Heat stress in poultry explained

Due to climate changes worldwide, hot days are getting hotter and occur more frequently with heat waves becoming more common, while fewer cold days are measured. This is not only the case in (sub)tropical regions, but also in regions with a temperate climate. These high temperatures are damaging to livestock as they will experience heat stress. Heat stress is one of the most important environmental stressors challenging poultry production worldwide as it has detrimental consequences on growth and production, leading to significant economic losses. For example, in the United States, heat stress in the poultry industry results in estimated total annual economic losses of approximately $128 to $165 million.

What is heat stress?

Diagram of thermoneutral zone. LCT: lower critical temperature, UCT: upper critical temperature, MAX: point of maximum heat loss.

Birds are homeothermic animals, which means they can maintain a constant body temperature (approximately 41 °C) within certain boundaries. However, birds are highly sensitive to high environmental temperatures due to their high metabolic activity, their feather coverage and the absence of sweat glands. When birds are subjected to high environmental temperatures, especially in combination with high relative humidity and low air speed, they experience heat stress. Heat stress occurs when difficulties in achieving a balance between body heat production and body heat loss arises.

Birds are able to maintain a constant body temperature in their thermoneutral zone (A), which is the range between the lower and upper critical temperature (LCT and UCT). When the environmental temperature exceeds the UCT, birds will have to actively lose heat by panting, first slowly (B) and thereafter faster (C) as the temperature rises, which compromises the welfare of the birds. When the environmental temperature exceeds the point of maximum heat loss (MAX), birds are unable to control their body temperature and death is inevitable (D).

How do poultry cope with heat stress?

Heat loss mechanisms avaiblable to birds. Radiation (green), convection (red), conduction (yellow) and evaporation (blue).

1. Increasing heat loss

Radiation, convection and conduction are the three sensible heat loss mechanisms birds have to their disposal to maintain a constant body temperature.

  • Radiation: Electromagnetic waves transfer body heat through the air towards cooler objects. The higher the difference in temperature, the more heat is lost from the surface of the body.
  • Convection: Heat from body parts such as the comb, wattles and wings is naturally lost to cooler surrounding air. To increase heat loss through convection, birds increase their exposed surface area by lifting and spreading their wings and by widening their blood vessels (vasodilatation). The colour of body parts such as the comb and wattles becomes darker. Adequate air flow is essential to achieve efficient heat loss via convection.
  • Conduction: Heat is transferred from the body to a cooler surface when both are in direct contact. Birds can for example sit on litter or lean against cage wire to lose body heat.

Once the environmental temperature increases to a level exceeding the upper critical temperature, the sensible heat loss mechanisms become inefficient and birds actively lose heat by panting. Panting is shallow open-mouth breathing which allows heat loss via evaporation of water from the mouth and respiratory tract and is the major method of heat loss at these high temperatures. A condition for this to be effective, however, is that the humidity in the air is not too high.

2. Reducing heat production

Next to losing heat, birds will also reduce their own body heat production. Indeed, body heat is produced by processes such as maintenance, growth, muscular activity and meat/egg production. The amount of heat produced in the bodies of chickens is affected by factors such as age, breed, gender and physical activity. To lower their heat production, birds will actively lower their metabolism and feed intake resulting in a decreased weight gain and meat/egg production. Additionally, birds become less active and move away from other birds.

Measures to combat heat stress

Heat stress poses serious challenges and has severe consequences for poultry welfare, production and profitability. However, several measures to deal with heat stress can be taken.

  • Implement adequate adjustments to management practices (such as ventilation, stocking density, litter management… )
  • Design and orientate farm house to alleviate of heat stress as much as possible
  • Introduce intermittent feeding programs as well as well-considered dietary composition changes
  • Consider feed additives designed to mitigate the negative effects of heat stress and improve the bird’s performance

References available upon request.

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