Genetic effects of aging on egg quality traits in the first laying cycle of White Leghorn strains and strain crosses.

Three White Leghorn strains, their two-way crosses, and two commercial lines were used to evaluate the effects of aging on heterosis (H), reciprocal effects, and additive (A), Z-chromosome (Z), and heterotic effects and their variances on egg quality traits during the first laying cycle. Egg weight (EW), specific gravity (SG), Haugh unit (HU), and albumen height (AH) were measured at 240, 350, and 450 d of age from hens housed one per cage in a randomized block design. The mean heterosis was significant over time only for EW. For EW, heterosis increased in magnitude with age. The mean heterosis for both HU and AH was also influenced by age. Reciprocal effects were significant, on average, across periods for all traits and were influenced by age. The age-related changes in additive, Z-chromosome, and heterotic effects varied significantly among strains, indicating differences by genetic group in response to aging for egg quality traits. The heterotic, environmental, and phenotypic variances increased with age for all traits, except for AH. The additive and Z-chromosome variances did not always increase with age. Their age trend varied, depending on the trait. Heritabilities decreased with advancing age, suggesting that selection to improve lifetime performance of egg quality traits can be done early in the cycle.