Introduction

Heat stress (HS) is a major challenge in poultry production, occurring when birds cannot balance body heat production and loss. High ambient temperature is a significant factor, with poultry experiencing heat stress above 25°C (77°F) (Wasti et al., 2020). Broilers and laying hens have optimal performance temperatures of 18-22°C (64.4-71.6°F) and 19-22°C (66.2-71.6°F), respectively (Gouda et al., 2024). The modern breeds of broilers have been bred for high feed conversion efficiency, accelerated muscle growth, and high rate of mitochondrial metabolism. Still, they possess a limited capacity for heat tolerance (Uyanga et al., 2022). Similarly, laying hens produce high metabolic heat with their increased rate of egg formation, increasing their vulnerability to HS. Heat stress has far-reaching implications for poultry (Uyanga et al., 2022; Apalowo et al., 2024). Therefore, effective heat stress management is crucial to minimize economic losses and ensure animal well-being (Goel, 2021). By understanding how heat stress affects poultry, farmers and researchers can develop and implement targeted mitigation strategies (Apalowo et al., 2024).

This article will look into how heat stress affects the overall production, health, and welfare of broiler and layer chicken. The sole focus of breeding meat and egg chickens puts much pressure on their bodies, especially when accompanied by unfavorable environments where the birds are raised. This leads to poor performance and losses to the farmer. Therefore, the article will also have a look at different strategies that can help to alleviate heat stress in broilers and layers.

Effect of heat stress in broilers

Heat stress (HS) poses a significant threat to broiler chicken production, negatively affecting growth performance, immunity, carcass and meat quality, and overall welfare.

Effect of heat stress on growth performance

Feed Intake (FI): High temperatures lead to a reduction in feed consumption (Goel, 2021; Apalowo et al., 2024). Broilers tend to spend less time feeding when under heat stress (Jadhao et al., 2020).
Weight Gain (WG): Studies have reported significant reductions in body weight gain (BWG) in broilers exposed to high temperatures (Goel, 2021).
Feed Conversion Ratio (FCR): Heat stress typically increases the feed conversion ratio, indicating reduced efficiency in converting feed into body mass (Goel, 2021). Genetic selection for growth has made broilers more susceptible to heat stress, as their thermoregulatory systems haven't kept pace with their rapid muscle growth (Zaboli et al., 2019; Saracila et al., 2021). Broilers exposed to 34°C (93.2°F) for 6 hours daily showed an 8–9% reduction in FI, a 17% reduction in BWG, and a 9–10% increase in FCR. Reduced FI during HS could be attributed to a reduction in intestinal weight, length, and modified villus morphology in chickens (Goel, 2021).

Effect on immunity

Gut Health: HS compromises the integrity of the intestinal barrier, leading to dysfunction and inflammation (Cornescu et al., 2023). The activities of commensal intestinal bacterial populations also have a notable impact on the physiological and pathological conditions of the host (Awad et al., 2020). Heat stress impairs performance and induces intestinal inflammation in broiler chickens infected with Salmonella Enteritidis (Wasti et al., 2020; Apalowo et al., 2024).
Overall Immunity: HS causes immunosuppression, reducing the relative weights of lymphoid organs and total circulating antibodies (Awad et al., 2020). The production of excessive reactive oxygen species (ROS) due to HS leads to oxidative stress (Goel, 2021). Additionally, heat stress decreases expression of the cytokines, avian beta-defensins 4 and 6, and Toll-like receptor 2 in broiler chickens infected with Salmonella Enteritidis (Goel et al., 2025).

Carcass/Meat quality

Pre-slaughter stress reactions can lead to an increase in the speed and degree of glycogen breakdown, a drop in pH, and drip loss which leads to a low-quality carcass (Saracila et al., 2021).
Broilers experiencing acute or short-term heat stress just before slaughter, displayed pale, soft, and exudative meat changes in the quality of their meat. Heat stress can promote pale-soft-exudative meat in turkeys and heat shortening in broilers (Awad et al., 2020).
Chronic heat stress could adversely affect the meat quality by changing the aerobic metabolism, glycolysis, and intramuscular fat deposition, resulting in low customer acceptability (Zaboli et al., 2019; Awad et al., 2020).

