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High temperature periods often generate a high relative humidity. This usually happens during the summer months. Heat stress can deeply affect the productivity of a flock.
At ambient temperatures of over 33° C (91.4 °F), high mortality and great loss of production are apparent, though at less extreme temperatures we often overlook heat stress as a cause for poor growth or loss of egg production, as well as a lower eggshell quality.
Thermoregulation in poultry
The excess body heat is removed via four different mechanisms:
Convection
Loss of body heat through cold circulating air. Fowls enlarge the exposed surface area by lowering and extending their wings away from their bodies. Convection is aided by air movement, thus creating a cold air effect
Vasodilation
Blood swells wattles and combs, sending the core body heat to the surface, which is then lost to the surrounding air.
Radiation.
Electromagnetic waves transfer heat via the air to a distant object. Body heat is irradiated to colder objects in the barn (for example, the walls, the ceiling, or the equipment).
Evaporative cooling.
Fast, shallow breathing with mouth open increases loss of body heat by increasing evaporation of water in the mouth and the respiratory tract. Decrease of moisture in the air contributes to evaporative cooling.
Conduction.
Loss of body heat to colder objects in direct contact with fowls (for example, the litter, roost floor, cage wire). Fowls look for the coolest areas in the shed. Fowls lie on the floor or dig into the litter to find a cooler place.
Radiation, convection, and conduction together are known as sensible heat loss.
The thermoneutral zone in the fowl is generally between 18– 25°C. Within this temperature range, loss of sensible heat is suitable to maintain the normal temperature of the fowl at 41°C.
Above the thermoneutral zone, the efficiency of sensible heat loss mechanisms is diminished. At this point, water evaporation from the respiratory tract becomes the main heat loss mechanism in the fowl. The evaporation of one gram of water dissipates 450 calories of body heat.
At temperatures higher than the thermoneutral zone, the fowl uses its energy to maintain a normal body temperature and its metabolic activities. This takes energy from growth and egg production, resulting in a lower yield.
At high ambient temperatures, fowls start breathing with mouth open in a fast, shallow manner known as gular reflux, to i...
