Genetic effects of ageing on fertility and hatchability in the first laying cycle of three White Leghorn strains and their two-way crosses

1. Fertility and hatchability of 3 White Leghorn strains and their 2-way cr osses were examined in the 1st laying cycle to evaluate the effects of agei ng on mean heterosis, reciprocal effects, additive (A), Z-chromosome (Z) an d heterotic effects (H), as well as on their variances. 2. Fertility and hatchability were evaluated at 37, 47 and 70 weeks of age for hens housed 1 per cage in a randomised block design. The mean heterosis was significant over time and increased with advancing age for both traits . Reciprocal effects were not significant across periods and did not show a consistent trend with age. 3. The pattern of age changes in A, Z and H effects varied among strains, i ndicating genotypic differences in response to ageing. The additive, hetero tic, environmental and phenotypic variances increased at the end of the cyc le for both traits, except for the additive variance of hatchability, which increased until 47 weeks of age but declined thereafter. 4. Heritability increased with age for fertility but did not show a consist ent pattern across the cycle for hatchability. Genetic variance for fertili ty and hatchability increased at the end of the cycle, suggesting that impr ovement in performance during the 1st production cycle of layers could be a chieved by selecting animals at older ages.