Managing heat stress – Part 1 – Layers respond to hot climatic conditions

29-03-2010 | | |
Managing heat stress – Part 1 – Layers respond to hot climatic conditions

Layers in hot climates are exposed to high temperatures, which may lead to a loss of productivity. Proper measures can be taken to manage such conditions, but this demands an understanding of the bird’s physiology.

By Viola Holik, Lohmann Tierzucht, Tanzania

In many developing countries poultry production is based mainly on traditional extensive poultry production systems. It has been estimated, for example, that 80% of the poultry population is found in traditional family-based poultry production systems, contributing up to 90% of poultry products in low income countries like Ethiopia.

Heat stress often is a common problem in such countries. High ambient temperatures can be devastating to commercial broilers and layers. Coupled with high humidity, high temperatures can have even more harmful effects. Heat stress interferes with the birds comfort and suppresses productive efficiency.

During periods of heat stress the hens have to make major thermo-regulatory adaptations to prevent death from heat exhaustion. As a result, the full genetic potential of the layer is often not achieved. The purpose of this article is to review some of the effects of heat stress on layers, and to look at methods that can be used by the poultry producer to partially alleviate some of the detrimental effects of heat stress on the birds’ productivity.

Removing excessive body heat

The body temperature of a hen varies between 40 and 42°C, depending on the time of day (before and after feeding, night time), feather cover (in connection with moulting), brooding, and environmental temperature. The ideal environmental temperature for chickens is 18-24°C. If temperatures exceed 24°C, the bird has a number of possibilities to release excessive body heat:

Heat loss is proportional to the temperature difference between the body surface and the surrounding air. Poorly insulated, hot roofs will increase house temperature and heat stress on sunny days, similar to direct sun on free range birds without shade.

The hen’s hot body will release hot air into the surrounding environment. This process can be assisted by providing moving air fast enough to break down the layer of still air surrounding the body.

Heat may flow surface to surface, such as when the birds stand or sit on cool litter or cool water pipes. Usually the litter has a similar temperature as the house and water pipes are insulated.

Since the bird’s skin has no sweat glands, evaporation takes place through panting. This is only effective if the humidity is not too high. Hot and humid conditions are therefore much more stressful than hot and dry conditions. In order to lose 1 ml of water, the chicken uses 540 kcal, and this energy loss may result in a significant drop in production.

Body and environmental temperature

As illustrated in Figure 1, the rate of losing body heat through radiation and convection decreases with increasing temperature, and the bird has to rely mainly on temperature regulation via evaporation.

As mentioned in Figure 2, 18 – 24°C is the ideal environmental temperature for a hen. Up to a temperature of about 30°C the hen can still regulate body temperature, but when the house temperature reaches 40°C the body temperature will increase dramatically. When exposed to a 30°C environment, the hen will eat less feed, thereby resulting in smaller egg size and eventually lower egg production.

When the temperature increases from 30 to 38°C, shell quality is likely to deteriorate as indicated by increasing percentage of cracked eggs. Above 38°C the bird can only get rid of body heat through severe panting, which produces respiratory alkalosis. This physiological response is characterised by an increase in blood pH (more basic) along with a decrease in blood CO2concentration. It upsets the acid-base balance and produces a decrease in blood calcium and bicarbonate, which are necessary for the production of strong egg shells. As a result, more thin-shelled eggs will be produced.



From 41°C the risk of death is high and emergency measures have to be taken. A temperature of 47°C is lethal.
Rising temperatures will cause the hen to eat less feed, but also to drink more water. Water:feed intake ratio at 15°C, for example, is 1.82:1. However, when the temperature reaches 30 – 35°C, the water:feed ratio will be 4.9:1, which is quite a common situation with open houses in hot climates. Table 1 shows the effect of ambient temperature on production, egg weight and feed conversion.

Layers reared in high temperatures from a young age can adapt to some extent and may reach good productivity levels. These hens will typically develop larger wattles and combs. Additionally, these birds will have less fat and feather coverage. Figure 3 shows how the body temperature adapts to high temperatures within 14 days.

Practical measures

Good management practices include attention to the following possibilities to minimise heat stress.

Stocking density
Heat loss often depends on the difference between the body temperature of birds and the ambient temperature. If stocking density is high, the radiant heat between the birds accumulates and the temperature increases. Therefore, the birds cannot lose body temperature (Table 2).

