Heat stress has grown to be a significant concern in today’s livestock production systems, particularly in tropical and subtropical nations where higher temperatures and humidity have an effect on cattle productivity and well-being. When a cow’s heat load exceeds its capacity to disperse heat, thermoregulatory processes and the animal’s homeostatic balance are disrupted. This condition is known as heat stress. Dairy cattle generally maintain thermal equilibrium within a thermoneutral zone of approximately -0.5-20°C, with an upper critical temperature of about 25-26°C beyond which physiological and behavioural heat-dissipation responses are triggered; heat stress is commonly considered to begin once the temperature-humidity index (THI) exceeds a threshold of 72 (
West, 2003;
Herbut et al., 2018). It has a detrimental influence on the immune system, reproductive capacity, feed intake, productivity and efficiency, as well as a significant financial impact on the dairy and meat industries (
Baumgard and Rhoads, 2013). The heat stress causes a number of physiological reactions that disrupt regular bodily functions, such as an increase in breathing rate, a redistribution of blood flow to reduce body heat retention and a change in endocrine balance. At the cellular level, hyperthermic events encourage the production of ROS, which lead to oxidative stress, cell damage and aberrant cell/tissue functions.
Due to these intricate consequences, realistic and sustainable mitigation strategies for livestock under field conditions are required (
Das, 2018). Because dietary intervention programs directly influence cattle’s metabolic and physiological resilience to thermal load, their role in mitigating heat-stress-related production losses has garnered significant attention. Antioxidants, electrolytes and osmoprotectants have shown promise in strengthening an animal’s resistance to heat stress by preserving cell integrity, preserving fluid and electrolyte balance and stabilising the animal’s circulatory capacities
(Bernabucci et al., 2010). In addition to discussing the efficacy of particular nutritional interventions to reduce heat stress and, when feasible, detrimental effects on the cattle system and/or their performance, this review aims to compile recent data regarding the mechanisms underlying the development of heat stress in cattle.
Beyond its physiological toll, heat stress imposes a substantial economic burden on the dairy sector, a concern that is especially acute in tropical and sub-tropical developing economies such as India, home to the world’s largest dairy herd. Projections for the hot, humid Trans and Upper Gangetic Plains of northern India-a belt that accounts for a large share of national milk output-estimate an annual milk-production loss of around 361-377 thousand tonnes due to heat stress, equivalent to an economic loss of approximately INR 11.93-12.44 billion (
Choudhary and Sirohi, 2022). Because this region experiences a high temperature-humidity index (THI) for much of the year and is dominated by smallholder producers with limited access to structural cooling, low-cost nutritional mitigation strategies of the kind discussed in this review represent a scalable adaptation option for protecting milk yield, animal health and farmer livelihoods in the Indian context.
Heat stress and thermoregulatory physiology in cattle
Heat stress happens when the cow is unable to dissipate the heat produced by creating metabolic heat due to an environment that is too demanding, particularly the ambient temperature, humidity, sun radiation and lack of air circulation. As homoeothermic animals, cattle regulate heat production and loss to keep their core body temperature within a certain range. We maintain equilibrium between sensible heat loss (by conduction, convection and radiation) and latent heat loss (by evaporation through sweating and respiration) while we are under thermoneutral settings. However, sensible heat loss becomes ineffective when the ambient temperature approaches or surpasses body temperature and evaporative cooling takes over. In order to increase heat loss under these situations, cattle will attempt to raise their rate of respiration (panting) and sweating, although doing so will result in higher energy expenditure and a disruption of the acid-base balance (
West, 2003).
The hypothalamus, which integrates temperature information and triggers physiological and behavioural responses such as decreased feed intake, water intake, thermo-neutral shade seeking and decreased bodily activity, regulates thermoregulation in cattle. In addition to being an adaptive adjustment to lower metabolic heat output, decreasing feed intake also leads to decreased nutritional availability and decreased productivity. Additionally, peripheral vasodilation improves skin circulation and encourages heat dissipation, which can impair normal blood flow to vital organs and interfere with the flow of nutrients to tissues. Prolonged heat stress, which includes elevated body temperature, dehydration and imbalanced hormones including cortisol, thyroid hormones and insulin, causes chronic physiological strain. Thermoregulatory effectiveness is influenced by breed variances, coat type and acclimatisation capacity; native breeds are often more heat tolerant than high-producing exotic breeds
(Belhadj et al., 2016; Herbut et al., 2018).
