Effect of Heat Stress on Growth Performance and Serum Metabolites in Broilers and the Protective Role of Phytogenic Formulations

Pashu Sandesh, 21 Jan 2026

Dr. Smriti Sandhu, Dr. Sonali Mishra ⃰, Dr. R.D. Patil, Dr. Rishika Vij and Dr. Geetanjali Singh

Department of Veterinary Physiology and Biochemistry, DGCN COVAS, CSKHPKV, Palampur

 Corresponding author: sonalimishra2299@gmail.com

The poultry industry in India is one of the fastest growing sectors of animal agriculture, driven by rising population, increased demand for animal protein, and supportive government initiatives. According to the 20th Livestock Census (2019), India has a poultry population of about 851.81 million birds, of which nearly 480 million are broilers. The country produces approximately 4.5 million tonnes of broiler meat annually, providing livelihood opportunities to millions of small-scale, backyard, and commercial poultry farmers. Despite its rapid growth and economic importance, the poultry sector faces several challenges, including disease outbreaks, climatic variations, fluctuating feed prices, and occasional poor-quality feed.

Heat stress is a major constraint in broiler production, particularly in tropical and subtropical regions like India. Due to their rapid growth, high metabolic rate, and limited thermoregulatory capacity, broiler chickens are highly susceptible to high environmental temperatures. Heat stress reduces feed intake, growth rate, and feed efficiency, suppresses immunity, causes oxidative damage, deteriorates meat quality, and may lead to mortality and economic losses. Phytogenic preparations derived from herbs, spices, essential oils, and plant extracts have emerged as promising natural interventions due to their antioxidant, anti-inflammatory, immunomodulatory, and performance-enhancing properties. Evaluating their effects on growth performance and serum profile of heat-stressed broilers is therefore essential for developing effective heat stress management strategies.

Exposure to high ambient temperature is the major cause of heat stress in poultry, combined with high relative humidity, inadequate ventilation, high stocking density, poor access to drinking water, and transportation stress.Birds do not have sweat glands; they rely mainly on panting and peripheral vasodilation to dissipate excess body heat. But during the later stages of growth, when metabolic rate is high, these thermoregulatory mechanismsbecome inefficient under extreme environmental conditions to dissipate theexcess body heat, resulting in thermal overload and physiological stress to birds.

Physiological, Hormonal, and Metabolic Responses to Heat Stress

Exposure to heat stress causes a number of physiological and endocrine changes in broilers. Birds increase their respiration rate and pant as compensatory mechanisms to dissipate heat, which, often leads to excessive loss of carbon dioxide and respiratory alkalosis. The cloacal temperature of birds increases to> 41.5°C (normal is ~41°C) while circulating thyroid hormones, particularly triiodothyronine and thyroxine, decline, resulting in a reduced metabolic rate. Corticosterone, a stress hormone, and heat stress stimulate the release of corticosterone. This hormone negatively affects the protein synthesis, immune function, and growth of birds. These physiological alterations affect normal metabolic processes and result in reduced production efficiency.

Effect of Heat Stress on Feed Intake and Growth Performance

Reduction in the feed intake is the earliest and most pronounced effect of the heat stress, as birds attempt to minimize metabolic heat production. Decreased feed consumption results in inadequate nutrient intake, reduced body weight gain, poor feed conversion ratio (FCR), and lower carcass yield. Prolonged exposure to elevated temperatures may increase mortality rates, mainly under chronic or severe heat stress conditions. Consequently, overall growth performance of birds and economic returns are significantly reduced.

Effect of Heat Stress on Immune Function

The immune organ index will be expressed relative to body weight (BW) (g of organ weight/kg of BW). Heat stress causes a strong immunosuppressive effect in broiler chickens. Chronic exposure to high temperature leads to reduced development and weight of lymphoid organs such as the bursa of Fabricius, thymus, and spleen. Both humoral and cell-mediated immune responses are impaired due to heat stress. It also results in decreased antibody production and reduced immune cell activity. Due to immune suppression susceptibility to infectious diseases increases and vaccine efficacy also reduced, further aggravating production losses under heat stress conditions. 

