Water and Your Horse: Beat the Heat and Keep Your Horse Healthy and Hydrated

An exercising horse’s muscles produce a tremendous amount of heat. And that heat can build up to dangerous levels that can damage organs or even cause death. The horse has to have a way to eliminate that heat from his body and, of course, by far the most efficient method of heat loss is by evaporation of sweat from the horse’s skin.

The dilation of blood vessels close to the skin brings hot blood to the surface so heat can be lost by convection, where heat is transferred to and carried off by the air currents passing over the horse’s body. The Catch-22 is that sweating means fluid loss, and fluid loss easily leads to dehydration.

To perform, your horse has to control body heat. To do that, he must sweat and will lose tremendous amounts of water and electrolytes in the process. The horse’s body is about 60% water, but most of that is inside the cells. A 1,100-pound horse has about 100 liters of fluid in the spaces around cells and circulating in the blood, and about 25 liters of that is blood volume. When you consider that a horse working in high heat can be producing 15 liters of sweat an hour, it’s not hard to picture how easily they can get into trouble.

The Full Tank

The first step in keeping these sweat losses manageable is to make sure you start out with as much on board as possible. The horse’s body carefully regulates the amount of salt/sodium and water it contains, but the horse does have an on-board tank you can take advantage of: his intestinal tract.

The horse’s large intestine holds 20 to 24 gallons of fluid. How much the horse has in there has a lot to do with his diet. Horses on high-fiber diets containing generous amounts of hay will drink considerably more and have more water in their intestinal tract than horses fed large amounts of grain or pelleted diets. Horses on fresh grass have a ”prehydrated” diet since live plants contain about 85% water. Although this ”internal canteen” is no substitute for liberal watering during and after exercise, it can help supplement it to some extent.

Sweating also means loss of electrolytes, which must be replaced or the horse won’t drink well, even if significantly dehydrated. This is because a large part of the horse’s drive to drink comes from rising concentrations of sodium in the blood. If the sodium lost in sweat is not replaced, this rise doesn’t happen and the horse’s brain doesn’t register that he’s dehydrated.

Pre-exercise strategies and that ”internal canteen” intestinal tract can help you out here, too. An additional one or two ounces of salt per day on top of the horse’s minimum salt requirements (the salt intake baseline is two ounces of salt per day) for a few days prior to the time you know your horse will be facing a long or hard day’s work in the heat will help guarantee he drinks maximally. Giving 2 oz. of a balanced electrolyte solution that matches sweat losses with the morning meal on days the horse will work long or hard gives you a head start on meeting sweat losses with exercise. See our March 2007 issue for information on selecting an electrolyte (www.horse-journal.com).


Human athletes replace water, electrolytes and carbohydrate/sugar frequently when exercising, as often as every 15 minutes. We’ve all seen these liquids being handed off to marathon runners on the fly. This would be ideal for horses, too, but since they can’t drink and keep moving at the same time, we are resigned to playing catch-up at rest stops, between classes, or between phases of a competition.

Plain water consumption without replacing electrolytes isn’t ideal. However, there are several different schools of thought, and even conflicting studies, regarding electrolyte supplementation during prolonged exercise. Part of the problem, again, is playing catch-up rather than staying on top of the problem. One thing is for sure, though. Pasting/syringing in concentrated electrolyte solutions without the horse being able, or willing, to drink adequate water to match can make things worse.

A middle-ground solution favored by many researchers is to provide salted water of the same or somewhat lower sodium concentration as the blood and extracellular fluids. Horses offered saline drank more and were better hydrated. Even the temperature of the water has an effect:

• A 1996 Swedish study published in the Equine Veterinary Journal found that horses during an endurance ride drank better when they were offered saline (0.9% salt) water vs. plain water alone or plain water and paste salt supplements.

• A Michigan State study published in 2004 in the Veterinary Journal found that horses consumed the most saline when offered tepid (68?° F), versus cold (50?° F) or warmer (86?° F) water. They hypothesized that the cooling effect in their mouths from the colder water quenched the drive to drink quicker, just like an icy drink is more satisfying for us than a warm one when we’re hot.

• Another Michigan State study appearing in 2002 in the Equine Veterinary Journal looked at the effects of offering either plain water, a 0.45% salt-solution or 0.9% salt-solution as the initial drink to dehydrated horses. After their initial watering, the horses all had free-choice plain water. The horses offered the salt solutions first clearly drank significantly more water after that.

Unfortunately, while salt does a great job with replacing sodium and gaining hydration, the electrolyte potassium will likely still come up short if you use only saline. You can substitute a correctly balanced sweat replacement electrolyte product for the plain salt, but limit the amount of sodium per gallon to about 5 grams for maximum absorption if using treated water during exercise.

You can calculate needs using these rules of thumb:

Rule 1: Sodium losses will vary from about 8 grams/hour to as high as 20 grams/hour, depending on work intensity and environmental temperature. Estimate your sodium loss by multiplying the hourly loss X the number of hours worked.

Rule 2: One gallon of normal saline contains 13.4 grams of sodium. If substituting a balanced electrolyte product for plain salt, use an amount per gallon that delivers up to 13.4 grams of sodium.

Example 1: You trained two hours at a steady, strong trot, including hills, on a hot summer day. Your horse sweats profusely. You estimate a 20 gram/hour sodium loss X 2 hours = 40 grams sodium. To replace this, the horse would have to drink just under 3 gallons of normal saline or equivalent electrolyte mixture. If he only drinks 2 gallons before you have to switch to plain water, that’s 26.8 grams of sodium, leaving you with about 13 grams you need to syringe in.

Example 2: You are out on a long trail ride in August. The pace isn’t fast but the horse is sweating enough to feel wet because of the heat. This horse is likely losing between 10 and 15 grams of sodium pe r hour.

When you stop to water your horse, check how many hours it has been since the last watering. If 2 hours, you need 20 to 30 grams of sodium. Add the correct amount of electrolyte supplement (e.g. If your supplement has 5 grams of sodium per ounce, you’ll need 4 to 6 ounces) and add that to the water so that it contains no more than 5 grams of sodium per gallon. Using the same example of a 5 grams sodium/oz supplement, you would add 1 oz of the supplement per gallon of water.

Example 3: Use the same scenario on the trail as in Example 2, but the horse will not drink the electrolyte supplemented water, or won’t drink enough. You can syringe in the needed electrolytes but only do so after the horse has consumed plain water. Use a bucket so that you know how much he drank, then syringe in the electrolytes at a rate of 5 grams of sodium per gallon of water consumed.

Bottom Line

Limit sodium to no more than 5 grams/gallon during work. More concentrated solutions, up to 13.4 grams of sodium/gallon, may be used initially following work, but make sure the horse has access to fresh plain water after this.

Maximize your horse’s pre-exercise hydration by a regular diet with generous fiber and salt/electrolyte supplementation, although you should avoid high-fiber feeding within a few hours of starting exercise. Use a soupy meal, like wet beet pulp, when you know your horse will face hard work on a hot day.

Consider adding a balanced electrolyte solution or even plain salt to water given during the day when working, up to 5 grams of sodium per gallon. Check the product labels, but most concentrated commercial electrolyte supplements contain 4.5 to 6 grams of sodium/ounce.

Calculate your likely sodium losses after work, using a range of 8 to 20 grams/hour, (low work intensity/low heat to high intensity/high heat). If you supplemented during the day, subtract the amount the horse has already received from the calculated total daily deficit.

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