Age changes in genetic and environmental variation in laying hens.

Effects of age on the genetic and environmental variation of egg production, egg weight and egg quality were investigated in two populations of laying hens. The first part of the study was based on data from a crossbred population in Sweden that allowed the estimation of sire and residual but not of dam variance components. Sire, dam and residual components of variance were estimated in the second part of the study that used data from eight generations of two unselected control and four high egg production-selected Canadian strains of chickens. - Although the data did not allow a clear-cut separation of the various genetic and environmental variance components, the results indicated that new genetic variation appeared parallel to an increase in the environmental variation with age of the birds. This was interpreted as a suggestion that the deteriorating process of aging impaired the organism's ability to cope with environmental conditions and this resulted in the observed increase in environmental variation. The simultaneous increase of the genetic variation was caused by the turning on of new genes in order to induce reactions counteracting the effects of aging. Alternatively, reduced accuracy of DNA transcription in older birds may cause impairment of the functional efficiency of metabolic systems thus increasing environmental variation parallell to an increase of genetic "error variation".