Genetic parameters of growth curve parameters in male and female chickens

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Abstract
1. Individual growth curves of 7143 chickens selected for the form of the growth curve were fitted using the Laird form of the Gompertz function, BWt=BWoxe(L/K)(1-e-20) where BWt is the body weight at age, BWo the estimated hatching weight, L the initial specific growth rate and K the maturation rate. 2. Line and sex effects were significant for each parameter of the growth curve. In males, L, BWo, age and body weight at inflection (TI and BWI) were higher whereas K was lower than in females. Lines selected for high adult body weight had higher BWo and BWI whereas lines selected for high juvenile body weight had larger estimates of L and lower estimates of TI. 3. Data from 38,474 animals were included in order to estimate the genetic parameters of growth curve parameters in males and females, considering them as sex-limited traits. Genetic parameters were estimated with REML (REstricted Maximum Likelihood) and an animal model. Maternal genetic effects were also included. 4. Heritabilities of the growth curve parameters were moderate to high and ranged between 0.31 and 0.54, L, BWo in both sexes and BWI in males exhibited significant maternal heritability. Heritabilities differed between males and females for BWI and TI. Genetic correlations between sexes differed significantly from one for all parameters. L , K and TI were highly correlated but correlations involving BWo and BWI were low to moderate. 5. Sexual dimorphism of body weight at 8 and 36 weeks and of L , K and TI was moderately heritable. Selection on growth curve parameters could modify the difference between sexes in precocity and thus in body weight at a given age.