Genetic effects of aging on fitness and nonfitness traits in laying hens housed three per cage.

Three White Leghorn strains, their two-way crosses, and two commercial lines were used to study the effects of aging on several parameters related to performance of fitness and nonfitness traits during the first laying cycle of hens housed three per cage. Egg number of the survivors (EPF) and egg number including mortality and morbidity (EPM) were divided into 12 periods of 28 d each, starting at age at sexual maturity. Egg weight (EW), specific gravity (SG), and albumen height (AH) were measured at 240, 350, and 450 d of age. Mean heterosis was significant over time, except for AH, increasing in magnitude with age for EPF, EPM, EW, and AH. Reciprocal effects were more important for egg quality than for egg production traits and were influenced by age. The age-related changes in additive, Z-chromosome, and heterotic effects varied among strains, showing that strains differ in their genetic schemes in response to aging. Nonadditive, environmental, and phenotypic variances increased with age for all traits. Additive variance increased with age for EPF, EW, and AH. Z-chromosome variance increased with age for EW and AH. Heritabilities decreased with age, except for EPF and AH. On average, genetic variance increased with advancing age. Improvement in lifetime performance may be obtained by selecting birds at older ages. As the relative increase with age in additive variance was larger for egg production than for egg quality traits, selection for the latter could be performed at early stages.

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.

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 crosses were examined in the 1st laying cycle to evaluate the effects of ageing on mean heterosis, reciprocal effects, additive (A), Z-chromosomne (Z) and 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, indicating genotypic differences in response to ageing. The additive, heterotic, environmental and phenotypic variances increased at the end of the cycle 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 consistent pattern across the cycle for hatchability. Genetic variance for fertility and hatchability increased at the end of the cycle, stggesting that improvement in performance during the 1st production cycle of layers could be achieved by selecting animals at older ages.