Genetic analysis of growth curve parameters for male and female chickens resulting from selection on shape of growth curve

The objectives of this research were to evaluate gender differences and sel ection on body weight as they affect growth curves of chickens. Marginal po sterior densities of growth curve parameters were studied by Gibbs sampling on 10,671 male and female chickens originating from five lines. Line X-+ w as selected on low body weight at 8 wk (BWS) and high body weight at 36 wk (BW36), line X+- on high BW8 and low BW36, X++ on high BW8 and BW36, X- on low BW8, and BW36, and X00 was an unselected control line. Growth was model ed by a Gompertz function. Heritabilities and genetic correlations among pa rameters of the Gompertz curve were estimated. Marginal posterior densities were drawn for parameters of the growth curve and for sexual dimorphism at ages ranging from hatching to 1 yr. Lines selected for a higher BW8 had hi gher initial specific growth rates (L), higher maturation rates (K), and lo wer ages at inflection (T-I). Lines selected for a higher BW36 had higher a symptotic body weights (A). Estimates of A, L, and T-I were higher in males and K was higher in females. Difference between sexes for A was greater in the line selected for a lower BW8 and a higher BW36. Dimorphism for L and K was the lowest in lines X++ and X-, respectively. The greatest difference in T-I was observed in the line selected for lower BW8 and BW36. Sexual di morphism of body weight was lower at most ages in the lightest line. Before 15 wk, sexual dimorphism in X++ line was lower than in the line selected f or higher BW8 and lower BW36. The increase in sexual dimorphism with body w eight could be reduced by selecting animals on body weight at two ages inst ead of one, as is usually done in commercial lines.