EFFECTS OF DIURNALLY CYCLING VERSUS CONSTANT TEMPERATURES ON CHICKEN GROWTH AND FOOD-INTAKE

1. Male broiler chickens were exposed in separate experiments to: (a) constant ambient temperatures (Ta) ranging between 10 and 35 degrees C ; (b) 12 h:12 h diurnal high-low temperature of 10:30 degrees C, 15:35 degrees C and 15:30 degrees C in different trials; and (c) 12 h of 30 or 35 degrees C and 12 h of variable lower temperature. 2. A progress ive decline in weight gain and food intake was obtained as Ta increase d from 18 to 35 degrees C. Under diurnally cycling temperature, weight gain and food intake were lower than in the average corresponding con stant temperature, with the exception of chickens exposed to 15:30 deg rees C, where weight gain and food intake were not significantly diffe rent from those of the constant average temperature. 3. As the diurnal cold period was made colder, chickens exposed during half of the diur nal cycle to high temperature (30, 35 degrees C) demonstrated a signif icant (P less than or equal to 0.05) increase in weight gain and food intake.4. There was a good correlation between plasma triiodothyronine (T-3) and Ta. The correlation between T-3 and food intake was highly significant, but that between T-3 and weight gain was poor. 5. The res ults indicate the difficulties in predicting the performance of broile rs exposed to diurnally cycling temperatures from knowledge of the ave rage temperature.