Selection for Body Weight at Eight Weeks of Age: 12. Egg Production in Selected and Relaxed Lines

Abstract

Correlated responses in egg production patterns were studied in lines of White Plymouth Rocks selected bidirectionally for juvenile body weight and in their respective relaxed lines. The production traits measured were the numbers and percentage hen-day production of normal, double-yolked, broken, ovulatory defective, and total defective eggs and yolks. Increasing the photoperiod beyond 14 hours in a 24-hour period from 119 days of age did not have a consistent effect on either body weight or reproductive traits. The frequency of various defective egg-types was significantly higher in the high weight than in the low weight lines. Relaxing selection for high body weight resulted in fewer double yolked and the total number of defective eggs, while relaxing selection for low body weight has no influence on any of the abnormal egg-types.The average heritability estimates for percentage hen-day production of normal eggs and yolks were similar. Heritability estimates of the total defective eggs were moderate to high and the genetic correlations among traits were generally larger than the phenotypic correlations. The negative phenotypic correlation between age at sexual maturity and percentage hen-day production of normal eggs was due to a negative environmental correlation between these traits. Early sexual maturity was genetically associated with a higher rate of normal egg production when feed restriction was moderate, while it tended to be opposite under a relatively severe feed restriction. Environmental factors that delayed sexual maturity had no significant influence on the production of defective eggs. The positive association between the production of normal eggs and yolks to January 1 was largely genetic, whereas the negative relationship between normal and defective egg production was primarily environmental. The low magnitude of phenotypic and environmental correlations among body weight and defective eggs suggest that under restricted feeding regimes defective egg production is largely independent of growth rate. There was evidence to indicate that non-additive genetic effects contribute to environmental correlations of percentage hen-day production of defective eggs with body weight and with age at sexual maturity. Furthermore, it is inferred that a genetic approach to the problem of defective eggs is realistic.