Genetic parameters of the twisted legs syndrome in broiler chickens

Abstract

Genetic parameters of two types of angulations, described in the twisted legs syndrome as 'valgus' and bilateral or unilateral 'varus', were investigated in two commercial broiler strains. In the first line, 14 264 chickens of both sexes born from 111 sires, 76 maternal grandsires and 768 dams were studied. In the second line, corresponding figures were 8 164 chickens of both sexes born from 94 sires, 71 maternal grandsires and 553 dams. Chickens were classified at 6 weeks according to the type of pathology. Since deformities under study were unordered categorical traits, a generalized linear model using a multinomial logistic transformation as link function was applied. Location parameters were estimated by 'maximum a posteriori', and variance components by 'maximum marginal likelihood' using a tilde-hat approximation. The model of analysis took into account the fixed effects of the hatch and the sex as well as the random effects of the sire, maternal grandsire and dam within maternal grandsire. 'Pseudoheritability' of latent susceptibility to valgus was equal to 0.16 and 0.29 in lines A and B respectively, when estimated from the sire/maternal-grandsire component, and to 0.40 and 0.35 respectively, when estimated from the dam component; for varus, estimates of the pseudoheritability were equal to 0.21 and 0.24 in lines A and B when estimated from the sire/maternal-grandsire component and to 0.30 and 0.26 when estimated from the dam component. Higher values of the dam heritabilities could suggest the existence of maternal or dominance effects. The average estimated genetic correlations between valgus and varus obtained from sire/maternal-grandsire and dam components were small to moderate (-0.31 in line A and 0.07 in line B). This agrees with clinical and anatomical evidence which suggests that each deformity could be linked to distinct causes. Moreover, this result questions the practice of pooling deformities when selecting against leg disorders.