Poultry belongs to the group of farm animals with a very intensive metabolism, which is the foundation of their high productivity. The rapid growth of broilers, high egg production in laying hens, and good reproductive performance of breeding stock are the result of long-term genetic selection, high-quality nutrition, and proper flock management. At the same time, however, this high level of performance means that the birds’ bodies operate with substantial demands for energy, nutrients, and precise regulation of physiological functions.

For this reason, poultry are among the animal species most sensitive to high temperatures. Heat stress, a condition in which the body cannot dissipate internally generated heat quickly enough, affects not only animal behaviour but the entire organism. Changes occur in feed intake, digestion, nutrient metabolism, immune function, and reproductive performance.

From the farmer’s perspective, the problem is often first noticed through subtle signs. Laying hens may produce fewer eggs or eggs with poorer shell quality. Broilers grow more slowly and require more feed per kilogram of weight gain. Turkeys reduce feed intake, and ornamental poultry may lose feather quality. These changes are not merely the result of discomfort caused by heat; they are the consequence of complex physiological processes occurring within the body.

Unlike humans, poultry cannot cool themselves by sweating. Instead, they rely on other mechanisms that are less efficient during prolonged exposure to high temperatures and place additional strain on the body.

Proper housing conditions, an adequate supply of high-quality water, and a well-balanced diet therefore represent the fundamental tools for helping poultry cope with periods of high temperatures while minimizing negative impacts.

Why are poultry sensitive to high temperatures?

Birds naturally maintain a higher body temperature than mammals. In adult chickens, body temperature typically ranges between 40 and 42°C. This elevated temperature is essential for proper metabolism, digestion, and immune function. However, it also means that the body has less capacity to tolerate further increases in temperature. When heat production exceeds the bird’s ability to dissipate heat to the environment, heat stress develops.

Feathers:  an advantage and a disadvantage

Feathers are one of the most important characteristics of birds. During colder periods, they serve as an excellent insulating layer, reducing heat loss. In hot environments, however, this same characteristic becomes a disadvantage. Dense feather cover limits direct skin contact with the surrounding air and makes heat dissipation more difficult.
The following are generally more susceptible to heat stress:

• Heavy meat-type breeds
• Fast-growing broilers
• Densely feathered breeds
• Long-feathered ornamental poultry

How do poultry cool themselves?

Poultry do not possess functional sweat glands and therefore cannot cool themselves through sweating as humans do.

Their primary cooling mechanisms include:

• Increased respiratory rate (panting with an open beak)
• Behavioural changes, such as reduced activity, seeking shade, lying down, and holding the wings away from the body
• Reduced feed intake

When does heat stress occur?

Heat stress is not determined by temperature alone. Several factors contribute to its development:

• Ambient temperature
• Air humidity
• Air movement
• Stocking density
• Age of the birds
• Body weight
• Health status

Under normal conditions, poultry begin to experience increased thermal load at temperatures above approximately 25°C. When temperatures remain around 30°C or higher for extended periods, the risk of heat stress increases significantly. High humidity is a major concern. When the air is saturated with water vapour, evaporation from the respiratory tract becomes less effective, making cooling more difficult.

What happens in the body during heat stress?

Heat stress is not simply a response to an uncomfortable environment. It is a complex condition affecting nearly all organ systems. One of the body’s first responses is a reduction in feed intake. Digestion and nutrient utilization generate metabolic heat. By consuming less feed, birds reduce the amount of heat produced during digestion.

Effects of heat stress on different poultry categories

Broilers

Broilers are among the most heat-sensitive poultry categories. Modern broilers have been genetically selected for extremely rapid muscle growth, resulting in a high metabolic rate and substantial heat production.

High temperatures can lead to:

• Reduced feed intake
• Slower growth
• Poorer feed conversion ratio (more feed required per kilogram of weight gain)
• Greater flock unevenness
• Increased mortality risk

The final weeks of the growing period are particularly critical because birds have reached their highest body weight and produce the most heat.

Practical example: A broiler farmer observes a marked slowdown in growth during several hot days. Birds spend most of their time sitting, visit feeders less frequently, and remain near drinkers for longer periods. Although feed is available, consumption declines. This is not a feed quality issue but rather a natural physiological response aimed at reducing additional heat production.

Laying hens

In laying hens, heat stress is especially problematic because egg production requires a substantial amount of energy. Under high temperatures, hens reduce feed intake.

This can result in:

• Lower egg production
• Smaller egg size
• Poorer shell quality
• Increased losses due to egg breakage

One common issue is the production of thin-shelled eggs. This is caused not only by reduced calcium availability but also by metabolic changes associated with increased respiratory activity.

Turkeys

Turkeys are particularly sensitive to heat stress. Their large body size results in high heat production, which becomes difficult to dissipate under hot conditions.

