater death loss in cages (14.3%) than in floor pens (3.5%) (P<.01) for the 40-week period in the laying house.…”

“…Those stocks consisted of two unselected control subpopulations or strains (Ci and C 2 ) and two strains selected for part-year egg mass (Yj and Y 2 ). The Y strains have become more productive because of selection (Craig et al, 1982).…”

“…Analyses of variance were used in testing for genetic and treatment effects using appropriate error terms as detailed by Craig et al (1982).…”

“…It was not the intent of this study to compare cage and floor housing environments as being representative of those found in commercial practice. Instead, our environments were specifically chosen, on the basis of results from a previous study (Craig et al, 1982), to represent strongly contrasting housing situations. The difference between selected and unselected stocks, although nonsignificant in this comparison, is as large as usually found between these systems of selection.…”

Tonic Immobility Responses of White Leghorn Hens Affected by Induction Techniques and Genetic Stock Differences

“…Statistical models and expected co mp ositio n of mea n squares fo r anlayses of variance followed the pattern outlined by Craig et al (1982 ) and are prese nted in detail elsewhere (Kuji yat, 1983).…”