Response of turkeys to relative humidity at high ambient temperature

1. The effects of relative humidity on growth rate and thermoregulation at high ambient temperature were evaluated in turkeys. Male turkeys were exposed to ambient temperature (Ta) of 35 degrees C and relative humidity (RH) of 40% to 75% and 50% to 85%, at ages of 13 to 19 weeks (trial 1) and 10 to 15 weeks (trial 2), respectively. 2. Body weight and food intake in both trials increased as RH increased up to 70% to 75%. When RH increased further to 80% to 85% (Trial 2) both body weight and food intake declined significantly. 3. Blood CO2 partial pressure (pCO2) did not change significantly at different RHs (trial 1). However, in trial 2 it dropped significantly at 80% to 85% RH. This response coincided with an increase in blood pH. 4. The effect of RH on body temperature (Tb) was not significant in either trial. 5. Triiodothyronine (T3) concentrations increased with the increase in RH up to 70% to 75% RH (trials 1 and 2). A significant decrease was observed when RH was further increased to 80% to 85% RH (Trial 2). Positive linear correlations between T3 and food intake or weight gain were observed in both trials. 6. The results indicate that turkeys can thermoregulate efficiently in the face of extreme changes in RH and that only at high RH (above 75%) did the performance of turkeys deteriorate.