Not all horses are alike in their needs for electrolyte replacement after strenuous exertion. Some deplete their body supplies more quickly or more extensively than others. Endurance horses generally have different needs for electrolyte replacement than do sprinters or cutting horses, and there also are individual differences among horses in any given sport in their need for electrolytes. Some can be helped immensely by judicious use of oral electrolyte supplementation to combat the effects of sweating and stress, while others rarely need it.

skin pinch test

The skin pinch test can be used to determine if a horse is dehydrated.

It makes a difference whether or not a horse eats and drinks before or during a competition (as in the case of a long endurance ride), for example whether he is a nervous or calm individual, and so on. A horse's individual metabolism also plays a role. The endurance horse doing aerobic (with oxygen) exercise has different needs than a racehorse doing anaerobic (without oxygen) exercise, or short, fast work. The properly conditioned event horse works aerobically like an endurance horse rather than anaerobically like a racing Thoroughbred.

The sprinter, or any horse which exerts strenuously for a short distance, relies upon energy stored within his muscles to supply the power for his efforts. The endurance horse, by contrast, or any horse which must keep up his effort for a long distance, cannot rely just upon stored reserves within the muscles, but utilizes the air he breathes to provide the oxygen necessary for the on-going "combustion"--the chemical process that produces the fuel for his working muscles. In other words, his exercise is aerobic, utilizing oxygen in the energy production process, while the sprinter's exercise is anaerobic, dependent upon the muscle's stores instead of oxygen.

Dissipating Heat

The equine athlete, like the human athlete, sweats when he exerts in order to cool himself. A horse trotting at seven miles per hour can lose up to 1 1/2 gallons of fluid per hour, and within that amount of fluid will be the equivalent of about three to four tablespoons of electrolytes, according to Frederick Harper, PhD, an Extension Horse Specialist with the University of Tennessee.

Working muscles produce extra body heat as one of the by-products of stepped-up metabolism and energy production. For his size and shape, the horse is not especially well designed to handle hot temperatures, so unless well conditioned to work in a hot environment, he is very vulnerable to heat exhaustion. As little as 20-50% of his energy is actually used to accomplish work (travel); the rest is converted to heat. Evolution programmed the horse to conserve heat more easily than dissipate it, since the horse evolved primarily in the cooler areas of North America (the primitive early horse), the steppes of Central Asia, and in northern Europe (except for the Arab-type horse which developed more recently in the hotter, drier North African climate).

Thus, a horse, designed to be good at conserving heat, must be conditioned to dissipate heat more efficiently when working in higher ambient temperatures. This by-product of his body's efforts must be released.

After trotting for one hour at 10 miles per hour, a horse has generated enough heat to boil 25 gallons of water. About two-thirds of this heat is released through the skin with the aid of sweat evaporation. Sweat cools the horse nicely, but only if the sweat can evaporate. High humidity can thwart the evaporation process and reduce its cooling effect by about 90%.

However, the horse is adaptable and can be conditioned to be more efficient in his sweating. Blood vessels in his skin gradually multiply and dilate as he trains in hot weather, bringing more blood closer to the body surface for cooling. Proper conditioning is the key. According to Tom Ivers, racehorse trainer and author of The Fit Racehorse, "If you condition, exercise will build heat tolerance in any horse."

Kerry Ridgeway, DVM, an endurance rider and the top veterinarian for many endurance rides, agrees. He says that hot weather acclimatization "takes place in 10 days, with larger, more efficient sweat glands, and a lower threshold for sweating."

The well-conditioned body is very efficient and might not sweat as much as the unconditioned body, but as exertion continues or increases, extra heat is produced and the body must be cooled to stay at a healthy temperature. Sweating is the best way to "keep the engine cool" and the body temperature within healthy limits.