Pathophysiology of heat stress-induced systemic dysfunction
Heat stress and its associated bovine lesions often result in various organ system dysfunctions that impact the animal’s overall physiology and reduce its physiological stability. The animal’s stimulus response mechanisms become maladaptive if the temperature exposure lasts for a long time. This includes increased respiration and perspiration as well as peripheral vasodilation, which eventually causes systemic malfunction. One of the primary consequences of excessive fluid loss is dehydration, which lowers plasma volume and compromises circulatory efficiency (
Collier and Gebremedhin, 2015). As a result, the tissues receive insufficient oxygen and nutrients, which impairs their ability to control body temperature and raises their stress levels. The internal organs, particularly the gastrointestinal system, receive reduced blood flow when blood flow is redistributed to the skin to facilitate heat dispersion. Hypoxia, decreased gut barrier function and increased gut permeability, also referred to as “leaky gut”, may arise from this, which may allow endotoxins to enter the bloodstream and trigger an inflammatory reaction
(Belhadj et al., 2016).
Heat stress stimulates the production of reactive oxygen species, primarily through increased electron leakage at complexes I and III of the mitochondrial electron transport chain, which overwhelms the cell’s enzymatic (superoxide dismutase, catalase, glutathione peroxidase) and non-enzymatic antioxidant defences
(Belhadj et al., 2016). At the cellular level, it destroys DNA, proteins and lipids at various levels. This oxidative imbalance impairs cell processes and encourages tissue deterioration. Moreover, heat stress has an impact on the endocrine system in a specific way, affecting the cortisol and thyroid activity that, in turn, influences the immunological and metabolic rates
(Nardone et al., 2010). The immune system, being compromised in heat stress, becomes less active, leading to less “cytokine” production and increased risk of infection and illness in the animals. Heat stress alters whole-body metabolism by increasing maintenance energy requirements while reducing nutrient retention and utilisation efficiency, so that a larger share of dietary energy is diverted toward maintaining homeostasis rather than growth, reproduction and milk synthesis, often resulting in a negative energy balance.
Oxidative stress and antioxidant defence mechanisms
Oxidative stress is one of the most important clinical signs of heat stress in cattle and arises from the increase in the generation of reactive oxygen species (ROS) over the animal’s capacity to neutralise them. ROS are continuously generated during cell metabolism, particularly in mitochondria and regulated by the antioxidant system of the body. High temperatures, on the other hand, increase metabolism and the activity of mitochondria, causing excessive generation of Reactive Oxygen Species (ROS) like superoxide radicals, hydrogen peroxide and hydroxyl ion. The extra production of ROS leads to oxidative damage to cellular components such as membrane peroxidation, protein oxidation and damage to DNA, all of which negatively affect the integrity and function of the cells
(Rhoads et al., 2009).
Cattle’s antioxidant defence is highly evolved such that both enzymatic and non-enzymatic antioxidants can be found and serve to block and neutralise these harmful effects. It is recognised that the enzymatic antioxidants, such as superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx), work together to transform harmful ROS into less harmful forms, which are either oxygen or water. Vitamin E, vitamin C, glutathione, carotenoids and micronutrients (such as zinc and selenium) play an important role in scavenging free radicals and maintaining the redox balance
(Xiao et al., 2021). When the body requires higher amounts of antioxidants during stressful heat, however, it may not be able to produce enough of them, leading to some imbalance in oxidative stress and/or dysfunction of antioxidants.
In addition, oxidative stress is intricately associated with immune suppression and inflammation, as ROS could harm immune cells and trigger pro-inflammatory signals. This leaves the coping mechanism of the animal weak and creates more tissue damage. Thus, an optimal antioxidant defence system (
Sordillo and Aitken, 2009), a healthy immune system, cell homeostasis and the minimisation of the negative effects of heat are dependent on each other. The processes may provide a scientific foundation for a feeding program that will increase heat-stressed cattle’s antioxidant capabilities.
Dietary antioxidants and their role in heat stress mitigation
By boosting the cells’ capacity to neutralise ROSs and maintain cell integrity, antioxidant qualities of the food are essential for enhancing the body’s capacity to withstand the detrimental effects of heat stress in cattle. Sometimes endogenous antioxidant defences are insufficient to handle the high oxidative load during temperature stress, necessitating dietary supplementation. Supplementing with vital antioxidants such as zinc, selenium, vitamin C (ascorbic acid), vitamin E (alpha-tocopherol) and other plant-based polyphenols is protective. As an antioxidant, lipid-soluble vitamin E prevents lipid peroxidation, preserving the structure and functionality of cell membranes. In addition to producing vitamin E active forms in concentrations in the cytosol and extracellular fluids, where vitamin E is stored in its less active form, vitamin C is a water-soluble antioxidant that works in concert with vitamin E by directly neutralising free radicals
(Chauhan et al., 2014).
