In tropical and subtropical countries, the layers are subjected to important heat stress.
This heat stress can occur for long periods. The lower feed intake noticed during these periods is the result of the bird's reduced ability to lose heat. Pullets and laying hens should maintain their body temperature constant by adjusting their food intake according to their ability to lose heat.
The lower growth rates during rearing and the reduced production during lay are only consequences of the reduction in feed consumption when the birds are incapable of regulating their temperature.
A better understanding of the mechanisms of thermoregulation helps us to reduce the impact of heat on growth and production.
To evacuate extra heat, the bird will increase the heat loss by direct exchange with the immediate exterior environment (air, litter, and radiation). As the environmental temperature becomes more and more difficult and as the upper critical temperature is reached, the body temperature starts to increase.
Heat elimination is then brought about by respiratory means. Heat losses are reduced as the humidity is increased. A relative humidity above 70 % renders thermoregulation very difficult in hot humid conditions.
Energy and protein synthesis decrease as soon as the body temperature increases. The production of extra heat during the process of digestion lowers the upper critical temperature, increases body temperature and reduces the resistance to heat.
When the conditions are too severe, there will be a reduction in the quantity of food consumed.
Birds have to match their feed intake to their ability to eliminate the heat produced.
The energy intake is the limiting factor for growth and production:
The dietary energy level has no effect on growth, thus showing that thermoregulation is responsible for the lower growth.
During the laying period, energy consumption is not modified by changing the energy level of the diet.
Growth and production are reduced at the same time as temperature is increased.
Egg weight falls by about 0.4 percent peroc between 23 and 27oc; above 27oc the decrease is about 0.8 percent peroc.
Growth at point of lay is reduced above 24oc, and is extremely low above 28oc.
Rate of lay is generally only affected above 30oc.
Feed conversion ratio is minimum at a temperature of about 28oc; above 28oc it increases, because of the failure in production.
The article also discusses how the adverse effects of heat could be reduced during the rearing and production period.
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Article made possible through the contribution of Hendrix Genetics