According to Lon Lewis in his book Feeding and Care of the Horse, about 75-80% of energy produced in the horse's body is given off as heat, and heat production increases greatly during exercise. Even at gentle exercise (a trot, canter, or lope at nine to 11 miles per hour), heat production "increases 10 to 20 times over that produced at rest, and during sprinting it can increase 40 to 60 times. Without heat loss (through sweating or other means) this would increase the horse's body temperature to a life-threatening level above l06° Fahrenheit within four to six minutes. Even at 50% of exertion capacity, the horse's body temperature without cooling would approach the critical level after l0 minutes."

With the horse's normal methods of heat dissipation, these in-creases in body temperature are not so drastic. The horse sweats more the longer and harder he is worked, particularly if it is a hot day. To restore the fluids lost through sweating, he must increase his water and electrolyte intake. Otherwise, severe dehydration can lead to serious consequences.

The body tries to keep a constant water level and a stable temperature. Normal temperature ranges from about 99-l00.5° Fahrenheit. (Each individual horse will have a very consistent "normal" somewhere within that range.) Temperature rises with exertion; after a long ride or fast work, it's quite normal to have a horse show a temperature of l02° or even slightly higher. But if his temperature climbs above l03°, the horse is getting into an abnormal situation and his body will show signs of stress. He usually has sweated a lot by the time he is this hot (in an attempt to cool himself), for when faced with the choice of overheating or dehydrating, the body chooses to dehydrate, using up water to try to keep cool. A temperature that stays over l03° for very long or keeps climbing can be dangerous; if it goes above l08°, the horse could die.

Metabolic rate increases about 7% for each degree of temperature elevation. As temperature rises, the heart loses its ability to contract properly, beats more rapidly in an effort to move more blood to the skin for cooling, and might beat weakly and erratically. If high temperature persists, the body wears itself out from the increased metabolism and the attempt to cool itself.

Without the body's cooling mechanism, which utilizes part of its inner water supply, even moderate exercise would raise the horse's temperature about three degrees per hour, and the exertion would soon kill him.

There is a limit to the horse's water reserves, and as he begins to dehydrate from too much sweating (running out of extra body fluid to keep himself cool), his temperature climbs. So on a long ride, the effects of dehydration must be reversed or the horse can suffer serious health trouble.

Heat loss is im-paired in the poorly conditioned horse, the fat horse, and the horse which still has his winter coat. Lean horses dissipate heat best, and they also use less energy to move the body; hence, they don't tire as quickly. The lean, fit horse does much better on a long ride than the unfit or fleshy horse. This was shown in a study done during the l990 Race of Champions (a l50-mile, two-day endurance ride), evaluating the horses' body weight and condition. Horses were weighed before and after the ride and at veterinary checks. They also were given body condition scores (how lean or fat), and rump fat was measured in thickness. Average body fat of all the horses competing was estimated at 7.8%.

The most competitive horses--the top seven finishers--were leaner and had less rump fat than the horses which were eliminated due to fatigue and dehydration. Sixty-one horses started the race, and only 25 finished. The top finishers had only 6.5% body fat, while the horses which could not finish had 11% body fat. The lean, efficient horse, with less of his own weight to carry around and less insulating fat to interfere with heat dissipation, did best on the long, hard ride.

In another study, rectal temperatures increased 50% more in unfit horses than in fit horses after the same amount of exercise.

A horse which is not physically fit, or properly conditioned for hard work in hot weather, not only will sweat profusely, but also will lose more electrolytes and proteins through sweating than will a conditioned horse. The unfit horse has sticky, smelly sweat that lathers easily--because it contains more waste products and more lost electrolytes--while the well-conditioned horse's sweating is more gradual, more efficient, and cleaner and clearer.

Sweat is the main source of water loss during hot weather or exertion, and when exercising and sweating, a horse can lose up to four gallons of water per hour from his 70-plus gallon supply (the total water content of body tissues). Sweat glands are activated by heat and exercise and sometimes by fear and nervousness. They secrete water filtered from the small capillaries near the skin's surface, mixed with salt and other electrolytes, along with small amounts of protein, urea, and ammonia.