Zinc is also involved in synthesising antioxidant enzymes (superoxide dismutase) and selenium is involved in glutathione peroxidase, which are very important enzymatic defence systems. Other phytogenic ingredients such as flavonoids and tannins from plants are also attracting attention, owing to their strong antioxidant and anti-inflammatory activities. The substances exhibit benefits for gut health, reduce oxidative stress and regulate immune function, all of which are often compromised in heat-stressed situations. The addition of dietary antioxidants has been demonstrated to positively work on intake, milk yield and production composition, as well as reproductive rate in heat-stressed cattle. Besides, antioxidants are helpful in lowering the chance of infections and metabolic ailments and counteracting hormonal imbalances caused by stress.
Role of mineral antioxidants
Trace minerals act as essential cofactors of the enzymatic antioxidant system rather than as free-radical scavengers themselves. Selenium is incorporated as selenocysteine into the catalytic centre of glutathione peroxidase, enabling the reduction of hydrogen peroxide and lipid hydroperoxides, while zinc, copper and manganese are structural cofactors of superoxide dismutase isoforms that convert superoxide radicals to hydrogen peroxide. Supplementing heat-stressed cattle with selenium, zinc, copper and manganese, particularly in combination with vitamin E, has been shown to raise glutathione peroxidase and superoxide dismutase activity, lower circulating markers of lipid peroxidation and improve immune function and reproductive performance
(Xiao et al., 2021; Sordillo, 2013).
Natural versus synthetic antioxidants
Dietary antioxidants used in heat-stress mitigation broadly fall into two classes. Synthetic antioxidants, such as DL-alpha-tocopheryl acetate (a synthetic form of vitamin E) and sodium selenite, are chemically defined, offer predictable dosing and remain the mainstay of supplementation programmes. Natural, plant-derived antioxidants, including polyphenols, flavonoids and tannins from sources such as green tea, grape pomace and other phytogenic extracts, act both directly, by donating hydrogen atoms to neutralise free radicals and indirectly, by upregulating endogenous antioxidant enzyme expression and modulating the rumen microbiome. Although natural antioxidants are generally regarded as safer and more consumer-acceptable, their bioavailability and potency are more variable than those of synthetic forms and current evidence suggests that combining the two approaches, rather than relying on either alone, gives the most consistent protection under heat stress
(Chauhan et al., 2014; Xiao et al., 2021). Comparable benefits of natural phytogenic and plant-derived supplementation under heat stress have been reported across species, including improved physiological and skin-temperature responses in indigenous and crossbred cattle supplemented with Chlorophytum borivilianum
(Devi et al., 2024), mitigation of heat-induced stress with phytogenic plant extracts in broilers
(Merchant et al., 2025) and improved performance following supplementation with rough lemon juice, amla juice and aloe vera gel in heat-stressed broiler chickens
(Gowri et al., 2022).
Electrolyte supplementation for maintaining fluid and circulatory balance
Nutrition for maintaining adequate fluid balance and circulatory stability plays an important role in managing cattle during heat stress and electrolyte supplementation is an important nutritional management technique. Several key electrolytes-sodium (Na
+), potassium (K
+), chloride (Cl
-) and bicarbonate (HCO
3-) ions-are lost more rapidly as ambient temperature rises, because elevated sweat output and increased respiration rate accompany higher temperatures. These losses alter the normal functioning of physiological processes, especially associated with the activity of the circulatory and neurological systems and with muscle function and alter the osmotic balance and plasma volume. As dehydration progresses, the heart workload increases and the viscosity of blood slowly decreases and tissue perfusion and oxygen supply decrease. In order to preserve homeostasis and promote the preservation of vital activities, it is crucial to restore electrolytes on time (
Collier and Gebremedhin, 2015).
The carbon dioxide loss from heavy panting that results in respiratory alkalosis is linked to the significance of electrolytes in the control of acid-base balance during HSS. Bicarbonate and other buffering agents help to stabilise blood pH and prevent metabolic disruptions that would otherwise hinder the activity of enzymes and the metabolism of nutrients
(Hall et al., 2016). Chloride affects osmotic pressure and stomach function, whereas sodium and potassium are particularly important for maintaining fluid quantity and quality in the extracellular and intracellular compartments, respectively. Since electrolyte imbalances may adversely impact microbial activity and the rumen fermentation process, which may lead to a decrease in feed efficiency and nutrient absorption, maintaining a healthy electrolyte balance is also crucial for the health of the rumen
(Shakeri et al., 2020).
