Genotype-by-environment interaction with broiler genotypes differing in growth rate. 4. Association between responses to heat stress and to cold-induced ascites

Males and females, selected from a commercial line to represent its phenotypic variation for BW, were mated with similarly ranked mates to produce sire families representing a wide genetic variation in potential growth rate (GR). Following 5 wk of rearing at normal ambient temperatures, birds representing all sire families were exposed to cold (Days 37 to 47, Trial 1) or hot (Days 43 to 48, Trial 2) environments. Birds exhibiting ascites syndrome (AS) in the cold environment (Trial 1) were counted, and the incidence of AS (%AS) per family was calculated. Sire families' least-square means of BW at 37 d of age in Trial 1 and BW at 43 d of age in Trial 2 represented the families' potential GR (i.e., GR under normal conditions). A significant positive correlation was found between potential GR and %AS (r = 0.479, Trial 1), indicating that families with higher potential GR under normal conditions are more likely to suffer from AS under cold stress, compared to families with lower GR. Heat stress markedly reduced weight gain in all families (Trial 2); however, the genetic potential GR was negatively correlated with actual GR under heat stress (r = -0.411, Trial 2). Since offspring of the same sire families were exposed to the two stressful environmental conditions, correlations between sire families' means under the two environments could be calculated. A negative correlation was found between growth under heat stress (Trial 2) and %AS (Trial 1) (r = -0.439), indicating that families whose GR is more depressed under heat stress are more likely to suffer from AS under cold stress. These results suggest that the two stress responses may share similar control of the genetic variation in each trait and their negative genetic correlation with potential GR.