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Heat Stress in Poultry

Signs, symptoms, prevention and treatment

High temperatures can impact bird performance and reduce profits for poultry producers. Heat stress is linked to changes in the birds’ behavior, respiratory and cardiovascular systems, immune function, intestinal integrity and nutrient use.

Poultry producers commonly face the challenge of heat stress either seasonally or year-round. Poultry farmed in hot and humid countries are genetically derived from strains originally bred in, and selected for, the cool climates of Europe and North America. Rearing birds outside of their thermal comfort zone could mean failing to achieve full genetic potential. Producers in warmer climates or those in cooler zones who adjust their shed temperatures to their own comfort levels, not to that of their birds, should consider the impact of heat stress on flocks. 

The negative effects associated with heat stress include:  

  • reduced feed intake  
  • reduced growth 
  • poor feed efficiency 
  • decreased egg production 
  • higher mortality 
  • compromised meat and egg quality  
  • higher incidence of disease 

Extreme cases cause suffering and death in all poultry breeds. It is also thought that with advancing poultry genetics there are higher metabolic demands being placed on birds and this may increase the sensitivity of modern strains to heat stress. This, combined with the fact that the poultry industry will continue to grow in the warm regions of the globe, various complementary strategies--including housing, managment and nutritional strategies--to alleviate the negative effects of heat stress are required.  

Signs of heat stress  

For any stage and any species, heat stress occurs when birds have difficulty balancing body heat production and body heat loss. Within the thermoneutral zone illustrated in Figure 1, birds are able to lose heat at a controlled rate by modifying their behavior. There is no heat stress at this point and their body temperature remains constant. At or above the upper critical temperature birds begin to pant to actively lose body heat.  

Panting is considered a normal response to heat, but as temperatures continue to increase, the rate of panting increases concurrently. If heat production becomes greater than maximum heat loss either in intensity (acute heat stress) or over prolonged periods (chronic heat stress), birds very quickly begin to die. The body temperature of the broiler must remain very close to 41°C. If body temperature rises above 45°C, death starts to occur rapidly. 

Poultry typically demonstrate the following symptoms when heat stressed:  

  • Breathing difficulty 
  • Lethargic behavior 
  • Diarrhea 
  • Seizures or convulsions 
The thermoneutral zone and consequences of heat stress
The thermoneutral zone and consequences of heat stress

Housing and management measures to prevent heat stress

A number of preventative measures may be taken to alleviate heat stress and these approaches may be related to housing conditions, management practices, and nutritional factors.  

Housing and management:  

  • Provide adequate ventilation for number of birds housed  
  • Reduce stocking densities 
  • Ensure adequate supply of clean, cool water 
  • Insulate sheds sufficiently to avoid solar heat gain 
  • Position fans to optimize wind speed an air circulation 
  • Use evaporative cooling pads or atomizing nozzle 

In addition to the various housing and management practices, there may be a number of nutritional strategies to also consider.  

Housing and management measures to prevent heat stress
Housing and management measures to prevent heat stress

10 nutritional strategies to manage heat stress

1. Diet Density 

As feed consumption decreases, providing a more dense diet may assist in maintaining nutrient intake. Higher energy feeds may be required due to an increase in ME requirement and low feed intake.  Increasing the energy and amino acid content of diets may assist to improve performance (Jiang et al, 2007). 

3. Fats/Oils  

Providing more energy in form of a high-quality fat can be one method to reduce heat produced by the bird and improve performance (Hassan et al, 2008). Fat levels can be increased by 2-3% to replace a portion of the energy from starch. Ensure that the fat or oil is of good quality and adequately stabilized with an antioxidant.  

2. Amino Acids (AA) and Crude Protein  

Heat stress markedly affects the AA requirements of poultry. Increasing the digestible AA levels in the diets of broiler chickens reared under high temperatures improves performance. Providing a well-balanced amino acid supply with moderate levels of crude protein assists to minimize energy loss and helps chickens to cope with heat stress (Soares et al, 2020).  

4. Digestibility

Improving the digestibility of the feed may reduce the energy required for metabolism and reduce the heat increment of digestion thereby directing more energy to growth and productive purposes. Utilizing high quality raw materials and use of feed additives such as enzymes and phytogenics may be methods to enhance diet digestibility.  

5. Feed Form  

If possible, offering feed as pellets is a method to reduce energy expenditure for eating. The physical nature of pellets allows the bird to consume feed with less wasted energy and assists in improving bird performance (Hosseini, 2007). Pellets should be of proper quality with minimum amount of fines to gain optimal benefits.  