By boosting palatability and arousing thirst, among other physiological actions that help avoid dehydration, the addition of electrolytes also encourages the consumption of water. Maintains proper blood volume and blood pressure needed for blood circulation under heat stress, which also helps to maintain appropriate heart function. Practical suggestions are to provide a blend of electrolytes in drinking water and/or feed and, in some cases, some form of carbohydrate such as glucose or an energy source.
Osmoprotectants and cellular homeostasis during heat stress
The role of osmoprotectants or appropriate osmoprotective solutes in maintaining cellular homeostasis for protecting intracellular osmotic balance and cell structure in cattle during heat stress has already been demonstrated. The loss of water through breathing and perspiration causes heat stress, dehydration, too-high osmotic pressure and cell shrinkage. The effect of these is on the integrity of the membranes, the functions of the enzymes and the structure of proteins that lead to reduced cell utilisation
(Ratriyanto et al., 2019). Osmoprotectants are small molecules like betaine, taurine, proline and glycerol which impose their osmolarity on the cell without disrupting normal function. Their action is through maintaining the volume, stability of osmolytes and denaturation of macromolecules under stress
(Shakeri et al., 2020).
Trimethylglycine, commonly referred to as betaine, is one of the best studied osmotica in animal feed. It is a methyl donor that helps maintain cell water balance by preventing loss of water and conserving energy for osmotic regulation
(Ratriyanto et al., 2019). Besides, betaine increases cellular heat stress resistance and protects protein and lipid membranes. Moreover, taurine and proline play a role in antioxidant defense and membrane stabilization thus decreasing the chances of oxidant damage. These substances will maintain gasto-intestinal integrity and will prevent hyperpermeability and hypoabsorption of the stomach during heat stress. Furthermore, osmoprotectants can decrease the maintenance energy demand and enhance metabolism, decreasing the physiological burden associated with heat stress.
Feeding with osmotic protectants has been linked to better feed efficiency, better hydration state, higher milk production and reduction in the levels of stress markers. Their importance is enhanced in combination with electrolytes, antioxidants and a unique nutritional approach is developed for counteracting heat stress. Osmoprotectantsare generally considered as a cellular defence mechanism towards heat stress, which helps sustaining systemic adaptability and stability of physiological functions and production of the organism. The dose-dependent improvements in rectal temperature, respiration rate, water intake, plasma osmolality, milk yield, feed intake and cortisol level with increasing betaine supplementation are summarised in Table 1.
Role of dietary fibre, fat and protein in heat production and thermoregulation
Beyond antioxidants, electrolytes and osmoprotectants, the macronutrient composition of the basal diet is itself a major determinant of the metabolic heat load that cattle must dissipate, a dimension that has received comparatively little attention in mitigation-focused reviews. Fibrous carbohydrates (neutral and acid detergent fibre) are fermented by rumen microbes to volatile fatty acids, a process that generates a substantially higher heat increment of feeding than the enzymatic digestion of starch, fat, or protein
(Kadzere et al., 2002; NRC, 2001). Because high-fibre, low-digestibility forages therefore contribute disproportionately to internal heat production, moderately reducing dietary fibre content and replacing a portion of it with more rapidly and efficiently digested, energy-dense ingredients during periods of heat stress lowers the fermentative heat load without compromising rumen health, provided adequate effective fibre is retained to sustain normal rumination and buffering.
Dietary fat is the macronutrient with the lowest heat increment per unit of metabolisable energy supplied, since its digestion and absorption largely bypass the extensive microbial fermentation associated with fibre and, to a lesser extent, protein. Partially substituting rumen-fermentable carbohydrate with rumen-inert or rumen-protected fat sources therefore allows energy density to be maintained, or even increased, while reducing metabolic heat burden and has been associated with improved dry matter intake and milk yield in heat-stressed cows (
NRC, 2001;
Kadzere et al., 2002).
Protein metabolism also contributes materially to heat production, particularly when rumen-degradable protein (RDP) is supplied in excess of microbial requirements: the resulting surplus ammonia must be converted to urea in the liver, a metabolically costly and heat-generating pathway. Formulating diets that closely match RDP supply to microbial demand, while meeting the animal’s amino acid requirements through appropriately balanced rumen-undegradable protein (RUP), reduces this avoidable component of heat increment. Collectively, moderating dietary fibre, favouring fat over fermentable carbohydrate as an energy source and tightening RDP:RUP balance constitute a practical, low-cost nutritional lever for reducing metabolic heat production that complements the antioxidant, electrolyte and osmoprotectant strategies discussed above
(Kadzere et al., 2002).
