With higher temperatures in the summertime, horses are prone to heat stress. This can contribute to an increased risk of colic, ulcers, weight loss, fatigue and dehydration during this time of year. [1]

Horse owners should be vigilant for signs your horse is overheated, which include excessive sweating, lethargy, elevated breathing and heart rate, and decreased appetite.

If your horse appears at risk of heatstroke, contact your veterinarian right away and cool them down with fans and cool water.

You can keep your horse safe and comfortable in hot weather by ensuring they are drinking enough water, replenishing electrolytes, avoiding intense exercise during peak heat, and keeping them out of direct sunlight for long periods.

Follow the 14 feeding and management tips in this article to support your horse’s well-being all summer long. Consult with our equine nutritionists for help with formulating your horse’s diet through the summer.

Horses in Hot Weather

Horses are extremely efficient at producing heat through digestion and muscular activity. When horses eat forage, microbes in the gut break down cellulose and hemicellulose. This process of microbial fermentation generates heat that needs to be dissipated. [2]

The horse’s gut is designed to constantly digest plant fibre. But this process can work against your horse in extreme heat, compounding the risk of heat stress and associated issues.

Additionally, horses in work generate heat through their working muscles. [2] This makes it harder for horses to thermoregulate when exercising in hot and humid environments.

How Horse Stay Cool

For all mammals, core body temperature is maintained in a narrow range by continuously balancing heat production with heat loss. A stable core body temperature is maintained by regulating: [15]

  • The body’s rate of heat production
  • The transfer of heat to the external environment
  • The efficiency of evaporative cooling

The rate of heat production in the body can be adjusted by increasing or decreasing metabolic rate. However, both extreme cold and extreme heat result in a faster metabolic rate as the body activates various systems to try to regain a normal core temperature.

There are 4 main ways that horses can lose heat to the environment:

1) Conduction

Conduction involves direct contact between the skin and a cooler surface, such as cool water applied to the skin.

This process is more efficient with greater temperature differences between skin and the other surface. It is impaired by insulating materials such as a thick coat.

2) Convection

Convection refers to heat lost to the surrounding air. This process is most efficient if the warmed air is moved and replaced by cooler air which occurs when the horse is running or when they are standing in windy air.

This heat transfer to air can also occur in the respiratory tract when they breathe in cool air.

3) Radiation

Radiative heat loss refers to electromagnetic radiation emitted or absorbed at the skin. The most common form is a gain of heat from sunlight which can contribute up to 15% of heat gain in a horse exercising in sunny conditions. [16]

When a horse’s internal body temperature increases above baseline, blood vessels in the skin expand (vasodilate) to enable heat to dissipate into the atmosphere via convection and radiation.

This cooler blood then travels to the internal organs and muscles, reducing internal temperature. [3]

4) Evaporation

This is the most important form of cooling for horses and many other mammals that can sweat.

This involves evaporation of sweat from skin and water exhaled from the respiratory tract. The water droplets hold heat which is lost into the surrounding air when the liquid evaporates.

Sweating is activated by hormonal signals (ephinephrine) that increase during exercise and stress. It is also activated by increased core body temperature which is sensed by thermoreceptors in the skin, abdomen, skeletal muscle and brain.

It is estimated that 1 L of sweat can dissipate 580 calories of body heat which is approximately the amount of heat generated in 2 minutes of high-intensity exercise or 6 minutes of moderate-level exercise. [15][17]

Considering that an exercising horse can lose 10 – 15 L of sweat per hour, sweat provides a major avenue for dissipating heat in the horse.

In low- to mid-humidity conditions, sweat evaporates and cools the skin as well as blood close to the surface. [3]

Extreme Heat & Humidity

This cooling process becomes less efficient in extreme heat and humidity. When temperatures and the humidex are high, sweat cannot evaporate into the air.

The humidex is an index of the humidity in the air, measuring the moisture held in the atmosphere. When the air is already holding an abundance of water molecules, water does not evaporate as quickly.

This means that your horse cannot dissipate as much heat through sweating, impairing the cooling systems of the horse and increasing the risk of heat stress.