Welfare

Heat stress significantly impacts the welfare of broiler chickens:

Broilers spend less time feeding and more time drinking and panting. The thermal imbalance caused by environmental factors and the birds' internal characteristics can be fatal if internal heat production exceeds heat loss (Jadhao et al., 2020; Gouda et al., 2024).
HS leads to conditions such as oxidative stress, acid-base imbalance, and immunosuppression, which further exacerbate poor performance and increase mortality (Wasti et al., 2020; Nawaz et al., 2021; Wang et al., 2023).

Effect of heat stress in layers

Feed intake, feed conversion efficiency, and egg production

Heat stress substantially reduces both feed intake and egg production in laying hens (Jadhao et al., 2020; Goel et al., 2025). As temperatures rise above 30°C, hens exhibit signs of heat stress, leading to a decline in feed consumption and, consequently, egg output (Wang et al., 2023). Chronic heat stress over a 5-week period has been shown to significantly diminish egg production. Reduced feed intake impairs egg production, which requires an adequate nutrient supply (Apalowo et al., 2024). High temperatures stimulate appetite-related hormones, potentially resulting in anorexia. A study by Goel, 2021 indicates that laying hens reared at 35°C for 6.5 hours daily experience decreased feed intake, directly impacting their ability to sustain egg production (Goel, 2021). Furthermore, heat exposure can affect gene expression of feed intake regulatory peptides in laying hens (Cornescu et al., 2023). A 5% reduction in feed consumption can be expected for every 1°C rise in temperature between 32-38°C (Jadhao et al., 2020). Heat stress increases the Feed Conversion Ratio (FCR), meaning the hens become less efficient at converting feed into eggs (Goel, 2021).

Egg quality, size, shell thickness, and nutrition

Egg Size and Weight: Elevated temperatures often lead to a reduction in egg weight (Wang et al., 2023).
Shell Thickness: Heat stress can decrease eggshell thickness, increasing the risk of damage. High ambient temperatures influence the eggshell quality and calbindin-D28k localization of the eggshell gland and intestinal segments of laying hens (Ahmad et al., 2022; Wang et al., 2023).
Shell Weight: Acute heat stress can decrease shell weight and increase FCR (Goel et al., 2025).
Nutritional Content: Extreme heat stress can reduce serum immunoglobulin (Ig) IgG and IgM levels, impacting the overall nutritional value of the eggs (Awad et al., 2020). Heat stress impairs the nutritional metabolism and reduces the productivity of egg-laying ducks (Ahmad et al., 2022).

Gut health and overall immunity

Gut Health: HS can disrupt the intestinal barrier, leading to inflammation and reduced nutrient absorption (Shakeri et al., 2020). Heat stress impairs performance and induces intestinal inflammation in chickens. (Olfati et al., 2018; Wasti et al., 2020; Apalowo et al., 2024)
Overall Immunity: Heat stress can suppress the immune system, making hens more susceptible to diseases. Immunosuppression has been observed in layers exposed to heat stress. Reduced lymphoid organ weight and lower levels of circulating antibodies, including specific IgM and IgY, have been detected in heat-stressed birds (Wang et al., 2023). Heat stress decreases expression of the cytokines, avian beta-defensins 4 and 6, and Toll-like receptor 2 in chickens. (Goel et al., 2025).

Welfare

Behavioral Changes: Hens under heat stress may exhibit panting, reduced activity, and altered feeding patterns. They spend less time feeding and more time drinking (Jadhao et al., 2020).
Physiological Stress: High temperatures can cause physiological stress, leading to increased corticosterone levels and oxidative stress. The generation of oxidative stress due to the production of excessive reactive oxygen species (ROS) is another problem that arises due to HS (Goel, 2021; Wang et al., 2023).
Mortality: In severe cases, heat stress can result in increased mortality rates, particularly if hens cannot dissipate heat effectively. Heat stress can be fatal if internal heat production exceeds heat loss (Jadhao et al., 2020; Gouda et al., 2024).

Mitigation strategies for heat stress

Mitigating heat stress in poultry, encompassing both layers and broilers demands a comprehensive strategy integrating refined management practices, nutritional adjustments, strategic use of additives, and genetic selection. Let us look into some of these solutions:

Routine management practices

Ventilation and Housing Adjustments (Apalowo et al., 2024):

Optimising ventilation is paramount to reduce heat and humidity levels within poultry houses. Proper management of poultry housing is critical in reducing heat stress, a possible stressor impacting productivity.
Implementing cooling systems, such as cool cell pads, sprinklers, and tunnel ventilation, can effectively lower ambient temperatures. Achieving effective heat stress management in poultry production necessitates a meticulous examination of the diverse heat generation sources present in the broiler house, as well as the development of solutions to alleviate the detrimental impacts these sources have on the birds.
Employing light-coloured roofing materials helps reflect solar radiation and reduce heat absorption. Dark-colored surfaces absorb a greater amount of heat from direct sun radiation than light-colored surfaces.

