ESTIMATION OF THE GENETIC CORRELATIONS BETWEEN TWISTED LEGS AND GROWTH OR CONFORMATION TRAITS IN BROILER-CHICKENS

Genetic correlation between two types of leg deformities, valgus and v arus angulations, and some growth or conformation traits were estimate d in two commercial broiler strains. 14264 chickens of both sexes in l ine A were measured for leg defects at 6 weeks and body weight at 3 (B W3) or 6 (BW6) weeks. The same measures were taken in line B on 8164 c hickens, as well as breast angle (BRA) and breast meat yield (BRM) at 6 weeks on 70% of the male birds. The multinomial logit model previous ly developed for the genetic analysis of valgus and varus deformities was extended to deal with the joint analysis of one unordered categori cal trait and one continuous variable. The model assumed a competition between latent susceptibility variates related to the various deformi ties and linearly dependent on the continuous performances. Location p arameters for latent susceptibilities and continuous trait were estima ted by the 'Maximum A Posteriori' approach and dispersion parameters b y the 'Maximum Marginal Likelihood' using a tilda-hat approximation. T he genetic model took into account the effects of the sire, maternal g randsire and dam within maternal grandsire. As described in a previous study, leg deformities showed moderate heritabilities. Mean heritabil ity estimate for both lines, based on the sire/maternal-grandsire (S/M GS) component, was equal to 0.22 for valgus and varus; when based on t he dam component, mean estimates were equal to 0.37 and 0.29 for the t wo deformities respectively. Except for BRA, heritability of growth an d conformation traits appeared to be smaller when based on S/MGS compo nent (from 0.18 to 0.47) than on dam component (from 0.41 to 0.63). Ve ry low genetic correlations were found between susceptibilities to leg deformities and growth performances: average estimates for both lines of the generic correlation with BW3 were -0.03 and -0.05 for valgus a nd varus respectively. Respective genetic correlations with BW6 were e stimated to be +0.05 and +0.01. According to a simulation study these small estimates were unlikely to be due to the negative back effects o f severe disorders on growth performances. According to these results, including leg defects in breeding schemes would not delay improvement on growth through unfavourable generic correlations. Susceptibility t o valgus deformity appeared to be genetically independent of conformat ion traits (genetic correlation was estimated to be -0.06 and -0.08 wi th BRA and BRM respectively), whereas moderate unfavourable genetic co rrelations were found for varus (+0.16 and +0.19 with BRA and BRM resp ectively). Care must be taken when considering the impact of the actua l intensive selection for greater conformation on the incidence of var us deformity.