Layers occasionally die as a result of heat stress others adapt to the stress.
Above the temperatures of 27°C, the bird’s body temperature rises and a much more dramatic reduction in feed intake can be anticipated.
As feed intake declines, the first production trait to respond is egg weight, resulting in increase in medium and small eggs. This is largely due to the reduction in the absolute amount of protein ingested at the higher temperature.
As temperatures rise above 32-35°C, egg production levels may also decline, as total nutrient intake is insufficient to support normal rates of lay.
The most frequent result of heat stress is a decline in eggshell quality, due to inadequate calcium intake. The maximum daily calcium intake recommended for most layers is 4.0 g. This would be obtained if the hen consumed 90 g of a feed containing 4.4% calcium, or a feed with 3.6% calcium plus 20 kg oyster shell supplement per tonne.
As layers pant to keep cool, excess carbon dioxide is exhaled. This causes the blood to become more alkaline and reduces its ability to hold and carry calcium for shell formation.
Various blood minerals are affected by heat stress. Among them is phosphorus, and the requirement for this element is increased at high temperatures.
Marginal phosphorus levels, when combined with heat stress, can lead to increased mortality rates, particularly among older birds.
As a result of the increased water intake, moisture content of droppings escalates in hot weather. This can provoke difficulties in the handling and storage of manure where the system depends on dry manure. It can also lead to increased soiling of egg shells, either directly from soiled feathers, or from splashing in shallow manure pits.
There is considerable evidence that the immune system of the bird is adversely affected by heat stress. Therefore, administration of vaccines to pullets or layers is not usually recommended during very hot weather.
Furthermore, where vaccines are spray administered, the closing down of the ventilation system to permit proper vaccine distribution is impractical in extremely hot conditions. If vaccinations cannot be avoided, they should be restricted to the coolest time of the day, when the adverse effects of reduced ventilation are at a minimum.
High temperatures may also result in less efficient utilization of vitamins, and certain vitamins in feed are themselves less stable in these circumstances.
The effects of heat stress will all be aggravated by other environmental factors such as increased bird density, feed and/or water deprivation, inadequate ventilation, vaccine reaction and the presence of diseases or parasites.