Oxidative Stress and Cellular Damage

Generation of reactive oxygen species enhances due to heat stress, leading to oxidative stress and cellular damage. Excessive free radical production results in lipid peroxidation, protein oxidation, and DNA damage, impairing normal cellular functions. The activity of endogenous antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase is reducedsimultaneously. Oxidative stress plays a central role in mediating the negative effects of heat stress on growth performance, immunity, gut integrity, and metabolic stability.

Effect of Heat Stress on Serum Metabolite Status

Heat stress causes severe alterations in serum biochemical and metabolite parameters of broilers, reflecting disturbances in energy, protein, lipid, and electrolyte metabolism. Due to decreases feed intake and alteration in carbohydrate metabolism, serum glucose level reduced. The level of serum AST and ALT generally increase with muscle or liver damage. Total protein and albumin concentrations decreased in heat stressed broilers, indicating impaired hepatic protein synthesis and increased protein catabolism. There are also alterations in serum cholesterol and triglyceride levels which reflect disruptions in lipid metabolism. Electrolyte imbalance and elevated corticosterone in heat stressed broilers further indicate physiological stress. Over all these changes serve as important indicators of metabolic dysfunction under heat stress.

Effect on Gut Health and Meat Quality

The gastrointestinal tract is highly sensitive to heat stress. High temperature affects intestinal morphology by reducing villus height and increasing intestinal permeability, as heat stress causes villus atrophy, epithelial erosion, and reduced absorptive surface leading to impaired nutrient absorption and altered gut microbiota. Due to these changes, growth performance and immune competencenegatively affected. Heat stress also cause severe affects the meat quality by reducing breast muscle yield and altering post-mortem muscle metabolism, resulting in pale, soft, and exudative meat. All these changes results in reduce processing quality of meat, consumer acceptance, and market value of broiler meat.

Effect on hematological parameters

Heat stress induces significant hematological alterations in broiler chickens due to reduced feed intake, haemodilution, hormonal imbalance, and oxidative stress. Prolonged exposure to high temperature results in decreased haemoglobin concentration, packed cell volume (PCV), and total erythrocyte count (TEC), reflecting suppressed erythropoiesis and increased erythrocyte destruction. Erythrocyte indices are also affected, with mean corpuscular volume (MCV) tending to increase or remain normal, while mean corpuscular haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC) usually decrease, indicating normocytic to macrocytic, hypochromic anemia. Heat stress further causes a reduction in total leukocyte count (TLC), mainly due to corticosterone-induced lymphopenia, leading to immunosuppression and increased disease susceptibility in affected birds.

Efficacy of Phytogenic Feed Additives in Heat-Stressed Broilers

Phytogenic feed additives are plant-derived products such as herbs, spices, essential oils, and plant extracts containing biologically active compounds. These compounds possess antioxidant, anti-inflammatory, antimicrobial, and digestive stimulant properties. Numerous studies have demonstrated the beneficial effects of phytogenic feed additives on growth performance of heat-stressed broilers. Supplementation with phytogenic formulations has been shown to improve feed intake, body weight gain, and feed conversion ratio under high temperature conditions. These improvements are attributed to positively influence serum metabolite profiles in heat-stressed broilers. Improved serum glucose, total protein, and albumin levels indicate better nutrient utilization and metabolic stability. enhanced nutrient digestibility, improved gut integrity, reduced oxidative damage, and modulation of stress-related hormonal responses. Their antioxidant activity supports liver function and reduces oxidative damage, thereby maintaining normal serum biochemical parameters and improving overall metabolic homeostasis.

Conclusion

Heat stress severely affects growth performance, immune competence, gut health, meat quality, and serum metabolite status of broiler chickens, leading to significant economic losses in poultry production. Alterations in physiological, hormonal, and metabolic processes under heat stress compromise productivity and health of birds. Phytogenic feed additives represent a promising natural nutritional strategy to mitigate the adverse effects of heat stress. By improving growth performance, stabilizing serum metabolite profiles, enhancing antioxidant defence, and supporting immune function, phytogenic formulations offer an effective approach for improving productivity and sustainability of broiler production under hot climatic conditions. Further research is required to optimize formulation, dosage, and combinations of phytogenic compounds for maximum efficacy.