Summer heat can create serious, preventable welfare risks for horses, especially in hot, humid conditions. The piece explains how horses cool themselves, how to recognize heat stress and anhidrosis, and why water, shade, and timing exercise matter.

It also summarizes research showing that continuous water application while the horse is stationary cools fastest, while walking alone is slow. The post ends with practical management advice for hydration, shade access, cooler work times, and heat acclimation.

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Summer is the season most horse people look forward to most. The days are long, the ground is firm, and there’s more time to ride. It is also the season that quietly poses some of the most serious and preventable welfare risks in horse management, particularly in climates where heat and humidity combine in ways that compromise your horse’s ability to cool themselves down.

Most horse owners have a working understanding of the basics. Provide water. Offer shade. Avoid riding in the middle of the day. But the physiology behind why these things matter, and the research on what interventions actually work when a horse is in heat stress, is more detailed and in some cases more surprising than the conventional wisdom suggests. Understanding the mechanism makes you a more effective manager, because it changes how you prioritize and respond.

How Horses Actually Regulate Body Temperature

Healthy adult horses maintain their body temperature within a narrow range of 37.5 to 38.5 degrees Celsius when resting in their thermoneutral zone, which spans roughly 5 to 25 degrees Celsius. When conditions push beyond that zone, or when the horse exercises and generates significant metabolic heat, they need active thermoregulatory mechanisms to bring temperature back down.1

Horses rely primarily on evaporative sweating to dissipate heat. Their sweat glands are densely distributed across the body surface and are capable of moving large volumes of fluid to the skin, where evaporation carries heat away. This system is highly effective under the right conditions, but it has two significant vulnerabilities.

The first is humidity. When the relative humidity is high, sweat cannot evaporate efficiently because the air is already saturated with moisture. A horse working in 35-degree heat at low humidity can manage the heat load reasonably well. The same horse working in 30-degree heat at 90% humidity is in a far more dangerous situation, because the primary cooling mechanism has been functionally impaired. This is why the temperature-humidity index, which combines both variables into a single risk metric, is a more reliable guide to safe working conditions than temperature alone.2

The second vulnerability is that horses have a comparatively low body surface-to-mass ratio compared to humans, approximately 1:90-100 square metres per kilogram compared to roughly 1:35-40 in people. This means their capacity to shed heat through the skin surface relative to the heat load generated by their muscle mass is inherently limited, particularly during strenuous exercise.3 A working horse is generating large amounts of metabolic heat in a body that is not especially well-shaped for getting rid of it quickly, which is why the external environment and post-exercise management matter so much.

Recognizing Heat Stress Before It Becomes an Emergency

The earliest signs of heat stress are worth knowing precisely because they appear before the obvious crisis, and catching them early is the difference between a horse that recovers with minimal intervention and one that is in genuine danger.

A normal resting rectal temperature in horses ranges from 37.5 to 38.5 degrees Celsius. Rectal temperature remains the most accessible and reliable indicator of core body temperature and the primary practical tool for detecting heat stress in field settings.4 A temperature above 39.5 degrees Celsius after rest indicates a problem. Above 40 degrees, the situation requires active cooling immediately. Above 41 degrees, veterinary attention is urgent.

Other early indicators include respiratory rate, heart rate, and sweating pattern. Elevated respiratory rate, particularly the rapid shallow breathing sometimes described as panting, is a sign that the horse is attempting to dissipate heat through the respiratory tract because evaporative sweating is insufficient. Elevated heart rate that does not come down within a reasonable time after exercise has stopped signals that the cardiovascular system is still working hard to manage heat load. A horse that stops sweating, or that is sweating dramatically less than expected for the conditions and work being done, is in one of the most immediately concerning situations possible.

The condition where sweating stops or is severely reduced is called anhidrosis, and it represents a failure of the primary cooling system. A comprehensive review of heat stress in horses noted that anhidrosis is normally seen in exercising athletic horses or stabled horses exposed to hot and humid climates for long periods, and that it may cause severe impairment of thermoregulation in the equine athlete.¹ Research on the genetic basis of chronic idiopathic anhidrosis has identified links to a defective potassium channel subunit that likely hinders sweat function, and the condition may affect between 2% and 6% of horses in affected regions.5 Horses with anhidrosis will maintain a dry coat even during heavy exercise and in high temperatures when other horses around them are visibly sweating. This is not a subtle sign, and it warrants veterinary attention and a careful reassessment of that horse’s workload and management in hot weather.

What the Research Actually Says About Cooling

This is where some of the most useful and surprising science sits, because several conventional cooling practices turn out to be less effective than widely believed, while the most effective approach is considerably simpler than many people expect.