Water Management (Apalowo et al., 2024):

Ensuring unrestricted access to cool, clean water supports thermoregulation. Broilers exhibit heightened water usage to adapt to elevated temperatures.
Providing chilled drinking water can offer additional relief from heat stress.

Stocking Density Reduction (Jadhao et al., 2020):

Lowering stocking densities minimizes heat generation within poultry houses. Insufficient stocking density and ventilation can worsen heat stress.
Consideration should be given to density allowances for broilers.

Feed Manipulation and Feeding Regime Adjustments

Adjusting Feeding Schedules (Wasti et al., 2020):

Restricting feed during peak temperature periods reduces metabolic heat production, commonly applied in poultry production. Restricting the feed during the hotter period of the day has been a common practice in poultry production.

Dietary Modifications:

Increasing dietary fat content can reduce heat increment. The influence of dietary energy and poultry fat on the response of broiler chicks to heat stress can be considered (Nawaz et al., 2021).
Concentrating energy, protein, amino acids, electrolytes, and vitamins A, E, and C in feed formulations can mitigate the detrimental effects of heat stress (Jadhao et al., 2020).
Supplementing diets with antioxidants (e.g., vitamins E and selenium) helps combat oxidative stress. Alleviating the environmental heat burden on laying hens by feeding on diets enriched with certain antioxidants (vitamin E and selenium) individually or combined can be done (Abbas et al., 2022; Ahmad et al., 2022; Hu et al., 2024).
Balancing amino acid profiles ensures optimal nutrient utilization and reduces metabolic heat production (Jadhao et al., 2020; Apalowo et al., 2024).

Probiotics and Organic Additives

Probiotic Supplementation (Jiang et al., 2020, 2021; Nawaz et al., 2021; Ahmad et al., 2022):

Adding probiotics to feed enhances gut health and immune function. Dietary supplementation of a mixture of Lactobacillus strains enhances the performance of broiler chickens raised under heat-stress conditions.
Bacillus subtilis supplemented in diets of heat-stressed broilers, improved their performance, intestinal morphology, and microflora composition.

Herbal and Organic Additives (Apalowo et al., 2024; Fayed & Bawish, 2024):

Incorporating herbs with thermoregulatory properties can help maintain body temperature. Herbs as thermoregulatory agents in poultry can be considered.
Dietary supplementation with phytogenic feed additives containing Terminalia bellirica and Andrographis paniculata showed positive results in mitigating heat stress-induced alterations in broiler chickens.

Genetic Adjustments

Selective Breeding (Jadhao et al., 2020; Shakeri et al., 2020; Goel et al., 2025):

Breeding programs should focus on selecting heat-resistant breeds and strains. Selection for breeding heat-resistant birds also provides merits for improving the germplasm of the strains.
Genetic variations alter physiological responses following heat stress in strains of laying hens.

Utilising Specific Genes (Wasti et al., 2020):

Employing Naked Neck (Na) and Frizzle (F) genes can enhance heat tolerance. The potential use of Na and F genes, along with proper nutrition, housing, and management should be beneficial in mitigating heat stress.

Additional Solutions

Early Age Thermal Conditioning (Perini et al., 2021; Goel et al., 2025):

Exposing chicks to early-age thermal conditioning can improve their resilience to heat stress later in life. Early-age thermal conditioning also helps in developing resistance for HS.

Active Substance Supplementation During Incubation; (Goel, 2021)

Recent advancements include supplementing active substances during incubation, potentially enhancing thermotolerance in newly hatched chicks. The most recent advancement is the supplementation of active substances during incubation.

Conclusion

Effectively combating heat stress in broilers and layers requires a holistic approach that integrates improved management, strategic nutritional adjustments, and genetic solutions. While current strategies offer substantial relief, there is a need for further research to optimize the synergistic effects of different interventions. Key areas for future investigation include understanding the molecular mechanisms of heat shock proteins, optimizing the use of probiotics and organic additives to modulate gut microbiota, and refining genetic selection for enhanced thermotolerance. Furthermore, research should focus on developing cost-effective, sustainable solutions applicable across diverse farming conditions to ensure both poultry welfare and economic viability.