Heat stress may cause:

• Markedly reduced feed intake
• Slower growth
• Poorer reproductive performance
• Higher mortality rates

Ducks and Geese

Waterfowl are often considered more heat-tolerant due to their association with aquatic environments. In reality, their tolerance largely depends on access to cooling opportunities. Without access to water that allows body cooling, ducks and geese can suffer from high temperatures just as severely as other poultry species.

Ornamental and exhibition poultry

In hobby flocks, productivity is often less important than overall condition and appearance.

Heat stress may negatively affect:

• Feather quality
• Feather shine
• The moulting process
• The reproductive performance of males

Breeds with particularly dense or long feathers are often the most susceptible.

How to recognize heat stress in poultry

The earlier a farmer identifies the first signs of heat stress, the greater the chance of preventing significant losses. Initially, symptoms may be subtle and easily mistaken for normal reductions in activity during hot weather.

Common early warning signs include:

• Reduced feed intake
• Increased water consumption
• Reduced activity
• Seeking shade or cooler areas
• Wings held away from the body
• Increased respiration rate
• Open-beak breathing
• Frequent lying down

In advanced stages, severe overheating (hyperthermia) may occur. Birds may lose coordination, lie on their side, become unresponsive, and eventually die.

Nutrition as a tool for preventing heat stress

A properly balanced diet cannot eliminate high environmental temperatures, but it can significantly improve the bird’s ability to cope with heat stress.

Nutritional strategies during hot weather aim to:

• Ensure adequate nutrient intake despite lower feed consumption
• Support gut health
• Protect cells against oxidative damage
• Maintain mineral balance
• Support the immune system

Because poultry eat less during hot weather, feed must provide adequate levels of energy, high-quality protein, amino acids, vitamins, and minerals. The goal is not simply to increase dietary energy density but to ensure an optimal nutrient balance. Essential amino acids, particularly methionine, lysine, and threonine, play a vital role in supporting growth, feather quality, intestinal health, and immunity.Heat stress also increases the production of free radicals that can damage cells. Therefore, antioxidants such as vitamin E, vitamin C, and selenium become especially important. Supportive compounds such as betaine and selected phytogenic additives may also help maintain cellular function, gut health, and nutrient utilization.

Water: the most important nutrient

During heat stress, water is the most important nutrient.At high temperatures, water consumption may increase several-fold. Birds require water not only for normal metabolic functions but also for cooling through respiration.Water should always be:

• Freely available
• Hygienically safe
• Fresh
• Regularly monitored

A common mistake in hobby flocks is assuming that a full drinker automatically means an adequate water supply.Practical example: A poultry keeper checks the drinker in the morning and finds it full. In the afternoon, however, some birds are panting heavily and eating less. The cause may be a blocked nipple drinker or excessively warm water. Although water is technically available, the birds are unable to use it effectively.

During hot weather it is therefore advisable to:

• Check drinker functionality regularly
• Clean water containers frequently
• Prevent drinkers from overheating in direct sunlight

The importance of electrolytes during heat stress

Rapid breathing alters mineral balance within the body.

Losses primarily involve:

• Sodium
• Potassium
• Chloride

These minerals, collectively known as electrolytes, are essential for proper muscle function, nerve activity, and maintenance of fluid balance.

Housing environment and management

Even the best feed cannot compensate for poor environmental conditions.

During hot weather it is important to provide:

• Adequate shade
• Good air circulation
• Protection from direct sunlight
• Regular monitoring of water supply
• Sufficient space
• Minimal handling during the hottest part of the day

In outdoor systems, direct solar radiation is often the greatest challenge. While outdoor access provides many benefits, birds must have access to shaded areas where they can seek shelter.

Common management mistakes

The most frequent mistakes include:

• Insufficient water availability during hot weather
• Placing drinkers in direct sunlight
• Poor ventilation in poultry houses
• Excessive stocking density
• Handling birds at midday
• Sudden dietary changes
• Feeding inappropriate food scraps instead of complete feed

Another common mistake is attempting to improve performance by feeding excessive amounts of grains. While grains provide energy, they do not adequately supply essential amino acids, vitamins, or minerals.

Conclusion

Heat stress is one of the most important risk factors in poultry production. Its effects extend far beyond a temporary reduction in activity and influence the entire organism, from feed intake and digestion to immunity and reproduction. Fast-growing broilers, high-producing laying hens, heavy turkey breeds, and some densely feathered ornamental breeds are particularly vulnerable. Effective prevention relies on the combination of appropriate housing conditions, an adequate supply of high-quality water, and balanced nutrition. Antioxidants, electrolytes, high-quality amino acids, and gut health-supporting additives play especially important roles during periods of heat stress. A simple rule applies for poultry producers: when birds consume significantly less feed, exhibit heavy panting, or show changes in productivity during hot weather, these signs should not be dismissed as normal responses to the weather. Early intervention can significantly reduce losses and help maintain the health and condition of the flock.

 

Sources used in preparing this article are available from the author, Ing. Lucie Humeni.