Contrasting heat stress sensitivity in different broiler breeder lines

During commercial production broiler chickens may be exposed to elevated thermal loads inducing acute heat stress, for example, during transport from farms to slaughter. Under these conditions birds will experience transient hyperthermia and disturbances in acid-base balance associated with skeletal muscle damage and increased mortality. It is not known if genetic selection for increased growth rate and food conversion efficiency have influenced the susceptibility of broilers to such acute heat stress. The present study, therefore, has compared thermoregulatory success and effort in three grandparent broiler breeder lines (A, B and C) during exposure to acute heat stress (AHS). Ten 6 week old birds from each of the 3 lines were placed in simulated transportation conditions (climate chambers) at 30°C / 70% RH (AHS treatment) for 3 hours. Rectal temperatures were measured and venous blood samples taken before and after AHS and analysed for pCO2 and pH. The mean 6 week body weights for the three lines did not differ significantly. Under the heat load imposed during HS, all 3 lines exhibited significant hyperthermia (p<0.001) and hypocapnia (p<0.01) with a tendency towards an associated alkalosis. In response to the thermal challenge line A exhibited a more marked hyperthermia than the other 2 lines (p<0.05), line C exhibiting the smallest response. The largest disturbances in pCO2 and pH were observed in lines B and A (p<0.05) whilst line C exhibited only a mild hypocapnia and no significant alteration in pH. It may be concluded that line A is the most heat stress susceptible of the three and line C the most heat tolerant. Line C is better able to thermoregulate than line A at the imposed heat load with an apparently reduced thermoregulatory effort. Lines used in the production of broilers possess different degrees of thermotolerance and this may contribute to the thermoregulatory capacities and characteristics of the commercial meat birds. These findings may have implications for future selection strategies.