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

Three White Leghorn strains, their two-way crosses, and two commercial line s were used to evaluate the effects of aging on mean heterosis, reciprocal, additive, Z-chromosome, and heterotic effects and their variances in two e gg production traits during the first laying cycle. Egg number of the survi vors (EPF) and egg number including mortality and morbidity (EPM) were eval uated from hens housed one per cage in a randomized block design. For analy ses, egg number was divided into 12 periods of 28 d each. Synchronization o f the records was achieved by starting recording at age at first egg. The m ean heterosis was significant over time and increased with age for both tra its (P < 0.05). Reciprocal effects were not significant across periods but increased in magnitude with age for EPF. The pattern of age changes in addi tive, Z-chromosome, and heterotic effects varied among strains, indicating genotypic differences in response to aging. These differences increased, on average, with age. The additive, heterotic, environmental, and phenotypic variances increased with age for both traits. A decline with physiological age was observed in heritabilities, which was due to a faster increase in e nvironmental variance than to an increase in additive variance. Genetic var iance of egg production increased with age, indicating that improvement of lifetime performance of layers is possible. This improvement could be achie ved by selecting animals at older ages or by favoring individuals with bett er DNA repair or those who had more genes turned on or off during the cours e of aging.