TECHNICAL ARTICLES Broilers/Layers/Breeders

Optimizing Broiler Production During Heat Stress

We are entering a period of the year when heat plays an important role in broiler production. Significant mortality has occurred in recent years.

Perhaps even more importantly, many tons of broiler meat will be lost from broilers that survive heat distress.

High performance commercial broilers are particularly susceptible to heat stress because metabolic heat production increases with growth rate while heat dissipation capacity tends to decrease with the size of the bird. Broiler heat production is particularly high because the efficiency of caloric use is only about 40°C. Sixty percent of all calories consumed are lost as heat.


There are two routes for heat dissipation in the broiler. Evaporative and non-evaporative. Non-evaporative cooling is heat radiation or dissipation from the service of the body. Non-evaporative cooling is the principle means of heat dissipation at low and intermediate ambient temperatures. It is the most energetically efficient means to dissipate heat. Birds manipulate non-evaporative cooling by increasing surface area and blood flow to the body’s surface. The authors suspect that such blood shunting also affects digestion rate, which lowers feed metabolism and, therefore, reduces the metabolic heat production during heat distress. Feed metabolism and, therefore, heat production are increased during subsequent recovery periods. Ventilation systems should be used during the evening hours to remove waste heat as quickly as possible so that non-evaporative cooling potential is restored and maximum time is provided for compensatory growth. The evening hours can provide considerable opportunity for broilers to regain lost growth potential.

Non-evaporative cooling is a reflection of the ratio of surface area to total bodyweight. Because surface area becomes smaller in proportion to total bodyweight as the broilers become heavier, the potential benefits of non-evaporative cooling also decrease as broilers become heavier. Evaporative cooling is the other route of heat dissipation. As ambient temperature exceeds the bird’s thermoneutral zone, non-evaporative cooling declines and evaporative cooling becomes the principal route of heat dissipation.

In broilers, evaporative cooling is primarily the result of the vaporization of water through the lungs during respiration. Increased respiration rates increase evaporative cooling. The increased respiration rate also requires additional energy expenditure and adds calories to thc bird’s heat load, further increasing its heat dissipation requirements.

Thermobalance is the combined result of heat production and heat dissipation. In thermoneutral environments heat production has no adverse consequences. During heat stress, the excessive heat production becomes potentially life threatening for the broiler. In an effort to survive, the broiler attempts to lower heat production by consuming less feed. The poultry producer attempts to encourage feed consumption in an attempt to avoid the growth suppression that is usually associated with heat stress. When the poultry producer is successful in increasing feed consumption the results are frequently unsatisfactory because of increased mortality. Previous exposure to heat stress is another variable that influences the broiler’s susceptibility to heat stress. The broiler’s ability to survive acute heat stress is dramatically increased by prior heat stress exposure. The body temperature of acclimatized broilers is lower than unacclimatized broilers during heat stress. Part of the acclimatization response is the result of reduced feed consumption. Research in the authors’ laboratory suggests the broiler is also able to repartition daily heat production to cooler periods of the day.

Management options

There are many management options that can have an effect on the reaction of the broiler to heat stress. Some techniques are effective for enhancing growth rate while other techniques are more effective for improving bird survival. Seeking a balance between the many options in order to achieve maximum product yield is the difficult choice.

Facilities: The broiler house should have an east-west orientation with enough roof overhang to keep direct sunlight (a significant heat source) from entering the building. Roof height should be approximately 5 meters to enable heat to move freely up and away from the birds. Roof insulation should be R-30 or greater. Ventilation is critical. All obstructions should be removed from around open-sided buildings to achieve maximum benefit from prevailing winds. Grass cover around the building is desirable in order to reduce reflected heat. Fans should be located throughout the house to facilitate air movement.

Feed management: Many studies have suggested the largest proportion of economic loss associated with heat stress is the result of lowered feed intake. Many management procedures have been used by poultry growers to offset the reduced nutrient intake that occurs during heat stress (running automatic feeders more frequently, physically shaking feeders, pelleting feed, continuous lighting, high nutrient-dense rations, high fat levels). Efforts to offset the physiological response of reduced feed intake may be counter-productive. Increasing feed intake increases the broiler’s heat load and results in an increased risk of mortality during acute heat stress.

Since increasing feed consumption tends to increase mortality during acute heat stress, one can anticipate that reducing feed consumption would favour bird survival. Fasting intervals as short as three hours prior to the initiation of broiler heat stress enhance bird survival, while removing feed after the onset of heat stress appears to be of little value. Time is required for the feed to clear the bird’s digestive tract and reduce the substrate availability that results in heat production as a result of digestion. Six to eight hours of fasting prior to heat stress initiation is probably the maximum amount of fasting time that can be achieved under practical conditions. Fasting birds for six hours prior to heat stress, in combination with a six-hour heat stress period, means the birds will he without feed for a total of 12 hours.

