Impact of strain and duration of thermal stress on carcass yield, metabolic hormones, immunological indices and the expression of HSP90 and Myogenin genes in broilers

The aim was to elucidate the effects of strain and the duration of thermal stress (36 degrees C) on the carcass yield, metabolic hormones, immunological parameters and the relative expression of liver heat shock protein 90 (HSP90) and Myogenin genes. Two hundred and seventy day-old chicks (135 of each breed; Ross 308 and Cobb 500) were used in this trial. The birds were divided into 3 equal groups (CON: thermoneutral condition; S-1 and S-2 groups were subjected to 4 and 6 h of daily thermal stress, respectively. The relative bursa weight in the thermonuteral Ross broilers, but not Cobb broilers was significantly greater than both heat-stressed groups (P = 0.001). The serum Na and phosphorus levels were significantly decreased when both broiler strains subjected to 6 h of thermals stress (P = 0.020 and 0.022, respectively). A linear decrease in the serum T-3 level was recorded in both broiler strains as the duration of thermal stress increases (P = 0.001). In both broiler strains, heat stress up-regulate the expression of liver HSP90 gene compared with the thermoneutral group (P = 0.012). Meanwhile, the S-2 group of Ross broilers showed a significantly lower IgG level (P = 0.027). Moreover, thermal stress for 6 h down-regulate the relative expression of Myogenin gene compared with the S-1 and thermoneutral groups (P = 0.005). In conclusion, thermal stress for 6 h down-regulate the mRNA expression of liver Myogenin, concomitantly with an increase in the expression of HSP90 gene in both broiler strains. These results could be helpful in the markers assisted selection to develop more heat-tolerant broiler strains.