Dehydration and Electrolyte Losses in the Sport Horse

You can lead a horse to water but you can't make him drink. We're all familiar with this maxim that has long been used to underscore the apparent stubbornness of the horse. In the heavily exercising, or heat-exhausted horse, however, this refusal to drink has nothing to do with personality or temperament, and everything to do with physiology.

Dehydration occurs when your horse's body loses excessive amounts of water. Normally, both you and your horse lose body water on a continual basis, in the form of sweat, urine, and feces. However, on an ordinary basis, you replace these losses easily – your body tells you that you are thirsty, and you drink fluids. However, sometimes the losses are too great for the body to keep up with.

In the exercising horse, fluid loss occurs in the form of sweat. Early in dehydration, the horse can cope well with the fluid loss. Dehydration is estimated in terms of percentage of body weight that the horse has lost. It is very difficult to detect losses of 5 percent or less.

During severe dehydration, the skin becomes less elastic, which is seen with skin tenting: When you pull up a loose fold of your horse's skin, it takes a long time to return to its normal state; a horse with 10 to 12 percent dehydration, the skin fold takes 20 to 45 seconds to disappear. As dehydration progresses the heart rate rises, because there is less fluid in the blood vessels, so the heart has to pump the blood around faster to achieve the same effect. Your horse urinates less frequently, or not at all, and his performance deteriorates, as dehydration contributes to exhaustion. Eventually, with severe dehydration, your horse will no longer be able to perform, and may even collapse.


At the most basic level, electrolytes are salts, such as table salt, that dissociate into separate ions when they are dissolved in water. Electrolytes are integral to nerve and muscle function, as well as to almost every other physiological function in the body. The most important electrolytes include sodium (Na+), chloride (Cl-), potassium (K+), calcium (Ca2+) and magnesium (Mg2+). These electrolytes are distributed throughout the body in a highly ordered way, and any disruption of this order can result in severe bodily dysfunction. For example, when the body's electrolyte levels are disturbed, your horse may have heart problems, his gastrointestinal system may not work properly, his muscles may develop cramps, and he may not even be able to think straight, because his brain may not function normally.

Your horse – or you or any mammal – is approximately two thirds water. So, for an average-sized 1000 pound horse, water accounts for 660 lbs of his body mass, which translates into 80 gallons of water. Of that 80 gallons, one third is extracellular, which means it is outside of the cells of the body in the form of blood and lymph fluid. Two thirds is intracellular, which means it stays inside the cells.

The electrolyte composition of the extracellular fluid (ECF) and intracellular fluid (ICF) is very different. The ICF is very high in potassium, whereas the ECF is very high in sodium. We use blood values to estimate whole body electrolyte composition, but they may be quite misleading, because they don't tell us about what is happening with the intracellular fluid.

All athletes lose water as sweat during exercise – it's part of an adaptation for getting rid of excess heat. As the sweat evaporates, your horse's body cools down. This is the primary reason why it is more difficult to exercise in high humidity – the sweat is still generated, but it doesn't evaporate, and thus doesn't cool effectively, so even more sweating is stimulated.

Equine sweat glands are different from human sweat glands, with the result that horses lose more electrolytes during sweating than humans do. Horses lose large amounts of both sodium and chloride in their sweat, with smaller losses of calcium and potassium. Water losses are high during exercise – up to 10 liters per hour. The rate of water loss will depend on the intensity of exercise as well as the ambient temperature.

Your horse's body gets the signal that he is thirsty when one of two things happens: either the blood volume drops or the sodium concentration in the extracellular fluid increases – that is, it gets concentrated, or salty. Horses get the thirst signal more slowly than humans, because in humans, sodium is less concentrated in sweat, so sodium becomes more concentrated in the blood, and the thirst signal goes out quickly.

In horses, the sweat glands are very poor at conserving sodium so even though the horse has lost a large amount of water and sodium, the signal does not go out for the horse to drink until a serious drop in blood volume occurs due to dehydration. Consequently, even though your horse is clearly dehydrated, when you lead him to water you can't make him drink. He's not stubborn – his body is just not giving him an early enough warning signal.

What to Watch For

After a challenging cross-country phase, a few chukkers of polo, or an endurance ride, your horse is likely to be quite dehydrated. Losses tend to occur early in the ride, even though your horse may look normal. Signs may include:

Your veterinarian will look for:

As dehydration proceeds, the horse may eventually collapse. In addition to the dehydration, your horse is also likely to have a whole body loss of electrolytes. Signs of low electrolyte levels may include

Researchers at Washington State University have recently found out that the body's hormones still work to recover lost sodium the day after intense exercise – even though the horses shows no outward signs of electrolyte depletion.

Water requirements vary greatly according to the weather and the level of work that the horse is doing. For instance, if your horse is exercising in hot, humid weather, he may need 2 to 4 times the minimum amount. A good rule of thumb is that a horse needs at least a gallon of water per 100 lbs of body weight. For your average horse, this equals 10 gallons a day.

If you need to get your horse to drink more, the first, and most important thing, is to make sure that he has continual access to water. Also, horses tend to drink less water in the winter if the water is cold. Studies have shown that horses will drink more water if it is warm or tepid, so, get yourself a water heater, and don't expect your horse to drink the icy cold water.

On the road, some horses are very picky about foreign water. Some experienced competitors advise bringing enough water from home, and getting your horse used to drinking flavored water. Many horses enjoy water flavored with apple juice. Horses will drink more when it is held up to them after and during competition. You should try to offer your horse water in a quiet area, where he will not be disturbed by all the action around him.

One wet-down flake of hay can absorb 1 to 2 gallons of water. If you feed your horse well-soaked hay, you can make a real impact on his fluid consumption. Endurance riders take advantage of this by feeding horses soaked hay before long rides.


It is possible to give your horse too many electrolytes and they can be toxic in excessive amounts. Requirements for salt (NaCl) range from one tenth of a percent of the ingested feed for a broodmare, to three tenths of a percent of the ingested feed for a competition horse. The maximum amount of NaCl that a horse can tolerate before he becomes toxic is three percent of the ingested feed. It would be unusual for a horse to ingest that much salt, as the food containing it would have an unpleasantly salty taste.

If you start to give your horse to many electrolytes, you will start to notice that your horse is urinating frequently, and drinking much more water. Some veterinarians do not recommend adding electrolytes to water. If you choose to supplement this way, it is VERY important that you also always offer your horse a bucket of water with NO additives. Your horse cannot, and should not, take in enough water with electrolytes to sustain his fluid requirements, and will become dehydrated if you do not give him access to plain water. In general, it is a better idea to supplement electrolytes in the feed, rather than in the water.

Horses can generally be supplemented from 1 to 2 ounces of salt per day. Most commercial grains contain from 0.5 percent to 1 percent added salt, in addition to potassium and calcium. Because horses eat considerably more hay and other forage than grain, the horse's total ingested salt will be less than 1 percent. This is more than adequate for a pleasure horse, but is usually not enough for heavily exercising horses.

Salt can be supplemented in the form of a salt block (a trace mineral block is best), or as free salt added to the grain. This works best with a grain that has some sort of binding agent, such as molasses. The amount of salt your horse gets from a salt block depends on whether your horse has a taste for the salt block. Studies have shown that some horses fail to take in even the minimal requirements if they are simply given a salt block.

After Competition