“Obviously high environmental temperatures are a key concern, but prolonged or intense exercise and inadequate hydration may all contribute to heat stress,” said Kris Hiney, Oklahoma State University Cooperative Extension equine specialist.
Horses, like humans, dissipate the majority of their excess body heat through sweating. Horses can sweat as much as 10 to 12 liters per hour. Depending on the environmental temperature and the animal’s workload, it is possible for a horse to become dehydrated in as little as two to three hours.
An equine that does not have adequate access to water will not be able to sustain the same sweating rate as a horse with proper hydration. In addition, there are physiological aspects that are important to avoiding heat stress.
“When we humans sweat, our sweat is hypotonic and has fewer electrolytes in it than does our blood,” Hiney said. “Horses have either isotonic (the same) or hypertonic (more electrolytes) than is present in their blood. This allows horses to sustain sweating rates longer than humans.”
That is important because since a horse’s blood does not increase in electrolyte concentration with sweat loss, the animal may not have the natural stimulus for thirst.
“It’s the responsibility of the equine manager to ensure the horse is drinking sufficient amounts of water to maintain good health and performance, and to back off the animal’s level of activity when needed,” Hiney said. “A good rule of thumb is to add the values of the temperature and humidity to get the heat index when determining whether or not to reduce animal activity.”
Horses will cool themselves normally – unless dehydrated or fatigued – if the combined values are below 130. Conditions when the combined values exceed 150, such as 90 degrees Fahrenheit with 60 percent humidity, require more assistance in cooling. If the heat index exceeds 170, consider doing something else as these conditions can be dangerous to both horse and rider.
“Keep an eye on the weather forecast,” Hiney said. “If temperatures are soaring, consider watching a training video instead and give your horse a break. If you must ride, consider doing so in the early morning hours or late in the evening.”
Sweating obviously employs evaporation as a major way for the horse to dissipate heat. A well-hydrated horse is necessary to maintain stable sweating rates to dissipate thermal load. However, be aware high humidity levels will limit the effectiveness of evaporative cooling.
“Water applied to the horse can greatly aid in cooling,” Hiney said. “Applying cool but not cold water to areas of the horse that have large blood vessels near the surface of the body is the most effective. Concentrate on the legs and neck of the horse. Blood will cool as it passes through these areas and then return to the trunk of the body to help dissipate the heat load.”
The major blood vessels in a horse’s leg lie to the inside, so horse managers should apply water there. Continual application of cool water will prevent the warming of the water on the horse’s skin. Otherwise, use a scraper to remove the warmed water and increase the rate of evaporative cooling.
Convection – heat that is lost due to air movement – is another major way that an animal loses heat. Supplying fans or keeping the horse in an area with wind flow is ideal. Fans with higher velocities provide more effective cooling,
“If you live in a hot climate and have access to electricity, putting a fan near the arena will aid in cooling during rest periods,” Hiney said. “Always make sure that your horse’s heart rate and respiration rate have dropped before returning to work.”
Radiation sounds suitably Atomic Age scary but it simply refers to heat transfer through space. Keeping the horse in the shade or riding in shaded areas is a good way to mitigate radiant heating. In addition, a horse will transfer heat through space to any object cooler than it.
“Trees are useful,” Hiney said. “Standing under trees allows the horse to radiate some heat up to the leaves which are continually cooled by their own evaporation.”
Hiney said horse managers should take full advantage of conduction or the direct transfer of heat between objects of differing temperatures. An example of conductive cooling would be a dog lying on a cooling mat or digging into the cool earth.
“Any surface that is cooler than the horse and in direct contact with the equine’s body will aid in cooling,” she said. “It’s why cool water applied to the horse’s body helps to cool it. Remember the key is cool water and not cold. Cold water can result in vasoconstriction which can limit blood flow to the horse’s skin.”
If a continual supply of suitable water is not available, placing cool wet towels on the horse’s body can provide conductive cooling. Just remember that continual reapplication of cool towels is necessary as the horse’s body heat is transferred to the towels.
And don’t forget situations where a horse is the rider. For horse managers hauling the animal a long distance, it may be better to drive at night.
“Some trailers have inadequate ventilation to keep a horse cool so avoiding the heat of the day can be beneficial,” Hiney said. “Also, the muscular work of balancing as the vehicle moves down the road can put an additional heat load on the horse, so drivers need to keep that in mind as well.”
Additional information about horse care is available through the OSU Cooperative Extension Equine website at http://osuhorse.okstate.edu.
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