In animals, certain alterations occur as a reaction against those repeated stimuli that disturb their homeostasis or normal physiological balance. This phenomenon is known as stress. Among the different types of stress, heat stress is the most frequent. It importantly affects poultry farms and has a great impact on productivity and mortality.

How do birds keep their body temperature?

Birds temperature is about 41áµ’C, which is maintained thanks to their thermoregulative capacity. In industrial poultry, birds must be kept in the thermo-neutral zone, a range of temperatures where birds can lose heat through basic mechanisms (radiation, convection and conduction). In this thermo-neutral zone, there is no heat stress and the body temperature remains constant.

However, it is usual that, under certain conditions, the limits of this thermoneutral zone are exceeded and these basic temperature regulation mechanisms are insufficient. When this happens, the body temperature rises and heat stress begins. At this time, the birds lose heat by evaporation, a mechanism to fight this situation through panting. During this process, the birds promote the circulation of air on the surface of their air sacs, which contributes to increase gas exchange and heat loss.

This mechanism includes the activity of the nervous, endocrine and cardiorespiratory systems, among others. It leads to the intensification of essential functions, that is, it increases the respiratory and cardiac rate and, on the other hand, the reduction of other less important functions, such as immune function, growth or reproduction.

Main signs in birds affected by heat stress

Initially, the most common sign is that birds, while trying to recover their thermal balance, modify their behavior, and we can observe:

• Birds with distended wings
• Crouched on the floor
• Slowness, lethargy
• Stupor, wobbles
• wet faeces
• Increase in water consumption
• Decrease in feed consumption
• Birds begin with slow gasps and end up panting very quickly
• Deviation of blood from internal organs to the skin, which obscures the color of the skin
• Terminal seizures
• Increase in mortality

Consequences of heat stress in birds

Increased mortality

When heat stress occurs for a long time or animals are exposed to very high temperatures, none of the aforementioned mechanism can reduce internal body temperature and mortality begins to increase due to cardiorespiratory failure.

Decrease in growth and egg laying

To cope with heat stress, birds intensify organic functions that are essential for survival. This means that most of the energy is used to fight the stressful situation and ensure survival, and not for fattening.

In broilers, a decrease in daily growth is observed and, in addition, when temperatures are high, feed consumption is reduced, which accentuates growth stunt.

In the case of layers, this leads, initially, to a decrease in the size of the eggs and, subsequently, to a reduction in egg production.

Increase of broken eggs

During the panting of the birds in heat stress situations, there is an excess in the loss of blood CO₂ levels below their basal concentration, which leads to the increase in the blood pH, producing metabolic alkalosis. In order to restore the levels of CO₂ in blood, calcium carbonate (Ca₂CO₃) moves from the bones to the blood and, once there, is divided as we see in the following scheme:

CaCO₃ → CO₂ + CaO+ H⁺ + OH^–   → CO₃H^– + CaO + H⁺

Bicarbonate (CO₃H^–) is excreted through the kidneys to fight metabolic alkalosis, as well as calcium, which is excreted as CaO, which causes the depletion of this element in birds.

Due to the excretion of both molecules, bicarbonate and calcium, the ability of the hen to produce enough calcium carbonate to form the eggshell is reduced.

While calcium deposits are big enough, that is, this mineral is properly stored in the bones, there will be enough calcium to form the eggshell, because this element will be mobilized from these bone storages. Therefore, it is key to maintain calcium reserves in the bones through proper calcium intake and good calcium absorption in the intestine. On the other hand, when these reserves are not enough or they are depleted, calcium deficiency takes place and the percentage of broken eggs increases.

Musculoskeletal disorders

There is a higher number of hes suffering from cage layer fatigue syndrome, which is characterized by the inability of the hens to stand up due to the fragility of the bones after the calcium carbonate mobilization to the blood. For the same reason, animals also suffer from fractures of the femoral head.

Measures to prevent heat stress in poultry farming

There are two main types of measures to prevent heat stress:

1. Management measures:

• Ventilation: It is very important to have ventilation systems, and even cooling in those climates that require it, in order to increase animal welfare and maintain the temperature in a comfort zone.
• Water: Birds must have continuous water availability, and it must be kept below 30áµ’C Keep the birds as calm as possible during the hottest hours of the day, in order to avoid increased movements and, therefore, increased metabolic activity and heat production.
• Implement and adjust appropriate lighting programs.
• Implement roof sprinklers, to be able to suffocate heat in the shortest time in case of emergencies.
• Maintain an adequate density that prevents overcrowding and environmental overheating of the farm.

2. Nutritional measures:

• Monitor feed and water consumption in order to prematurely detect cases of heat stress.
• It is important that the diet is balanced regarding important nutrients such as aminoacids, calcium, sodium, phosphorus or vitamins, especially the water soluble ones.
• Feed the birds during the coolest hours of the day in order to avoid raising the temperature as a result of metabolism, animals can even be fed at midnight.
• Supplement highly digestible ingredients in order to provide all the necessary nutrients thanks to a better use of the diet, despite the reduction of the feed consumption.
• Add additives in drinking water rich in electrolytes and vitamins in order to compensate their loss.
• Provide carbonates in order to avoid the loss of bone calcium, necessary for the formation of the eggshell in periods of high production.

Conclusions

Heat stress is a very important problem in current poultry production, since the mechanisms of birds to avoid it derive energy to the maintenance of vital functions rather than production. Therefore, efforts to fight must be aimed at avoiding the environmental conditions that facilitate heat stress and management and nutritional measures, or a combination of both, should be implemented. All these measures are more effective as prevention, since controlling heat stress problems once  it has already tarted is very complicated and economic losses can have a great impact on the farms.