Fit Equals Efficient

The sweat of the fit horse works more efficiently to cool him, while compromising body chemistry the least. Horses which exert in hot weather should always have access to a salt/mineral supplement, because even moderate exercise can result in a daily loss of 50-60 grams of salt in the sweat.

Prolonged sweating can deplete electrolytes, throwing all body systems out of balance and hindering their functions. The horse that dies of "exhaustion" actually has run out of fluids and electrolytes; the body cannot keep functioning when it becomes too short of water and the salts that aid in proper movement of that water in and out of the cells.

In the exhausted horse, the combination of dehydration and loss of sodium results in "thickening" of the blood, decreased blood volume, and inadequate circulation. These factors can lead to impaired kidney function or even shutdown of the kidneys. The acid/base disturbance (due to depletion of calcium, potassium, and magnesium) can interfere with nerve signals, creating all sorts of problems.

Erratic nerve signals can contribute to digestive tract malfunction, irregular heartbeat, and muscle cramps or spasms. Some horses develop thumps (synchronous diaphragmatic flutter)--a jerking contraction of the abdomen in time with the heartbeat. Thumps is seen most frequently in endurance horses in hot weather, or horses doing prolonged hard work without adequate conditioning.

The horse has several water reserves--in the bloodstream, the digestive tract (about l8 to 20 gallons are stored there, half of which can be rapidly absorbed by the bloodstream if necessary), and between the body cells and within the cells themselves. When the bloodstream becomes short of fluid, it is replaced by some of the water from between the body cells, which in turn is replaced by water from within the cells. There is movement of fluids among all these areas, from one to the other, and this is how the horse compensates for large fluid losses--fluid is drawn from one area to replace the fluid lost in another. But if the horse has not had water for a time and the digestive tract is short on fluid, the cell water is not replaced and soon the functions of the cells begin to suffer.

Water can leave the bloodstream very quickly as sweat, but is replaced more slowly from fluids in and around the cells or the digestive tract. Heavy sweating uses water faster than increased drinking can replace it. To reverse the dehydration process, the horse must have plenty of water to drink, and the over-heating process and sweating must be slowed down. Having the horse drink along the trail at every opportunity is very important, but so is pacing the horse, allowing him time to adjust internally and not run out of reserves. Eating along the way (at any rest stop or vet check) also is good, since grass is a good source of water; it just takes longer for this moisture to become useful to his body.

You usually can't tell that your horse is dehydrated until he has lost at least four gallons of fluid, or about 32 pounds of body weight. By that time, his performance will be affected; he'll need to slow down. A loss of up to nine gallons (72 pounds of body weight) can be fatal.

A decrease in blood volume due to dehydration reduces blood flow to muscles and the skin. Decreased blood flow to the muscles hinders their performance and leads to fatigue or exhaustion. Decreased blood flow to the skin decreases heat dissipation by decreasing heat transport to the skin and by reducing sweating (which increases the risk and onset of heat stress or exhaustion).

A decrease in plasma chloride due to sweat loss can result in alkalosis (the pH of the body is changed too much toward alkaline). Alkalosis inhibits respiration, which results in less efficient excretion of carbon dioxide and less oxygen intake. This, in turn, reduces the availability of oxygen for energy production and speeds up glycogen depletion in the muscles and lactic acid accumulation. All this brings the horse to a point of inadequate energy production, fatigue, or exhaustion. Oxygen availability to the body tissues is further decreased by inadequate fluid flow into and around the cells, due to electrolyte depletion and the dehydration-induced decrease in blood volume.

A decrease in plasma potassium concentration decreases the strength and tone of the muscles. The decrease in potassium and sodium also decreases the horse's appetite and thirst response, which compounds the problem. He doesn't feel like eating or drinking enough to correct the electrolyte deficits and energy depletion.

Combatting Dehydration and Preventing Fatigue

Any time a horse will eat, he is not in serious trouble. He'll probably be more interested in green grass than in hay or grain. Grass can be as much as 50-90% water, depending on how lush it is, and it is the best possible food for a dehydrated horse after a long, hard ride. Hay, by contrast, is only 5-8% water, and can be pretty hard to eat if a horse is dehydrated. Grain is more apt to throw the stressed digestive system out of whack (and cause colic) than either grass or hay. At this point, the hard-worked horse is much more able to handle good-quality roughages than concentrates, and he might need his electrolytes replenished.