Heat Stress in Horses

Heat stress (sometimes called heat exhaustion) is an umbrella term for symptoms caused by a sustained high internal body temperature. Signs of heat stress include: [1]

  • Profuse sweating or less sweat than expected
  • Rapid breathing
  • Signs of dehydration such as loss of skin elasticity, tacky gums, sunken eyes, and reduced urine output
  • Rapid heart rate that does not resolve with rest
  • Muscle weakness
  • Extremely hot skin
  • Colic
  • Weight loss
  • Poor appetite or reduced feed intake
  • Gastric ulcers
  • Fatigue or lethargy
  • Heatstroke

Some horses are particularly susceptible to heat stress because they cannot adequately regulate their temperature in hot weather. This includes young foals, obese horses, older horses with PPID, and horses with anhidrosis (an inability to sweat).

Heat stress is clinically diagnosed by taking the rectal temperatures of the horse. Temperatures above 39.5oC (103oF) indicate heat stress. [13]

Rectal temperatures are typically lower than the horse’s actual core body temperature but are useful as a diagnostic tool.

If a rectal thermometer is not available, observing behaviours such as elevated respiratory rate and flared nostrils can be used as indicators of heat stress. [19]

If you think your horse is experience heat stress or exhaustion, it is critical to reduce their body temperature gradually until their internal temperature is with a normal range (99o to 101oF or 37.5o to 38.5oC) to avoid heatstroke.

Heatstroke

If heat stress devolves into heatstroke, horses can experience permanent internal organ damage or death. Signs of heatstroke include:

  • Dehydration
  • Extremely elevated internal temperature
  • Altered behaviour, either listless or panicky
  • Thick, dark urine
  • Stumbling
  • Muscle spasms
  • Seizures [4]

Heatstroke is clinically diagnosed with a rectal temperature greater than 40.5oC (105oF). Horses with heatstroke will also have a respiratory rate of greater than 40 breaths per minute and a heart rate of greater than 60 beats per minute. [13]

Heatstroke is a veterinary emergency. Call your veterinarian immediately and start cooling your horse right away to minimize the risk of long-lasting adverse effects.

Cooling your Horse

If your horse is experiencing heat stress or heatstroke, you can cool them down safely with the following strategies:
[13]

  • Move the horse indoors or into a shaded area and out of direct sunlight
  • Use fans or a misting device
  • Spray the horse’s head, back, and rump with cool water
  • Use ice water in cases of extreme heat
  • Provide fresh, cool water to encourage drinking

While rapid cooling can cause muscle cramping to occur, research shows that applying ice water to the skin can be used safely to cool down a horse. [13]

Your horse’s veterinarian may also administer IV fluids and electrolytes to combat dehydration.

14 Tips for Caring for your Horse in Hot Weather

Horses have evolved to live in some of the hottest regions of the world. Feral herds of the Namib Desert horse live in Namibia where daytime temperatures in the summer can exceed 45°C (113 °F).

Some horses are better suited to hotter climates (i.e. Arabians and Marwaris) while other breeds are less suited (i.e. Clydesdales and Icelandic Ponies).

Regardless of breed, you can ensure your horse is comfortable even in the hottest and most humid environments with the right feeding and management strategies.

Below, we outline 14 tips for caring for your horse in hot weather to keep them happy and safe from heat stress.

1) Encourage Adequate Water Intake

During particularly hot, humid days or intense exercise, horses can produce up to 15 litres of sweat per hour. If this water loss is not appropriately replaced, your horse can quickly become dehydrated. [4]

Encouraging water consumption helps to replace water lost through sweat and urination. Drinking water that has a lower temperature than your horse’s internal temperature can also help cool the body.

As the famous saying goes, “You can lead a horse to water, but you can’t make them drink”. However, you can encourage your horse to drink more water by:

  • Providing constant access to fresh, clean water
  • Offering water at a temperature of 45o to 65oF
  • Placing water in areas where your horse naturally spends their time (i.e. close to food or their favourite shady spot)
  • Providing multiple water sources to herds so that even horses low in the social hierarchy have constant access
  • Cleaning buckets or water troughs regularly
  • Providing water in blue buckets, which have been shown to increase consumption
  • Adding safe but tasty additives to water such as apple juice, small amounts of jello powder, or flavoured electrolytes

Giving your horse constant access to fresh cool water is the best way to ensure a happy, hydrated horse in heat.

