Genetics of changes in economic traits during the laying year

Abstract

Body weights, egg weights and egg numbers were measured at different ages in two distinct, fully pedigreed, random bred strains of White Leghorns over a period of 3 years. The heritabilities of the separate measurements and the genetic correlations between them were obtained by an analysis of variance and covariance. The principal findings were: (i) The heritability of weight gain declined sharply with age over consecutive periods after 12 weeks. The genetic correlation of body weight at 12 weeks with gain in subsequent periods was low. There appeared to be sufficient genetic variance in growth rate in these later periods to alter the growth curve substantially by selection. For example, it should be possible to breed strains with a rapid early growth and low mature body size. (ii) There was evidence of genetic variation in the change of egg weight with time. The genetic variation was, however, so low that in any selection programme to improve 28‐week egg weight without changing 44‐week egg weight the progress would be very slow. (iii) Body weight at 12 weeks was genetically negatively correlated with sexual maturity. Its correlation with production in the first 44 weeks was positive in both strains, but not significantly different from zero. (iv) Body weights after 28 weeks were apparently uncorrelated with egg production or sexual maturity. (v) Egg weight and production traits show considerable heterogeneity of variance between years, probably partly due to their distribution not being normal. (vi) The pattern of genetic variation appeared to differ significantly between the two strains so that correlated responses would not necessarily be the same. (vii) There was a suggestion of maternal effects acting upon body weight but the evidence was somewhat contradictory and inconclusive due to the high standard error of the estimates. (viii) None of the 15 traits measured regressed over the 3 year period of relaxation of selection.