Heat Stress: Using the Equine Heat Index to Keep Horses Safe
Horses cool themselves mostly by sweating, and sweat only works if it can evaporate. When the air is both hot and humid, cooling slows down and exercise gets dangerous. This guide explains the equine heat index, the signs of trouble and what to do about them.
How horses lose heat
Horses produce a lot of heat when they work, and they lose most of it by sweating, which cools the skin as the moisture evaporates. In humid air, sweat evaporates slowly or not at all, so it just runs off the coat without cooling. The horse also loses water and salts in the sweat, so dehydration and electrolyte loss add to the problem.
The equine heat index
The rule used by the American Association of Equine Practitioners and others is simple: add the air temperature in degrees Fahrenheit to the relative humidity as a percentage. A day with a temperature of 90°F and humidity of 60 percent has an equine heat index of 150. The Heat Index Calculator adds the numbers and shows the guidance for that reading.
| Temperature + humidity | What it means |
|---|---|
| Under 130 | Cooling by sweating works well. Normal precautions. |
| 130 to 150 | Caution. Sweating is less effective, so shorten or slow work, give water and shade and watch the horse closely. |
| 150 to 180 | Limit work to light riding in the coolest part of the day, and cool the horse actively afterward. |
| Over 180 | Horses cannot cool effectively. Do not ride, work or haul if you can avoid it. |
This rule of thumb is not the same as the human heat index, and it is only a guide. A fit horse used to the climate copes better than an unfit, overweight, older or recently transported one. Dark coats, heavy muscling, thick winter coats that have not shed, and horses with Cushing's disease or anhidrosis (the inability to sweat) all have extra trouble.
Signs of heat stress
- Heavy sweating that suddenly stops, or no sweating at all.
- Rapid breathing that does not settle within about 10 to 20 minutes after work.
- A rectal temperature above about 103°F (39.5°C) that does not fall quickly with rest. The normal resting range is 99 to 101.5°F.
- Dull, depressed behavior, weakness, stumbling or muscle tremors.
- Dark urine, dry gums or a skin pinch that is slow to flatten out, which suggests dehydration.
Heat stroke is a medical emergency. A horse with a very high temperature, a staggering gait or collapse needs cooling and a vet immediately.
Cooling a hot horse
- Move the horse into shade with airflow, and stop the work.
- Hose or sponge cold water over the large muscles, neck and between the hind legs, and keep it moving. Scrape the water off and keep applying more; the water absorbs heat only if it is replaced.
- Let the horse drink small amounts of water frequently, rather than letting it gulp.
- Use fans if you can.
- Check the temperature, and call the vet if it stays above 103°F or if the horse looks distressed.
Water and electrolytes
A horse normally drinks 5 to 10 gallons a day, and needs much more in hot weather or in work; some horses can drink 15 to 20 gallons or more. Offer clean, cool water at all times and check the troughs daily. Salt is the key electrolyte; make sure loose salt is available, and ask your vet before using electrolyte products or pastes for horses in heavy sweat, as they work only when the horse is also drinking. Use the Gallons to Liters Converter for tracking intake in metric units.
Managing the day
- Ride early or late, and shorten or lighten work on hot, humid days.
- Clip a heavy coat if the horse is working in hot climates.
- Give shade, a shelter with airflow, or fans in stalls.
- Avoid hauling in the heat, or plan for stops, ventilation and water; see Hauling and Towing Horses.
- Bring horses into the climate gradually, since acclimatization takes about two weeks.
Convert between temperature scales with the Fahrenheit to Celsius Converter, as many thermometers and forecasts use Celsius.
Common mistakes
- Judging heat by temperature alone and ignoring humidity.
- Pouring water on and leaving it, which traps heat.
- Withholding water from a hot horse.
- Pushing a horse that has stopped sweating.
Frequently asked questions
How do you calculate the equine heat index?
Add the air temperature in degrees Fahrenheit and the relative humidity percentage. For example, 85 degrees and 55 percent humidity gives 140.
What heat index is too hot to ride?
As a general guide, below 130 cooling works well, 130 to 150 calls for caution, 150 to 180 means light work only in the coolest part of the day, and above 180 horses cannot cool effectively. Fitness, climate adaptation and health matter too.
What is a horse's normal temperature?
About 99 to 101.5 degrees Fahrenheit at rest. A reading above 103 degrees after rest and cooling is a reason to call your vet.
Tools used in this guide
More guides
- Hauling and Towing Horses Safely
- How to Estimate Your Horse's Weight Without a Scale
- Feeding Horses: Hay, Grain and Supplements
- Body Condition Scoring with the Henneke Scale
- All HorseCalcs guides
Figures in this guide are general guidance for planning. Every horse is different, so work with your veterinarian, farrier or trainer on health, hoof and training decisions, and read our disclaimer.