A study published in the Journal of Equine Veterinary Science compared five cooling methods in Thoroughbred horses after exercise under hot and humid conditions, measuring the time taken for pulmonary artery temperature to return below 39 degrees Celsius. The five methods tested were: walking with no additional cooling, walking with fans producing an air current of 3 metres per second, walking with intermittent cold water application and scraping, walking with intermittent cold water application without scraping, and stationary with continuous tap water application via shower hoses. The most effective method was continuous tap water showering while stationary, which returned core temperature below 39 degrees in an average of approximately 2 minutes. Walking alone took approximately 25 minutes.6

Two widely held beliefs received a significant challenge from these findings. The first is that horses must be walked to cool down. Walking alone was by far the slowest cooling method. The second is that scraping off water between applications speeds up cooling. The study found no significant difference between intermittent cold water with scraping and without scraping, and both were slower than continuous tap water application. The essential variable is not the temperature of the water or whether you scrape, but that the horse is continuously covered in water that is cooler than its body temperature.7

The explanation for why continuous tap water outperforms intermittent cold water lies in the physics of heat transfer. In a humid environment, evaporation is limited. What drives cooling in those conditions is conduction, the direct transfer of heat from the horse’s body into the water on its surface. Cold water applied intermittently and then scraped off limits the duration of that conductive contact. Continuous tap water maintains it. The water does not need to be ice cold. It needs to be cooler than the horse and in continuous contact with the skin.

For horses in more moderate conditions after lighter work, a 2022 study published in Animals examining water cooling methods after medium-intensity exercise found that applying cold water to the lower legs specifically was effective under moderate air temperature conditions and can be recommended as a practical approach for leisure horses in less extreme circumstances.8

Practical Summer Management That Makes a Difference

The research on heat stress translates into a fairly clear set of management priorities that apply regardless of whether you have one horse or twenty.

Access to fresh water is the most fundamental. During hot weather and exercise, horses can lose significant fluid volumes through sweat, and dehydration compounds the thermoregulatory challenge considerably. Water should be clean, accessible at all times, and checked frequently. Automatic waterers require regular inspection in summer because a malfunctioning waterer that a horse stops drinking from may not be noticed quickly enough.

Shade matters more than many owners realize. Research comparing horses in shaded versus unshaded pens in hot, sunny environments found clear differences in physiological stress indicators and behavioral patterns.9 Shade access should be reliable and sufficient for the number of horses sharing a space, since a single shade structure that three horses must compete for provides meaningfully less protection than one that all three can use simultaneously without social conflict.

Timing of exercise is one of the most effective management tools available. Riding in the early morning or late evening, when temperatures are lower and solar radiation is absent, substantially reduces the heat load your horse accumulates during work. For horses in regions where summer temperatures are consistently high, adjusting the entire training schedule to cooler parts of the day is not a concession but a straightforward welfare decision.

Acclimatization deserves consideration for any horse being moved to a hotter climate, or for horses early in the summer season who haven’t yet adjusted to the heat. Heat acclimation genuinely changes equine physiology. A 2024 study published in Physiological Reports found that heat acclimation improved exercise performance in hot conditions in Thoroughbreds and increased the expression of heat shock proteins in skeletal muscle, which are proteins that protect cellular function under thermal stress.10 Horses need time to build these adaptations, and working a horse hard in early summer before that adaptation has occurred carries meaningfully more risk than working the same horse in the same heat later in the season.

The Bottom Line

Heat stress in horses is serious, well-researched, and substantially preventable. The physiology is clear: horses rely primarily on evaporative sweating to cool themselves, humidity undermines that mechanism, and a horse that stops sweating is a horse in acute danger. The research on cooling methods is equally clear: continuous water application while the horse is stationary is the most effective intervention, and walking alone does very little.

The summer months are some of the best time to be around horses. They are also the months that call for the most active management awareness. Knowing what to look for, knowing what to do when you see it, and building the small daily habits that reduce heat load before it accumulates are the things that keep summer enjoyable rather than dangerous for the horses in your care.

  1. Hyungsuk Kang et al., “Heat Stress in Horses: A Literature Review,” International Journal of Biometeorology 67, no. 6 (2023): 957–973, https://doi.org/10.1007/s00484-023-02467-7. ↩︎
  2. Ibid ↩︎
  3. Ibid ↩︎
  4. Ibid ↩︎
  5. Laura Patterson Rosa et al., “Genomic Association of Chronic Idiopathic Anhidrosis to a Potassium Channel Subunit in a Large Animal Model,” Journal of Investigative Dermatology 141, no. 11 (2021): 2639–2645, https://doi.org/10.1016/j.jid.2021.03.035. ↩︎
  6. Yuji Takahashi et al., “A Comparison of Five Cooling Methods in Hot and Humid Environments in Thoroughbred Horses,” Journal of Equine Veterinary Science 91 (2020): 103130, https://doi.org/10.1016/j.jevs.2020.103130. ↩︎
  7. Ibid ↩︎
  8. Marcin Trela et al., “Effect of Different Water Cooling Treatments on Changes in Rectal and Surface Body Temperature in Leisure Horses after Medium-Intensity Effort,” Animals 12, no. 4 (2022): 482, https://doi.org/10.3390/ani12040482. ↩︎
  9. Hyungsuk Kang et al., “Heat Stress in Horses: A Literature Review,” International Journal of Biometeorology 67, no. 6 (2023): 957–973, https://doi.org/10.1007/s00484-023-02467-7. ↩︎
  10. Yutaka Ebisuda et al., “Heat Acclimation Improves Exercise Performance in Hot Conditions and Increases Heat Shock Protein 70 and 90 of Skeletal Muscles in Thoroughbred Horses,” Physiological Reports 12, no. 10 (2024): e16083, https://doi.org/10.14814/phy2.16083. ↩︎