2) Ensure Adequate Salt Consumption

Salt has a number of important roles in the horse’s body, but one of the most important is promoting thirst.

In one study, doubling the salt in a horse’s daily ration from 50 mg/kg to 100 mg/kg body weight resulted in a 53% increase in water consumption. [14]

Feed your horse 2 tablespoons (30 grams) of salt per day with their feed and provide plain, loose free-choice salt at all times.

Loose salt is recommended over salt or mineral blocks because horses do not obtain enough salt from blocks.

3) Consider Electrolyte Supplementation

When your horse sweats, they don’t just lose water. They also lose electrolyte minerals – specifically high levels of sodium, potassium, and chloride. [5]

Horse’s sweat is hypertonic, which means that it has a higher concentration of electrolytes compared to blood. This makes horses particularly susceptible to excessive electrolyte loss in hot weather or with exercise.

If electrolytes are not replaced promptly, your horse can experience:

  • Fatigue
  • Muscle wasting
  • Weight loss
  • Muscle cramps
  • Reduced sweating
  • Slow recovery time following exercise

In extreme cases of electrolyte imbalance, horses can experience heart palpitations and seizures. [6]

When temperatures are high, and especially when exercising your horse in hot weather, feed an electrolyte supplement to replenish levels and restore electrolyte balance.

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4) Soak Grain, Hay Cubes, and Flaked Hay

Soaking your horse’s hay and grain is an excellent way to increase hydration, reduce the risk of choke and support your horse’s respiratory system during hot, dry seasons.

Horses that are dehydrated do not produce enough saliva when chewing (masticating) their food. Saliva is important because it moistens food and provides lubrication for food to travel down the esophagus.

If your horse’s feed is not adequately moistened by saliva, it can stick to the interior of the esophagus resulting in choke. Soaking the feed in water greatly reduces this risk and encourages salivation.

One flake of soaked hay can absorb 1 – 2 gallons of water, significantly boosting water intake while also reducing the inhalation of dust particles.

Extreme heat is also associated with dry spells and low rainfall. This can lead to dry, dusty ground, which can cause respiratory issues. Soaked hay is one less source of dust in an otherwise dusty environment.

Hay needs to be soaked carefully in hot weather to minimize mould growth. Soaking can also change nutrient composition of the hay and loss of minerals. Appropriate vitamin and mineral supplementation is requirement to balance soaked hay.

It’s recommended to soak hay in cool water for 30 minutes to reduce dust particles and support the respiratory system during hot, dry seasons. [7]

5) Gradually Acclimate your Horse to Hot Climates

If you’re moving to a new climate, it is recommended to gradually acclimate them to new weather conditions.

The University of Minnesota recommends acclimating horses over 15 to 21 days. During this period, exercise intensity should be limited to allow your horse to adjust appropriately. This will improve heat tolerance and exercise performance for horses from cool or dry climates relocating to hot and humid climates. [13]

6) Avoid Exercising in Extremely Hot Weather

Keep your horse safe by avoiding training or riding your horse when temperatures and humidity are high.

As a general rule, when the air temperature (in Fahrenheit) plus the relative humidity percentage is greater than 150, you should not exercise your horse. This threshold may be lower for horses that are not acclimated to hot climates. [13]

Air temperature (F) + Relative humidity (%) = Greater than 150

In hot weather when the combined air temperature (F) and relative humidity % are between 130 – 150, work your horse during cooler periods of the day, such as in the evening or early morning.

7) Turnout your horse overnight

Turning your horse out at night can ensure they still have time to graze in their paddock while reducing exposure to hot conditions and direct sunlight.

Particularly in regions where there is a big drop in nighttime temperatures, this can help give your horse the chance to express species-appropriate foraging behaviours while keeping them cool.

Turnout at nighttime also means fewer bugs, which can improve your horse’s comfort.

8) Adjust Energy Sources in the Diet

In hot and humid weather, consider switching your horse’s feeding program to one that is high in fat and low in protein.