Bird handling
During the hotter periods of the day any additional stress on the birds should be avoided. Vaccination, beak trimming, transfer, or any other kind of handling should be done during the coolest period of the day, if necessary at night. In any case, handle the birds as calmly and gently as possible.

Water temperature
Birds can reduce body temperature by drinking cool water. Researchers Leeson and Summers exposed layers to an Feeding time
Feeding at the right time of the day is very important to help the birds cope with heat stress. During late afternoon a significant rise in body temperature can be observed, which, in severe cases, can cause bird mortality. This is not the hottest time of the day, but it is the peak time of digestion if the birds have been fed in the early/mid morning. A good strategy to take an unnecessary heat load off the birds is to withdraw feed 8 hours prior to the anticipated time of peak temperature. One third of the daily feed ration should be given in the morning and two thirds in the late afternoon. An additional advantage is the availability of calcium in the digestive system during s hell formation at night and in the early hours of the morning. This will improve shell quality and prevent the birds from depleting bone calcium. So-called ‘midnight snacks’ are a good tool to give hens extra feeding time in the cooler parts of the night. This does not necessarily need to be done at midnight, but rather 3 hours of darkness before and after the extra 1-2 hours of light is essential to avoid disturbing the lighting programme.

Feed stimulation
There are some simple strategies to stimulate feed intake.
– Run the feeder chains more frequently. Empty them to avoid overflow if need be.
– Feeders should run empty at least once a day to enhance the appetite and to ensure that the fine particles of the feed (premixes, vitamins etc.) are consumed.
– The feed texture should not be too fine. Oil can be used to avoid “dusty” feed.



ME requirement decreases as ambient temperature increases to above 21°C, resulting from a reduction of energy requirements for maintenance. The energy requirement will decrease with the rise of temperature up to 27°C, above which it will start to increase again since the bird needs additional energy for panting to reduce body heat.

Including oil in the diet has long proved to be beneficiary in hot climates and shows better effects than in moderate climates. For example, the inclusion of oil increased feed intake by 17.2% at 31°C compared to only 4.5% at temperatures of 10-18°C. Digestion of fat produces less heat than the digestion of carbohydrates and proteins. Oil binds the fine particles in the feed and stimulates feed intake. Additionally, it increases the energy level in the feed, which is very important to compensate the reduced energy intake due to less feed intake during the hotter periods. Fat has also been shown to slow down feed passage through the gastro-intestinal tract, and therefore increases nutrient utilisation. Up to 5% oil can be used. An additional advantage of oil is the content of linoleic acid, which improves the production and weight of the eggs.

Whether protein levels should be increased or decreased in diets to minimise heat stress and maintain production has been studied with different results. Consensus appears to be that the key to good nutrition is to focus on daily intake of essential amino acids while reducing total digestible protein intake within the constraints of available raw materials.

Vitamins are unarguably very important components of a chicken’s diet. It should also be noted that unless a formulated ration is used, it is likely that deficiencies will occur, negatively affecting the bird (Table 3). Vitamin C is thought to support the birds in handling heat stress, but the effects are not yet fully understood. Some birds may not be able to synthesise sufficient ascorbic acid to replace the severe loss of vitamins during heat stress. Research in the early 1960s showed that an addition of vitamin C improved egg weight, shell thickness and egg production. Later experiments demonstrated that adding ascorbic acid in the feed formula improved feed intake and feed utilisation. The optimal effect was shown by adding 250-400 mg ascorbic acid/kg. Due to the lower feed intake at high temperatures, a sufficient supply of vitamins has to be guaranteed.

The electrolyte balance in birds is altered during heat stress due to panting. Panting increases carbon dioxide loss in the bird, which reduces the bird’s ideal water intake. By adding electrolytes to the feed or water, birds increase their water intake. This aids in keeping a constant body temperature, and also maintains an effective system of evaporative cooling.

During hot periods, mineral excretion is usually increased. It is therefore advisable to increase the mineral level in the formula. Since it is difficult to react fast enough through dietary changes, application via drinking water is recommended.

To meet the calcium requirements of laying hens, additional oyster shells or limestone chips can be offered.

* References are available on request

Natalie Kinsley Freelance journalist