Layer diets are normally provided in mash form, but consider pellets during summer. Although feed consumption might not be affected by pelleting the ration, egg production, feed efficiency and water intake can be significantly increased in laying hens. Increased water consumption and improved digestibility of the diet is probably responsible for the advantageous effect of pelleting. Broiler chickens, however, prefer to eat more feed with larger particle size in hot environments. When corn is fed as whole grains, broilers consume more protein and have an improved feed efficiency. 

6. Feed and Water Management

The proper allocation of feeders and watering space is critical under conditions of high temperatures. Allocate the correct number of spaces or reduce stocking density if needed. Withholding feed (controlled fasting) during periods of high temperatures and providing feed at times of relatively low ambient temperatures may assist in reducing heat related stress and improve performance.  

Broiler programs 

Temporary feed restriction before heat exposure can enhance thermal resistance of broilers. Feed withdrawal reduces heat production, increment speed of body temperature and mortality of broiler chickens. However, this strategy may also result in reduced growth rate, a longer growing period and a delay in slaughter age.  

The dual feeding program is another strategy used for broilers, which includes a protein diet during the cooler phase and an energy-rich diet during the warmer phase of each day. This maintains a nutritional balance by adequate proportion of the two diets. During a heat stress challenge, dual feeding reduces body temperature and mortality. 

Layer feeding 

In laying hens, partial feed restriction or a controlled feeding regime alleviates the harmful effect of heat stress on laying performance. Changing the feeding time from twice to just once daily is also favorable to the performance of laying hens. The best time to feed the flock would be in the afternoon near sunset. 

7. Feed Additives  

Employ the use of certain feed additives such as betaine, enzymes, antioxidants, mycotoxin binders, phytogenics, and probiotics to alleviate effects of heat stress. Also, during periods of high temperatures, avoid use of coccidiostats such as nicarbazin and monensin that may induce further stress or impact water intake.  

Phytogenic feed additives (PFAs, or botanicals) are able to alleviate the negative consequences of heat stress. A recent study was conducted in Germany with a flock of Lohmann Brown Classic birds that came to lay during the early summer months at a time of high temperatures recorded in the hen houses. The birds were divided into two groups, with the control group being fed with a basal diet, while the treatment group received the same basal diet but with the inclusion of Digestarom® Poultry –a phytogenic feed additive– at an inclusion rate of 150 g per ton of feed. The trial ran from the flock age of 23 to 63 weeks. 

The effects of the phytogenic feed additive during the heat stress period are displayed in Figure 2. The group of hens supplemented with Digestarom® in their diets consistently outperformed the control group throughout the 41 weeks in terms of hen day production, maintaining a steady peak amid the heat stress period, despite recording higher house temperatures, due to less efficient insulation and ventilation in the older building compared to that of the control group.  

Hen day performance and house temperatures
Hen day performance and house temperatures
Source: BIOMIN

The average hen day production of the Digestarom® group was 89.2% compared to 86.9% in the control group, representing a 2.3% increase in laying rate. There was also an improvement of feed conversion of 19 points in the birds fed diets containing the phytogenic feed additive, together with higher egg mass and average egg weights (Figure 3). The return on investment for the egg producer was 1:7. 

Digestarom® positively helped the flock of commercial layers to maintain their peak production throughout a period of heat stress. The birds receiving Digestarom® had better laying performance, better feed efficiency, improved feed conversion ratio, higher profitability, higher return-on-investment and better health status, with no reports of any disease outbreak. 

Due to its scientifically proven mode of action and unique benefits for the birds, Digestarom® successfully helped to combat heat stress, proving once again its potential as a next generation feed additive for innovative poultry nutrition and contributing to increased profits for the producer. 

Trial parameters and results of the heat stress study in commercial layers
Trial parameters and results of the heat stress study in commercial layers

8. Vitamins   

The daily requirements and availability of the vitamins to the bird may be impacted by high temperatures due to reduced feed intake and stress response. Chicks raised under high ambient temperatures show an increased requirement for thiamine (Vitamin B2). Heat stress also affects the adrenal glands and increases cortisol level and demand for Vitamin C. Supplementing ascorbic acid through the feed or water may be beneficial at these times. Additional Vitamin A and Vitamin E is effective in elevating antioxidant status and immune function (Niu et al, 2009). 

