Equine Electrolytes, Exercise, and the Heat

By Eleanor Kellon, VMD, staff veterinary specialist for Uckele Health & Nutrition

Many riders look forward to the summer season as the ideal time to ride or work their horse. Most understand the dangers of working horses under high heat and humidity conditions. However, horses can be compromised under less than sweltering conditions if you do not know how to protect them properly.

Dehydration through sweat loss is the major danger. Sweat loss totaling as little as a 3% of body weight can cause almost a 10% reduction in the horse's exercise tolerance. For a 500 kg (1,100 pound) horse, that's only 15 liters of sweat. Research has shown that a horse sweating heavily can lose as many as 16 liters of sweat in just one hour. Even horses working in milder weather conditions or horses turned out in hot weather and sweating can lose around four liters of sweat per hour.

Providing an adequate supply of palatable water is a huge factor in avoiding dehydration. However, water is only part of the answer. For the horse's body to hold onto the required amount of water, electrolyte levels must be correct.

Sodium, potassium, and chloride are the major equine electrolytes. Bicarbonate is also important but the horse's body can produce that from carbon dioxide and water as needed, and bicarbonate is not lost in sweat.

Calcium and magnesium are also lost in sweat but in much smaller amounts than sodium, potassium, and chloride. Sweat contains only a few hundred milligrams of these minerals, compared to thousands or tens of thousands milligrams of sodium, potassium and chloride.


Sodium is the electrolyte the horse's body "reads" in the brain to determine whether to send out the impulse to drink more water. If sodium concentration in the blood increases in relation to water, the brain will send out the message to drink. If sodium content in the body is low, blood concentration will not increase enough to trigger drinking even if there is a significant reduction in body water (dehydration).

A 1,100-pound horse loses 20 grams of sodium per day in bodily fluids, not including sweat. This equates to about 1 ounce of plain table salt (sodium chloride). A horse standing around and not exercising but under high heat conditions might require 2 to 3 ounces just to meet basic losses without even being exercised.

When selecting an electrolyte supplement, it is critically important to first meet those baseline needs. Some concentrated commercial electrolyte supplements might only supply about six grams of sodium per ounce (and others much less). Therefore, just to meet baseline sodium requirements a minimum of 3 to 9+ ounces of electrolyte mix would have to fed per day before it would start to replace the sweat electrolytes. Take this into consideration when selecting a supplement.


Potassium is present in sweat at about half the level of sodium. Potassium content in hay runs from two to over four times the baseline requirement, depending on the type of hay provided. A rule of thumb some veterinarians advocate is that a horse getting 2% of body weight/day (22 pounds for an 1,100-pound horse, for example) in hay or equivalent pasture time can exercise for up to two hours and not need any supplemental potassium.

If a horse consumes less than 2% of his body weight/day, he might need electrolyte replacement supplements sooner.

Low blood potassium is common in horses that are stressed by heat. However, this doesn't mean that potassium intake is inadequate. The body puts a priority on preserving sodium. If levels are low, the kidneys will excrete more potassium instead of sodium. To remedy low blood potassium, increase sodium to meet needs.


Chloride is especially important for horses that are working several hours in hot conditions, whether endurance racing or just trail riding. Chloride is lost via sweat at twice the rate of sodium (which, as mentioned, is twice the level of potassium).

If chloride drops, bicarbonate levels will rise and cause a condition called alkalosis (the body's pH changes too much toward alkaline). Alkalosis, in turn, binds up ionized/electrically charged forms of calcium and magnesium, which can result in colic or (http://www.thehorse.com/ViewArticle.aspx?ID=12446) thumps. Supplying calcium and magnesium ions intravenously corrects this condition, but ensuring the horse consumes adequate chloride ensures the condition doesn't happen in the first place.

Until the 2007 National Research Council recommendations were released, there was no target chloride intake for horses. That has now changed, however. Hay and pasture are major sources of chloride, and more comes from meeting sodium requirements by feeding plain salt (i.e., sodium chloride).

How to Deal with Hot Weather

A few simple steps can improve your horse's water and electrolyte status in the heat:

  • Do not rely on salt intake from licking. Add salt directly to meals and/or mix in water and spray on hay;
  • Feed about 2 ounces of plain salt or an electrolyte product daily;
  • Always let your horse drink freely during exercise lasting longer than two hours and immediately after stopping work. Research has shown horses that have their water aaccess restricted while cooling out do not drink as much in total as horses with unrestricted water access;
  • Choose an electrolyte product that has approximately twice as much sodium as potassium and twice as much chloride as sodium; and
  • Follow the instructions below for determining how much you need to give to replace sweat losses.

Choose an electrolyte sweat replacement supplement close to a potassium:sodium:chloride ratio of 1:2:4. Horses consuming generous forage can often use a supplement with lower potassium levels.

Take-Home Message

Ensuring your horse consumes his baseline sodium, potassium, and chloride requirements first and then using electrolyte supplements as needed to replace sweat losses will maximize performance and protect against problems caused by dehydration and electrolyte imbalances.

About the Author

Uckele Health and Nutrition

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