Horses in moderate work typically only require 8 – 12% protein in their diet. Excess protein in the horse’s diet gets metabolized in a process called deamination, which breaks down protein molecules into a useable energy source.

Deamination produces heat and urea as byproducts. Increased protein breakdown may require more sweating and higher calorie expenditure to eliminate the heat. Removing urea in urine requires water and can contribute to dehydration in the horse.

Fat is an efficient energy source for horses in hot temperatures. Compared to fibre digestion, breaking down fat and using it for energy produces little heat in the horse’s body. [9]

Test your horse’s hay to make sure it is not too high in protein and discuss nutrient requirements with an equine nutritionist. [8]

9) Place Feed and Water in Shady Areas

Horses that live outside most of the time will spend 12 – 14 hours of their day foraging or grazing.

Encourage your horse to spend time in cooler areas by placing forage and water sources in locations covered by shade or shelter. This will keep them out of direct sunlight and reduce the risk of sunburns and heat stress.

10) Provide Ventilation in Hot Weather

If your horse is stabled or spends most of its time indoors, providing adequate ventilation is important for their overall health and well-being.

Air quality can deteriorate quickly in unventilated barns when the temperature is high. Increasing airflow also helps to keep your horse cool.

Use fans and open windows to improve ventilation. In very hot climates, consider installing a stable misting fan to spray the air and horses with cool water.

11) Aim for a Leaner Horse

Fat (adipose tissue) is a highly efficient heat insulator, which can provide an advantage to horses carrying excess body condition in cold temperatures.

However, carrying extra body fat works against your horse in hot weather and can make it harder for them to dissipate heat. [10]

Regularly monitor your horse’s body condition score throughout the year. Body condition is scored on a scale of 1-9, with lower scores representing underweight horses and higher scores representing overweight horses.

To keep your horse comfortable at high temperatures, its body condition should be maintained at a score of 4.5 – 5. At this score, your horse will have ribs that are easily felt under a layer of fat, and there will be little to no crease of fat running down the length of their back. [11]

Keeping your horse at a lean body condition score ensures they have enough fat deposits to store energy and protect the organs, but not enough fat to trap excess heat in their body. [11]

12) Make Diet Changes Slowly

Hot weather can increase your horse’s susceptibility to digestive upset. If you decide to change your horse’s diet, make sure to do so gradually.

Sudden changes in feed can alter the gastrointestinal environment and induce colic symptoms. When introducing a new feed or forage, it is best to follow these guidelines for gradual introduction: [12]

  1. Replace 25% of the old feed with new feed for 3 days
  2. Replace 50% of the old feed with new feed for 3 days
  3. Replace 75% of the old feed with new feed for 3 days
  4. On the 10th day, 100% of the feed portion should be the new feed

13) Provide Protection from Sunburns

Horses can become sunburned when they are in direct sunlight for long periods of time without protection. Light-coloured horses are particularly vulnerable, but any horse can develop a sunburn on pink areas with exposed skin around the head and feet.

UV turnout sheets or fly sheets with UV-protective material can help to protect your horse’s skin but should be avoided when temperatures are really hot as they can make it harder for the horse to dissipate heat. There are also UV-blocking face masks that can be used to protect the head.

You can also buy sunblock make specifically for horses or use sunscreens intended for children. Your veterinarian can help you find the right product for your horse.

If your horse develops a sunburn, keep them out of sunshine until the skin heals to prevent further damage. Appropriate topical creams can be used to help soothe irritations and promote healing.

14) Keep your Horse Cool When Travelling

Transportation is a major source of stress for horses, which may be exacerbated in hot weather. Only travel when necessary and avoid long trips if temperatures are high.

It is recommended to travel early in the morning when temperatures are cooler. Use a trailer that provides adequate ventilation and keep the windows open. Give your horse plenty of water with electrolytes prior to travelling and as soon as you arrive at your destination.

Unique Challenges for Exercising Horses

When a horse is not exercising it can easily maintain a normal core body temperature by regulating heat production vs heat loss, especially in cooler climates.