Work with endurance horses, which are probably stressed as hard or harder than any other type of equine athlete, has shown that most endurance horses do best with all the good-quality roughage they can consume (primarily grass hay, since alfalfa is too high in protein--which creates more heat as it is broken down and metabolized) and not much grain. The roughage in the digestive tract helps the gut act as a big "vat," holding moisture. Thus the horse doesn't dehydrate as readily on a 50- or l00-mile ride. The horse fed heavily on grain and not much roughage is not able to keep this reserve of moisture (and is also more apt to have digestive problems due to the stress of the ride).

To combat dehydration, a person should start conditioning a horse for strenuous performance slowly and gradually, especially in hot weather. It's best if you can start earlier, with some workouts in the spring when the weather is cool. If a horse is already in shape by the time hot weather arrives, his body will adjust more easily and be better able to handle it. Keep in mind if you are taking him to competitions in different parts of the country that a horse conditioned for strenuous activity in a dry climate might not do as well in humid conditions.

According to Heidi Smith, DVM, an Oregon veterinarian who rides and breeds endurance horses and acts as an attending veterinarian at many endurance rides (as well as having headed the Pan American ride in Oregon on Sept. 13), conditioning is imperative for horses which will be competing in hot weather. She points out that you can condition the horse's sweat glands to almost 90% greater efficiency, for as you gradually increase his work in hot weather, he will adjust and his sweat will become 90% more diluted, losing fewer electrolytes. In other words, you are helping him become able to sweat mostly water. The big problem with most under-conditioned horses is that they are sweating to cool off and losing too many valuable minerals in the process.

Smith advises against force-feeding electrolytes unless you are sure your horse needs them. The horse's body is usually quite remarkable at adapting. Just as his urinary system tries to preserve his body reserves of salt when he is short (by excreting very little through the kidneys and urine, and flushing out more when he has plenty), his sweat glands also can adjust their output, with gradual conditioning. However, if you are giving a horse supplemental electrolytes continually, you interfere with his ability to adjust to shortages, she says. He becomes dependent upon the supplement, and his kidneys open up their channels for getting rid of the additional salts instead of shutting down those channels for conserving electrolytes.

The best use of an oral electrolyte supplement is as a replacement when the horse is becoming depleted, as on a strenuous ride in hot weather. Electrolyte replacement can be very beneficial during or after a strenuous event. Electrolyte supplements are generally of little value before the exertion, and actually could be harmful, cautions Smith. The horse which is given more electrolytes than he needs will just excrete them through the kidneys in his urine. The horse cannot store the extra minerals--he has to flush them out of his system to keep his body chemistry in balance--and in doing so uses up some of his water reserves.

When he is stressed and beginning to get dehydrated and becoming short on electrolytes during his exertion, replacement of these crucial mineral salts is of great value. Electrolytes can be given orally, during or after the ride, either in tablet form, paste, or powder dissolved in water, added to feed, or mixed with a carrier such as syrup or applesauce and squirted into his mouth.

Severe and dangerous dehydration best can be reversed by giving fluids and electrolytes in sterile solution intravenously, by stomach tube, or into the rectum, to be rapidly absorbed. A horse which is severely dehydrated should have veterinary attention. Moderate dehydration can be reversed by giving the horse water and allowing him free-choice electrolyte powder alongside his grain, dissolved in his water, or given orally by dose syringe.

