Growth Performance and Concentrations of Thyroid Hormones and Growth Hormone in Plasma of Broilers at High Temperatures

Abstract

Responses of broiler chickens to a high ambient temperature (35°C) were measured in two experiments. In one experiment temperatures were increased abruptly from 21°C to a daily range of 21-35°C whereas, in the other, temperatures were increased more gradually over 6 days. The high temperatures were maintained for 5 h/day. In' both experiments, birds exposed to the high temperatures ate less food and gained less liveweight than birds maintained at 21°C. Efficiency of food conversion to liveweight gain and body composition were not affected by high temperature but there was a tendency for thyroid weight to decrease. Overall, the plasma concentration of triiodothyronine (T 3) decreased and the plasma concentration of thyroxine (T4) increased, resulting in a decreased T/T4 molar ratio, during exposure to high temperature. The concentration of plasma growth hormone, but not plasma reverse T 3' was increased by high temperature. The initial responses to increased temperature were variable, with birds exposed more gradually adjusting relatively well until the maximum temperature was increased to 35°C. All heated birds readjusted quickly to the daily reduction in temperature to 21°C.