However, in comparison to other mammals, horses are more prone to exercise-induced heat stress. This is due to several unique features of the horse including: [15]

  1. Greater use of muscle for exercise: In fit horses, skeletal muscle represents 50% of their total bodyweight compared to 30-40% in other large animals such as cows. They also engage more of their muscle during exercise and therefore generate more heat through muscle contraction than other animals.
  2. Lower surface area for dissipating heat: The horse has a 50% lower surface area to body mass ratio than humans. This means they have less opportunity to dissipate heat through their skin.
  3. Respiratory coupling to locomotion: When a horse is running their breathing rate is coupled to their stride so they can not increase their breathing rate independently to allow more heat transfer through respiration

Horses have adapted somewhat to these limitations by adjusting their tolerance to an elevated core temperature. For example, humans choose to stop voluntary exercise when their core body temperature reaches 40oC. In comparison, horses will continue to exercise past this and will choose to stop voluntary exercise when their core body temperature reaches 42oC or higher.

Above these levels, heat stress would be highly likely and damaging to many tissues including the brain. In response, the brain activates several systems including coordinating blood flow to the skin, activating sweating and stopping exercise. [18]

Key Points for Hot Weather Care

Dealing with heatwaves can be stressful for you and your horse, but with a bit of preparation, you can ensure that your horse is comfortable during the summer heat.

The care and management practices discussed in this article will support even the most sensitive horses in hot and humid conditions. Remember to:

  1. Encourage water intake
  2. Feed salt and use electrolyte supplements
  3. Soak grain, hay cubes, and hay
  4. Provide shade and protection from the sun
  5. Exercise your horse responsibly
  6. Monitor body condition and reduce protein intake

Speak to a nutritionist for additional ways to optimize your horse’s feeding program for summer heat. You can submit your horse’s information online for a complementary diet balancing by our university-trained equine nutritionists.

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References

  1. Lindinger, M. Sweating, dehydration and electrolyte supplementation: Challenges for the performance horse. Proceedings of the 4th Euro Equine Health & Nut Congress. 2008.
  2. Medjell, C. et al. Caring for the horse in a cold climate—Reviewing principles for thermoregulation and horse preferences. App Animal Behaviour Sci. 2020.
  3. Osilla, E. et al. Physiology, Temperature Regulation. Nat Center for Biotech Info. 2021.
  4. Pankowski, R. et al. A review of heatstroke [in the horse, heat exhaustion]. Equine Prac. 1984
  5. Smith, F. Note on the Composition of the Sweat of The Horse. The Journal of Phys. 1980.
  6. Johnson, P. Electrolyte and Acid-Base Disturbances in the Horse. Vet Clinics of NA: Equine Practice. 1995
  7. Moore-Colyer, M. Effects of soaking hay fodder for horses on dust and mineral content. Animal Sci. 2010.
  8. Ellis, A. et al. The impact of nutrition on the health and welfare of horses. 5th European Workshop Equine Nutrition. 2010.
  9. Hintz, H. Digestive Physiology of the Horse. Journal of the South African Vet Soc. 1975.
  10. Langlois, B. Inter-breed variation in the horse with regard to cold adaptation: a review. Livestock Production Sci. 1994.
  11. Carroll, C. and Huntington, P. Body condition scoring and weight estimation of horses. Equine Vet Journal. 1988.
  12. Shirazi- Beechey, S. Molecular insights into dietary induced colic in the horse. Equine Vet Journal. 2010.
  13. Martinson, K. Caring for horses during hot weather. University of Minnesota Extension. 2020.
  14. Cymbaluk, N.F. Water. In Equine Applied and Clinical Nutrition. 2013.
  15. McCutcheon, L.J. and Geor, R.J.Thermoregulation and exercise-associated heat stress. In Equine Exercise Physiology. 2008.
  16. Guthrie, A.J. and Lund, R.J. Thermoregulation. Base mechanisms and hyperthermia. Vet Clin North Am Equine Pract. 1998.
  17. Iatridis, P.G. Human Circulation: Regulation during physical exercise. JAMA. 1987.
  18. Lindinger, M.I. Exercise in the Heat: Thermoregulatory Limitations to Performance in Humans and Horses. Can J Appl Physiol. 1999.
  19. Pritchard, J.C. et al. Validity of a behavioural measure of heat stress and a skin tent test for dehydration in working horses and donkeys. Equine Vet J. 2006.