9. Cation-Anion Balance  

Due to panting and the excessive loss of carbon dioxide, blood pH and acid-base balance of the bird can be impacted during heat stress. The addition of sodium bicarbonate (0.5%) to diet can enhance performance under heat stress conditions. Along with bicarbonates, electrolytes such as sodium and potassium are excreted in the urine which may affect water balance. Addition of electrolytes (through water) may also be beneficial to maintain osmotic balance.   

10. Trace Minerals  

Under conditions of heat stress, trace minerals may be excreted at higher amounts. The retention rates of manganese, copper, and zinc were found to be lower in chicks reared at high ambient temperatures (Belay and Teeter, 1996), thus, mineral supplementation may reduce the consequences of heat stress and have benefits to bird performance. As a required trace mineral, selenium is also known to be an effective antioxidant.  

Escherichia Coli
Escherichia Coli
Source: CDC/ Janice Haney Carr
Organic selenium to combat heat stress 

During periods of high ambient temperatures there is a release of stress hormones (corticosteroids and catecholamines) which initiates lipid peroxidation of cell membranes and generates free radicals or reactive oxygen species (ROS). The breakdown of various cells and tissues along with increased concentration of ROS may lead to increased intestinal permeability and immunosuppression of poultry.  

This gastrointestinal dysfunction alters the immune barrier and may make birds more susceptible to various pathogenic challenges from Eimeria, Clostridia, E.coli, and Salmonella (Quinterio-Filho, 2010). Supplementation of antioxidants, such as selenium, having antioxidant properties can be beneficial to mitigating this oxidative stress.  

Selenium (Se) is an essential trace element involved in various biochemical pathways and physiological functions. It is an integral component of several seleno-proteins involved in antioxidant activity and its use can be an effective method to combat the various stressors of poultry production, including heat stress.  

Biomin® TorSel™: A Novel Organic Se* - Seleno-homolanthionine (SeHLan)

Organic forms of Se are now typically fed in replacement (or as portion) of sodium selenite. This is often in the form of seleno-methionine (SeMet) that is largely produced by selenized yeasts though in recent years, there has been discovery of a novel organic selenium source in the form of seleno-homolanthionine* (SeHLan), branded as Biomin® TorSel™. There is evidence that this new form or source is more efficient at distributing selenium into animal tissue (Tsuji et al, 2010).  

In studies at the University of Sydney (Selle et al, 2013; Celi et al, 2013), two organic selenium supplements, one from predominantly SeHLan (Biomin® TorSel™) and the other mainly as SeMet, were fed to broiler chickens to compare the effects on performance, Se tissue concentrations, and oxidative status. The trial results illustrated that both SeHLan and SeMet provide significantly higher Se tissue concentrations in breast muscle and liver of birds relative to a negative control (Figure 1).  

As in Figure 2, SeHLan was shown to generate even higher Se concentrations in breast muscle tissue than SeMet (0.283 vs 0.257 mg/kg; P<0.05). Both sources of organic Se increased (P>0.01) plasma concentrations of glutathione peroxidase and superoxide dismutase, key antioxidative enzymes involved in scavenging of reactive oxygen species.  

This enhanced antioxidant enzyme activity indicates that under high ambient temperatures, feeding organic selenium is beneficial to reducing oxidative stress and may assist bird in coping with heat stress. Further studies with heat stressed poultry will be conducted to determine this effect and response.  

Effect of organic selenium supplementation on selenium concentration (mg/kg) in breast muscle tissue
Figure 1. Effect of organic selenium supplementation on selenium concentration (mg/kg) in breast muscle tissue
Source: University of Sydney (Selle et al, 2013; Celi et al, 2013)
Effect of organic selenium supplementation on plasma oxidative status as glutathione peroxidase activity (nmol/min.mL)
Figure 2. Effect of organic selenium supplementation on plasma oxidative status as glutathione peroxidase activity (nmol/min.mL)
Source: University of Sydney (Selle et al, 2013; Celi et al, 2013)


  • Exposure to seasonally high environmental temperatures is a major concern for the poultry industry, particular as the climate becomes more extreme over time. Proper housing and management practices are key to preventing the consequences of heat stress. There are also numerous nutritional strategies that may be employed to assist in reducing the heat generated by the bird or to supply bioactive compounds to restore physiological functions and reduce stress. Antioxidants, such as organic selenium, or phytogenic feed additives can play a role in reducing the effect of heat stress and offer a solution to poultry producers seeking to minimize performance losses during times of high ambient temperatures.