Different Horses, Different Needs

According to Smith, different horses have different genetic aptitudes for electrolyte conservation, and hence different needs. Some will require electrolyte supplementation if used strenuously, while others can adjust to the stress and manage fine with just access to salt or free-choice electrolytes. She suggests giving a horse constant access at home to loose salt, since he can eat it readily and replenish his sodium and chloride whenever needed. Since good roughage contains all the other necessary minerals, the horse which is a good eater often will do well with little or no supplementation, she says. This is why many endurance riders offer their horses all the good grass hay they can eat as their basic diet. The horse can load up on nutrients and electrolytes naturally the day before the ride. Grain also is used in the regular ration, but grass hay is the mainstay of the diet. The horse which keeps his ability to eat (grabbing some grass or hay at every vet check) and is not stressed out and off his feed from nervousness during a long hard ride, generally will perform better than the fussy or nervous individual who cannot relax or does not eat at every opportunity.

Smith says horses which are good eaters often do better on a ride if they are offered a free-choice loose salt-electrolyte mix. They then will eat only what they need to replenish their reserves, and you don't risk imbalancing their systems with force-fed electrolytes they don't need.

Eating on the trail is helpful to a horse which is not a good drinker. Green grass is about 70% water. It will take longer for this water to become useful to his body, but grass is a good source of water and electrolytes. Another way to get a horse to eat more and consume a little more water in the process is to water a horse's hay at the rest stops or vet checks. This makes the hay more palatable to the thirsty horse and can increase his moisture intake.

The nervous horse which won't eat, or the horse with a different genetic makeup and metabolism, can get into serious trouble without oral supplementation during exertion. Smith points out that the nervous horse is in double jeopardy if he gets nervous diarrhea before or at the beginning of a ride, losing both fluids and electrolytes in his many loose bowel movements. He cannot service both losses (diarrhea and sweating) at the same time, and he quickly depletes his stores, becoming dehydrated and short on electrolytes very early in the ride. This type of horse will do better--and not be at such a high risk of serious dehydration and exhaustion--if he is given oral supplements during the ride and will drink adequately at every opportunity.

Give the horse a chance to fill up on water before a long ride, and let him drink whenever he will during the ride. Some riders are afraid of watering a horse when he's hot, but unless the water is ice-cold, most horses are much better off to be allowed to drink whenever they want water so they won't become dehydrated. Even endurance horses which drink as much as possible develop slight to moderate dehydration on long rides in hot weather. Fluid losses of 6-l0 gallons or more are common.

A severely dehydrated horse might refuse to drink even though he desperately needs fluid. This is because he sweats out more salt than water in his fluid balance. The electrolyte "trigger" (rising salt content in body fluids) is what normally sends his brain the signal to be thirsty. But electrolyte depletion is now depressing his thirst alarm; his normal thirst response is out of order and fails to alert his senses to the crisis of his dehydration.

During and after a hard ride, especially if the day is hot and humid and the horse is sweating, you might want to sponge him with water to help cool him and conserve body fluids, so he won't have to sweat so much. Cool him out carefully, bringing his temperature down to normal and replenishing his water and electrolytes, but taking care not to chill him because this will add more stress to his fatigued systems. Cold water all over his body can chill the skin and stop his sweating reflex, interfering with his own cooling system. Use cool water over the areas where there are many large veins (bringing the overheated blood to the skin surface) such as the head, inside the upper legs, and under the neck. If a horse's temperature is l04° or higher after prolonged exertion, the fastest and safest way to cool him is to apply cold water to these specific areas, but not all over his body.

If the air is very dry or windy, causing moisture to evaporate quickly (which can over-cool a horse), most horses should not be sponged with cold water. Under dry or windy conditions, a horse generally will cool out rapidly on his own with just a normal rubdown and grooming. On cool or windy days, you might even need to walk or blanket the horse after strenuous exercise so he won't cool out too fast.

Anyone who competes in strenuous athletic events or uses a horse hard for any purpose needs to be very familiar with the horse's abilities, conditioning, and body needs. One also needs to be aware of the signs and effects of dehydration and fatigue, and how to help prevent and combat excessive fluid and electrolyte loss. Some degree of dehydration goes hand-in-hand with strenuous performance, so the good horse owner must know how to cope with this aspect of equine athletics.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More

Free Newsletters

Sign up for the latest in:

From our partners