Heat Stress

All classes of poultry experience heat distress when high temperatures, accompanied by high humidity, rise above their comfort zones. Birds, unlike mammals, do not have sweat glands. When the environmental temperatures are between 28°C and 35°C, birds use nonevaporative cooling (radiation, conduction, and convection) as their major means of heat dissipation.

Birds manipulate non-evaporative cooling in 2 ways: 1) increasing the surface area by relaxing the wings and hanging them loosely at their sides, and 2) increasing the peripheral blood flow.

Increased respiration rate alters the acid-base balance because the CO2 concentration in the blood decreases. The higher blood pH reduces the amount of ionized calcium in the blood, which is needed for eggshell formation. In consequence, laying flocks experience an increase in the number of thin-shell eggs.

The extent of the losses from heat stress are determined by the age, environmental history, the maximum temperature to which the bird is exposed, the duration of the high temperatures, the rate of temperature change, and the relative humidity of the air.

Every effort should be made to prevent or alleviate heat stress in the flock. During periods of high temperatures, adequate drinking water should be available. Techniques aimed at lowering the water temperature of heat-distressed birds will aid in heat dissipation.

The addition of vitamin and electrolyte supplements in the drinking water may help to replenish the minerals and correct the acid/base balance. The addition of vitamin E in the diet has shown a beneficial effect in layers; however, responses to the addition of Vitamin C in the diet have varied from no effect to substantial reductions in heat-stress consequences.

Increase air circulation in the house by running ventilation equipment at full capacity. Cooling the air inside the house can be accomplished by using sprinklers or spraying down the floor, walls, ceiling, and outside roof with cool water during times of extremely high temperatures.

Preventive measures consist of installing fans and foggers, properly constructing ventilating ducts, insulating the buildings and water pipes, installing roof overhangs to prevent the sunlight from shining into the house, and using white or aluminum paint on the outside to reflect heat.