In order for fasting to he effective the fasting period must be synchronized with the time that heat stress will occur. The start of heat stress may be defined as the time birds are first observed panting and exhibiting signs of discomfort.

The initiation of fasting should begin approximately six hours prior to the start of heat stress. Fasting should continue until the ambient temperature starts to decline. Fasting programs are not intended to increase growth rate, according to the authors, but are intended to maximize survival during severe heat stress. Results obtained in the authors’ laboratory indicate fasting has very little effect on growth rate. The principle effect of fasting is to shift bird feed consumption and growth to the cooler periods of the day. Growth rate is not markedly changed. In other research, feed was withdrawn four hours prior to heat exposure and returned to the birds one hour before the onset of heat stress. This research suggested the birds could tolerate the combination of feed intake and heat stress. It appeared this procedure stimulated water consumption, which was positively correlated with bird survival.

Acid-base balance

Increased respiration during heat stress may result in the loss of carbon dioxide and changes in acid-base balance. A number of attempts have been made to adjust the acid-base balance during heat stress (the addition of ammonia chloride, hydrochloric acid, potassium chloride, sodium chloride and potassium sulfate to the water or feed). The only common denominator between the various treatments and experiments appeared to be the positive correlation between growth rate and water consumption. The various drinking water supplements appeared to alter growth rate primarily by inducing the bird to increase water consumption.

Water management

Water consumption by the heat-stressed bird is critical. Heat-stressed birds dissipate more than 80% of their heat production through evaporative cooling. The addition of various salts to the drinking water alters the bird’s osmotic balance, increases water consumption and influences water balance. It has been observed that potassium chloride addition to the water during heat stress not only improves performance but also reduces serum corticosterone. Increased water consumption benefits the bird by acting as a heat receptor as well as increasing the amount of heat dissipated per breath. Data indicate that increasing water consumption 20% can increase heat loss per breath as much as 30%. These thermobalance effects are principally observed when water temperature is at or below 28°C. Birds in positive water balance are better able to maintain body temperature homeostasis.

Heat stress increases urine production independent of water intake and forces birds to sustain higher water consumption levels than would be required to simply replace the water lost due to evaporative cooling. There appear to he significant inter-actions between the addition of salt to drinking water and the drinking water temperature. Adding potassium chloride to the drinking water significantly increased feed consumption and growth rate when the temperature of the water was lower than the bird’s body temperature. This effect was not noted when the temperature of the water was similar to the body temperature of the bird. Lowering the water temperature without the salt addition also proved to be beneficial. The effects of lowering the drinking water temperature and adding salt were additive.

Ration composition

Some research has suggested that growth rate of heat-stressed broilers can be increased by adding fat to the ration. However. the effect of reducing dietary heat increment by replacing carbohydrate calories with fat calories is frequently associated with an increase in energy consumption. The increase in energy consumption is usually larger than the reduced heat increment. The result is that increasing dietary energy levels tends to increase mortality as well as weight gain during survival-limiting (severe) heat stress.

The poultry industry is faced with the dilemma of placing emphasis on growth rate enhancement or on mortality reduction. It is extremelv difficult to achieve both. If carcass composition is important the added fat gain associated with the higher caloric density may be enough to curtail the use of higher fat levels. Recommendations for the best protein level to use for heat-stressed broilers range from increasing the level of protein to decreasing the level of protein. The authors said the only approach they have observed that simultaneously improves both growth rate and survivability is to reduce the crude protein level while maintaining essential amino acid levels by adding synthetic amino acids.The authors reported that research in their laboratory indicated a positive response for weight gain, gain:feed ratio and survivability when antibiotic growth promoters were included in the diet of heat-stressed broilers.

The only antibiotic growth promoter studied was virginiamycin. The authors have hypothesized that the rather dramatic response to antibiotic growth promoters may be the result of either reduced immune challenge (which results in a fever response) and/or reduced heat production. Broilers consuming virginiamycin were observed to produce less heat and consume less oxygen per calorie of metabolizable energy consumed. The authors said, generally speaking, they had not observed that vitamin fortification will solve a heat stress problem. However. it is critical that broilers be provided a good vitamin premix. The authors recommended that the vitamins not be withdrawn. Withdrawing the vitamin premix from heat-stressed broilers resulted in a greater numerical reduction in performance than when the vitamin premix was withdrawn from broilers that were in a thermoneutral environment.

The Bottom Line

A wide variety of alternatives are available to the poultry industry to help in reducing the effects of acute heat stress. Choosing the right combination is a difficult task.


R Teeter and T. Belay, Oklahoma State University

William A. Dudley-Cash, Feedstuffs, June 4, 2001.

Date published: 2003-12-01

R Teeter and T. Belay
and William A. Dudley-Cash