Exposure of poultry flocks to ambient temperature above the zone of minimum metabolism results in an increase in endogenous heat production. Convective transfer of heat is the major thermo-regulatory mechanism of chickens and depends on movement of air by natural or fan-powered ventilation. An increase in convective heat transfer as a result of air movement is proportional to air velocity of up to 100 m/minute, provided ambient air temperature is below body temperature.
Hyperpnea (panting) occurs in mature chickens exposed to temperatures exceeding 30ºC. Respiratory rate can increase from 22 breaths/minute (bpm) to 200 bpm when ambient temperature is increased from 27ºC to 45ºC within 20 minutes. Panting facilitates evaporative cooling, and above 38ºC, chickens are almost entirely dependent on latent heat loss for thermo- regulation. Prolonged hyperpnea results in excessive excretion of carbon dioxide resulting in respiratory alkalosis.
Exposure to high ambient temperature has a profound economic impact on liveability, growth rate, egg production, egg shell quality, and feed conversion efficiency. Exposure to high environmental temperature for extended periods will suppress the humoral immune response of chickens, reducing antibody titer. It is presumed that a reduction in circulating antibody is associated with a corticosteroid-induced change in serum ions. Cellular immunity is also suppressed by prolonged exposure to temperatures in excess of 36ºC. This effect is mediated through T-cell or regulatory amplifier cell response.