Heat stress in horses: why understanding it matters more than ever

Understanding heat stress can help us support our horses’ wellbeing, comfort and performance.

As temperatures rise and heatwaves become more frequent, horses are being exposed to warmer environmental conditions than those they have historically experienced. This can pose a range of risks to horse welfare, health and performance – especially when combined with the more humid climate of the UK and Ireland.

Further understanding the impact of heat stress on horses will help the equine industry to continue to develop effective mitigation strategies, reducing the risk of adverse events at every level.

What is heat stress?

Heat stress is a serious welfare issue, not only during competition but also leisure riding and transportation during hot and humid weather. This can also be exasperated in cases where housing and management are not suited or adapted for warm conditions.

There is not a clear definition of heat stress in horses, or much data available regarding this condition. In relation to welfare, a horse is considered under heat stress when their body is accumulating more heat than it can lose through normal mechanisms like sweating.

A healthy horse can maintain normal body temperature (37.2-38.3° C) when the environmental, or ambient, temperature is within their thermoneutral zone of 5-24º C. It is essential to note that the upper limit of this range is not purely fixed on air temperature – it is also affected by humidity, solar radiation and air movement. For example, a horse will experience heat stress at a lower air temperature if relative humidity is high, like it often is in the UK and Ireland.

Keeping body temperature within this narrow range is essential to allow organs, muscles and enzymes to function most efficiently. However, when temperatures exceed this, their body must actively lose heat to prevent its core temperature from rising.

Horses lose heat through four main processes:

Evaporation – Horses depend primarily on sweating to cool themselves and have numerous sweat glands across their skin. As the sweat evaporates, it removes heat from the body, cooling the horse. In high humidity, sweat evaporates more slowly which significantly reduces the loss of heat.
Radiation – Heat naturally moves from the horse's body to cooler objects or the surrounding air without direct contact. This process is most effective when the surrounding environment is cooler than the horse.
Convection – Air moving over the horse's body, such as from wind or a fan, carries heat away from the skin. Increased airflow improves cooling by both removing heat and speeding up sweat evaporation.
Conduction – Heat is transferred when the horse comes into contact with a cooler surface, such as damp ground or cool water. Under normal field conditions, this contributes less to cooling than evaporation.

What makes horses vulnerable to heat stress?

Although horses are well adapted for athletic performance, several features of their physiology make them susceptible to heat stress, particularly during hot and humid weather.

Horses produce a large amount of metabolic heat. During exercise, only about 20-25% of the energy produced by muscles is used for movement, the rest (75-80%) of that energy produces heat. This means that horses can accumulate heat very quickly while exercising.
Horses rely heavily on sweating to cool themselves, which becomes much less efficient when humidity is high as sweat evaporates more slowly. If sweat cannot evaporate, the horse gains little cooling benefit while continuing to lose water and electrolytes.
Large animals have a lower surface area relative to their body mass than smaller animals, meaning they have less surface from which they can lose heat. As a result, their body heat can build up faster than it can be removed.

Minimising the risk of heat stress

Many elite competition venues have implemented cooling protocols following exercise to facilitate recovery – such as monitoring heart rate and respiration, fans, cold water and shaded recovery areas. This has distilled down to many horse owners and yards across the UK and Ireland, including the use of industrial fans to keep barns and stables cooler.

In the field, it’s important to ensure adequate access to shade – whether from natural tree cover or purpose-built shelters – to reduce heat exposure from the sun. Research demonstrates that horses standing in direct sunlight can experience substantially greater heat loads than those under shade, even in identical air temperature. If this cannot be provided, horses should be stabled with good ventilation during peak sunlight hours.

Unlimited access to clean, fresh water is also critical for helping horses cope with warm and/or humid weather. An adult horse may drink 25-55 L/day under normal conditions, however this can increase to 60-80+ L/day during hot weather or after exercise. Replacing electrolytes appropriately is another important consideration.

Additionally, exercise should be reduced during hot weather, or undertaken during the early morning or late evening when temperatures are cooler.

Signs of heat stress in horses

There are a range of signs to look for in horses at risk of developing heat stress:

Sweating profusely (or sweating less than expected)
Increased breathing rate (8-18 breaths per minute is normal)
Elevated heart rate (30-44 beats per minute is normal)
Mild dehydration (e.g. dry gums, 2-3 second capillary refill time, dark yellow urine, reduced skin elasticity)

Horses showing signs like this should stop exercising and be cooled as quickly as possible. Current research suggests the most effective way to do this is to apply large volumes of cold water immediately and continuously, such as with buckets or a hose pipe.

As prey animals, horses can also be highly stoic, meaning early signs of discomfort may be subtle and difficult to notice. Changes in appetite, behaviour, willingness to work or recovery following exercise may indicate that a horse is finding conditions more challenging than usual.

The role of the gut microbiome

As body temperature rises, it acts as a physiological stressor that triggers cooling mechanisms and activates a stress response, including cortisol production. This diverts blood flow away from the gut, disrupting the balance of the hindgut microbiome and compromising the integrity of the gut lining.

The hindgut is home to a complex and diverse community of microbes responsible for fibre digestion and the production of nutrients that support overall health and wellbeing. Maintaining a stable microbial population is essential for efficient digestion and normal gut function, but research suggests that periods of physical or environmental stress – including heat stress – can disrupt this balance, increasing the risk of damaging the gut barrier (also known as “leaky gut”) and an increased risk of colic.

The hindgut microbiome is also closely linked to the gut-brain axis, a two-way communication network connecting the digestive system and the nervous system. As our understanding of this relationship develops, researchers are gaining new insights into how changes in the hindgut during periods of stress may influence wider aspects of equine health, wellbeing and performance.

Can nutrition play a role?

Nutrition cannot eliminate heat stress, but it may form part of a broader strategy to support horses during periods of environmental challenge.

The horse's hindgut naturally generates significant heat due to the fermentation of fibre, making digestive efficiency an important consideration during hot weather. Maintaining a healthy and stable hindgut environment may help support overall digestive function when horses are exposed to increased thermal demands.

Alongside practical management, nutritional strategies that support hindgut stability may also contribute to a horse's overall resilience to heat and humidity. For instance, studies have suggested that diets incorporating grains, chopped fibre sources and fats may produce less heat during digestion than traditional high-fibre diets. However, care must be taken to not overload the digestive tract with starch.

Supporting a healthy microbiome is key to helping horses thrive in any environment, and products such as Actisaf® Sc 47 live yeast probiotic and Safmannan® premium yeast fraction can be incorporated into diets to support digestive function and hindgut stability.

Supporting horses in a changing climate

As temperatures continue to rise, heat stress is an increasingly important consideration for horse owners, riders, trainers and the industry at-large. By recognising heat stress and understanding the factors that influence it, we can help horses cope more effectively with warmer conditions and continue to thrive in an elite sporting environment.

The good news is that research continues to improve our understanding of how horses respond to heat (and other sources of stress), helping us make informed decisions that support health, welfare and performance through practical management and nutrition.