THERMOREGULATION AT HIGH AMBIENT-TEMPERATURE IN GENETICALLY FAT AND LEAN BROILER HENS FED AD-LIBITUM OR ON A CONTROLLED-FEEDING REGIME

1. Hens of hnes divergently selected for fatness and leanness, fed either ad libitum or on a controlled regimen, were compared for susceptibility to heat stress. 2. The rate of increase in deep-body (rectal) temperature during exposure to 32-degrees-C was used as the index of thermoregulatory ability. Comb and foot surface temperatures were measured as indicators of peripheral vasomotor tone. 3. Because body temperature control depends on the balance between heat production and heat loss, heat production was measured to determine whether fat line hens had a higher heat production, which they would then have to dissipate. 4. During the first hour of heat exposure, rectal temperature in the ad libitum-fed birds increased twice as rapidly as in the corresponding lean line sample and 6 times more rapidly than in the control-fed fat-line group. 5. Surface temperatures of comb and foot increased from 27-degrees-C to 37-degrees-C within the first hour at 32-degrees-C, with no effect of either genotype or feeding regimen on rate of increase. 6. Heat production was unrelated to genotype but was reduced by 23% by controlled feeding, largely because of the reduction in body weight. 7. The results demonstrated that ad libitum-fed fat-line birds are susceptible to heat stress and that this is related not to increased heat production, but to a decreased ability to lose heat. Elevation of blood viscosity by plasma triglycerides is suggested as a causal factor.