Thermoregulatory and metabolic heat production responses during acute heat stress in genetically improved broiler chickens

It has been proposed that artificial genetic selection for improved production traits in modern broiler lines is associated with both pathology and reduced effectiveness of some homeostatic systems. The high metabolic rate of rapidly growing birds may reduce their resistance to high ambient temperatures. The present study has compared the effects of acute heat stress upon deep body temperature regulation, metabolic heat production and some physiological indices of thermoregulatory effort in a fast growing broiler line (FG) and a slower growing genetic predecessor (SG). The metabolic heat production (MHP) responses to a 2 hour exposure to a temperature of 32°C + 70% relative humidity were determined in birds from the two lines at identical body weights by indirect calorimetry. Deep body temperatures were also recorded and venous blood samples obtained for determination of pCO2 and pH before and after heat stress. Exposure to high thermal load induced a profound hyperthermia in both lines, rectal temperature increasing 2.6°C and 4.8°C in SG and FG birds. During heat stress mean heat production increased by approximately 20% (p<0.05) in both groups, the absolute increase being 55% greater in the FG birds. The peak changes in MHP during heat stress were 57% (p<0.005) and 35% (p<0.01) in the FG and SG groups respectively. There was no evidence of a greater thermoregulatory effort in the FG birds as the degree of hypocapnic alkalosis induced in the two lines was similar. It is suggested that fast growing broiler chickens may exhibit inappropriately elevated MHP responses during heat stress and that the heat loss mechanisms to dissipate the imposed heat load are inadequate. It is concluded genetic selection for improved growth in broiler chickens has detrimental effects upon thermo-tolerance through regulation of MHP and heat loss and that this may compromise their productivity and welfare by limiting their capacity to respond to thermal challenge.