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.

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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