Nutritional modulation of circulatory and metabolic adaptations
The nutritional modulation has an important role in stressing energy balance, blood flow and cellular metabolism in heat-stressed cattle, which is a key factor in stabilizing blood flow and metabolic rates (
Collier and Gebremedhin, 2015;
Belhadj Slimen et al., 2016). The transport of nutrients to the internal organs and metabolic efficiency may be affected by circulatory changes brought on by heat stress, such as peripheral vasodilation and blood flow to internal organs. These benefits, such as the preservation of blood volume, enhanced vascular function and the appropriate transfer of oxygen and nutrients to tissues in the blood, can be attained by strategic dietary recommendations
(Marai et al., 2007). Effective supplements that can aid in circulation and thermoregulation include electrolytes and certain nutrients, such as arginine, which enhances vasodilation through nitric oxide generation.
Cattle under heat stress may physiologically consume less and produce more energy for maintenance rather than development and lactation. The goal of nutrition is to maximise nutritional availability and energy density while minimising metabolic increase. Bypass proteins, rumen-protected lipids and high digestible feeds all contribute to maintaining energy balances in the rumen and lowering feed heat increase. Additionally, micronutrients such as niacin and chromium have a critical role in controlling blood glucose levels and improving insulin awareness, which facilitates more efficient use of energy under stress. Additionally, antioxidants aid in preserving the oxidative stability of your metabolism, shielding your metabolic cells and enzymes from harm
(Sejian et al., 2018).
Additionally, insulin, cortisol and thyroid hormonesall of which are blocked during heat stressare among the hormones that dietary modulation helps maintain in balance. In order to maintain metabolic balance and increase physiological resilience, dietary techniques aid in moderating these hormonal reactions. Together, these tactics strengthen animals’ resistance to heat stress and preserve ideal circulation and metabolism, which improves the health and production of heat-stressed cattle cows.
Integrated nutritional strategies and emerging feed technologies
In order to provide complete mitigation of heat stress at the cellular, metabolic and systemic levels, integrated dietary solutions involve providing antioxidants, electrolytes and osmoprotectants. The recommended premise of current feeding programs is balanced supplemental feeding, which enhances antioxidant defence and guarantees fluid balance and electrolyte conditions, rather than relying on a single intervention (
NRC, 2001). When combined with electrolytes and osmoprotectants like betaine, vitamin E, selenium and zinc supplements have been shown to improve thermotolerance, immunological response and productive performance more effectively than when taken separately. Additionally, the best nutritional practices related to particular physiological stages, environmental conditions and breed needs are increasingly important. The comparative effects of antioxidant-only, electrolyte-only and integrated supplementation on rectal temperature, respiration rate, milk yield, dry matter intake, feed efficiency and plasma cortisol are summarised in Table 2.
Through nutrition delivery and bioavailability, new feed technologies are improving these tactics even further. The increasing use of minerals protected by ruminen, nano-mineral formulations, encapsulated antioxidants and phytogenic feed additives improves the stability and targeted release in the digestive tract
(Dunshea et al., 2013). These are achieved through using technology such as smart feeding systems, which can be adjusted in real time using heat load to determine the feeding systems required and to formulate the ration for the animals. In addition, gut health and overall resilience can be aided with probiotics and plant bioactives in functional feeds during stress
(Belhadj et al., 2016). They all play a role in sustainable livestock production
i.e. improved comfort of animals in hot areas, better assessment of heat stress losses and cost-effective gear.
Discussion and directions for future research
Taken together, the evidence reviewed above indicates that dietary antioxidants, electrolytes, osmoprotectants and macronutrient balance act through distinct but complementary physiological routes-limiting oxidative damage, sustaining fluid and acid-base balance, protecting cellular osmotic integrity and reducing avoidable metabolic heat production, respectively-so that their combined, rather than isolated, use is consistent with better outcomes for heat-stressed cattle. It should be acknowledged, however, that much of the supporting evidence for individual nutrients derives from a comparatively small number of trials, several of them conducted in sheep, in temperate research herds, or at experimental doses that may not translate directly to field conditions in tropical and sub-tropical smallholder systems; several statements in this review consequently rest on a single primary source rather than a convergent body of evidence. Larger, factorially designed field trials in Bos indicus and crossbred cattle under Indian and other tropical management conditions, systematic reviews or meta-analyses that formally pool dose–response data across studies and longer-term evaluations of production, reproductive and economic outcomes are therefore needed before the strategies discussed here can be generalised with confidence across breeds, climates and production systems.
