Page 50 - Poultry-Punch April 2020 edition

POULTRY PUNCH ARTICLE
Technical Update

UNDERSTANDING HEAT STRESS IN LAYERS:

Management Tips to Improve Hot Weather
Flock Performance

Periods of high environmental temperatures, often accompanied by high relative humidity, are common
in the summer months. Heat stress can profoundly affect the productivity of a flock. At environmental
temperatures above 33°C, high mortality and large production losses are readily evident, but at less
extreme temperatures, heat stress is often overlooked as a cause for poor growth or subtle losses in egg
production and shell quality.

THERMOREGULATION OF THE HEN
Excess body heat is removed by four different mechanisms (see Figure 1).
1. Convection 3. Evaporative Cooling
Body heat lost to cooler surrounding air. Birds will increase Rapid, shallow, open-mouth
exposed surface area by drooping and spreading wings. breathing increases heat loss
Convection is aided with air movement by creating a by increasing the evaporation
wind chill effect. of water from the mouth and
respiratory tract. Evaporative
Vasodilation – Blood-swollen wattles cooling is aided by lower
and comb bring internal body heat air humidity.
to the surface to be lost to the cooler
surrounding air.
Reduced Body Heat 4. Conduction
Production – Body heat loss to cooler
Birds become inactive objects in direct contact with
2. Radiation and listless, decreases the bird (i.e. litter, slats, cage
wire). Birds will seek cooler
Electromagnetic waves transfer heat feed consumption. places in the house. Birds will
through the air to a distant object. Body lie on floor and dig into litter to
heat is radiated to cooler objects in the find a cooler place.
house (i.e. walls, ceiling, equipment).

Figure 1. Heat loss mechanisms of the chicken.
EFFECTS OF HEAT STRESS
Radiation, convection and conduction together
are called sensible heat loss. The thermoneutral  Feed intake  Mortality
zone of the chicken is generally between 18–25°C.  Egg production (especially with
Within this temperature range, sensible heat loss  Egg weight acute heat stress)
is adequate to maintain the bird’s normal body  Shell quality  Cannibalism
temperature of 41°C.  Immunosuppression
 Albumen height
Above the thermoneutral zone, the efficiency of  Growth  Hatchability
sensible heat loss mechanisms diminishes. At this  Fertility in roosters
point, the evaporation of water from the respiratory
tract becomes the major heat loss mechanism of Production losses occurring from heat stress
the bird. The evaporation of one gram of water depend on:
dissipates 540 calories of body heat. 1. Maximum temperature to which the flock
At temperatures above the thermoneutral zone, was exposed
the bird has to expend energy to maintain normal 2. Duration of high temperatures
body temperature and metabolic activities. This 3. Rate of temperature change
diverts energy away from growth and egg
production, resulting in performance loss. 4. Relative humidity of air

50 April, 2020