INDUCTION OF THERMOTOLERANCE IN MALE BROILER-CHICKENS BY TEMPERATURE CONDITIONING AT AN EARLY AGE

Chicks were conditioned by exposure to heat stress (36 +/- 1 C; 70 to 80% RH) for 24 h at the age of 5, or 5 and 7 d. During conditioning we ight gain was depressed. Due to accelerated growth during the postcond itioning period, a complete compensation for lost weight gain was achi eved by the group conditioned at 5 d, but only a partial compensation was obtained in the group conditioned at both 5 and 7 d. At the age of 42 d, challenge with acute heat stress (35 +/- 1 C; 20 to 30% RH) res ulted in a large increase in cloacal temperature of the controls and a more moderate increase in the conditioned chickens. Mortality during the thermal challenge was significantly higher in the controls than th at of the previously exposed (conditioned) groups. Conditioning at an early age resulted in hemodynamic changes (significant decrease in hea rt weight and hematocrit) and reduced plasma triiodothyronine (T-3) co ncentration. The results suggest that reduced T-3 and hemodynamic chan ges may be part of the mechanism associated with improved thermotolera nce by early age temperature conditioning.