With the summer months approaching, it is time for a refresher on the impact of the warmer environmental conditions on your horse. Warmer temperatures and high relative humidity place added stress on horses during exercise because of their increased reliance on sweating for control of body temperature. A major consequence of sweating is the loss of large quantities of water and electrolytes. Without adequate replacement of those losses, a horse's performance will suffer and, worse, there is potential for development of serious medical problems during or after exercise.

In this article, I will discuss fluid and electrolyte losses in the horse during exercise, the impact of those losses on performance and health, and various strategies that can be used to ensure adequate replacement of fluids and electrolytes.

Water And Electrolyte Balance

Water makes up slightly more than 65% of a horse's body mass. For a 1,200-pound (550 kg) horse, this constitutes more than 350 liters of water! This water is distributed into two main fluid compartments. The first is the intercellular compartment and refers to water inside the body's cells. Fluids surrounding cells represent the extracellular compartment. Extracellular fluid includes blood plasma, the interstitial fluids (the fluid that flows in the microscopic spaces between cells), and fluid within the gastrointestinal tract. Two-thirds of total body water is in the intracellular compartment (see Figure 1 on page 88). Up to 60 or so liters of fluid can be present in the large intestine, and this fluid reservoir is drawn upon during endurance-type exercise. Diets high in fiber increase the size of the gut fluid reservoir, which is a real benefit for the endurance horse.

How much water does a horse need on a daily basis? As a general rule, a sedentary (couch potato) horse requires a bare minimum of about 30-35 ml of water per kg body weight per day for maintenance of water balance--this equates to about 17 to 20 liters or four to five gallons for a 1,200 lb horse (see Figure 2 below). Since feeds consumed by horses contain some water, a fair amount of this requirement is met by the diet. Horses at lush pasture will receive much of their water needs by eating grass. On the other hand, diets high in dry roughage sources (e.g., hay) increase water needs because of the water-holding capacity of fiber in the large intestine

Two factors that can increase a horse's water requirement greatly are the level of exercise and climate. As discussed, sweating is the main means by which horses lose heat during exercise. The hotter the climate, the greater the sweat losses--in fact, when it is hot, sweating becomes important for regulation of body temperature even at rest. Those losses must be balanced by an increase in water intake. Practically speaking, the horse is the best judge of his needs. Our job is to ensure that there is plenty of fresh water available to the horse. At least two buckets should be hung in a horse's stall--in hot weather, those will require refilling at frequent intervals.

What About Electrolytes?

The minerals sodium, potassium, chloride, calcium, and magnesium collectively are termed electrolytes. When dissolved in body water, those minerals are electrically charged particles called ions. Sodium and chloride are the primary electrolytes contained in blood plasma and extracellular fluid, while potassium is the chief intracellular electrolyte. These electrolytes are critical for a large number of body functions. For example, electrolytes modulate fluid exchange between the body's fluid compartments and regulate acid-base balance. Sodium and potassium are important for the establishment of proper electrical gradients across cell membranes. Calcium and magnesium also are important in this regard. These electrical gradients are vital for normal nerve and muscle function--electrolyte deficiencies or imbalances therefore can impair nerve and muscle function.

The kidneys are of prime importance in maintaining electrolyte balance. Some electrolytes are lost in manure, but most of the fine tuning is done by the kidneys. In general, sodium is highly conserved--very little is excreted in the urine. On the other hand, the kidneys excrete substantial quantities of potassium and calcium on a daily basis. This is partly due to the fact that dietary intake of potassium and calcium tends to be high.

For athletic horses, electrolyte balance is markedly altered by losses in sweat. Let's briefly review why horses sweat and the extent of water and electrolyte losses during exercise.

Temperature Regulation During Exercise

A large amount of heat is produced by contracting skeletal muscles during exercise because the process of converting stored energy (e.g., muscle glycogen) to mechanical work is only 20% to 25% efficient. This means that 75% to 80% of this stored chemical energy is converted to heat within the muscle cells. The rate of heat production is directly related to the intensity of exercise--the faster the horse runs, the greater the rate of heat production. Similarly, steep terrain, difficult footing, and weight carriage (the rider and tack), are all factors that increase the work effort and the rate of heat production.

To prevent dangerous increases in muscle and body temperature, this heat must be removed from the body. There are four basic means by which this occurs:

1) radiation--heat energy is emitted at the body surface;
2) conduction--heat is transferred to the blood flowing through the muscles and redirected to the skin via the circulation;
3) convection--heat transfer between the skin surface and the surrounding air; and
4) evaporation--conversion of a liquid--e.g., sweat fluid--to a vapor, with a resultant loss of heat energy at the skin surface.

Of these four methods for heat loss, evaporation is by far the most important for the exercising horse. Although evaporation of sweat on the skin surface accounts for up to 70% of heat loss during exercise, a substantial amount of evaporative heat loss also occurs through the respiratory tract. In fact, during low- and moderate-intensity exercise, up to 25% of the heat load is lost through the lungs and nasal passages.

Environmental temperature and relative humidity profoundly influence the effectiveness of these heat loss mechanisms. At best, heat loss by radiation and convection accounts for about 10% of the heat that must be lost from the body during and after exercise. As the air temperature increases, the effectiveness of these mechanisms decreases--when ambient temperature approaches the horse's skin temperature (low to mid 90s Fahrenheit), very little heat is lost by convection and radiation. In fact, the horse might gain heat from the surrounding air and the sun (radiant heat).

Therefore, in warm environmental conditions, evaporation is the primary means for heat loss. As body temperature rises, the horse will begin to sweat and, while trotting, there will be an increase in breathing rate to increase evaporative heat loss through the respiratory tract. Although these mechanisms for heat loss are very effective, there is a price to pay--heavy losses of essential water and electrolytes.

The effectiveness of evaporative heat loss is affected by the relative humidity--that is, the degree of saturation of the air with water. Efficient evaporation, both in the respiratory tract and on the skin surface, occurs when the air is very dry and therefore able to accept moisture in the form of water vapor. As relative humidity increases, this gradient for transfer of water vapor diminishes and, as a result, heat loss by evaporation is greatly decreased. We all have recognized this phenomenon--when it is hot and humid, it seems as if we (and our horses) sweat more compared to a hot, less humid day. Actually, sweating rates in these two climates are similar. Whereas sweat fluid quickly evaporates at low humidity, when it is humid, the skin surface remains wet and sweat will drip from the body. Importantly, dripping of sweat removes very little heat from the body, although it does remove water and electrolytes. Thus, loss of body heat becomes a serious problem for horses during exercise in hot, humid conditions.

Exercise-Associated Water & Electrolyte Losses

Just how much fluid and electrolytes are lost by the horse during exercise? Two main factors affect the extent of these losses--the duration and intensity of exercise and the environmental conditions. During steady exercise in cool and moderate conditions (less than 60-70° F), horses will lose about six to seven liters of sweat (1.2% of bodyweight for a 1,200-pound horse) per hour of exercise. However, the rate of sweat fluid loss increases in warmer conditions. Fluid losses can approach 15 liters (2.7% of bodyweight) per hour when it is very hot.

For an endurance horse performing under average summer conditions, sweat fluid losses are around 10 liters per hour. Even when water is consumed during the course of a 50- or 100-mile ride, horses can lose 40 kg (7% of body weight). During the speed and endurance test of a three-day event, body weight losses of 20-25 kg have been measured in equine competitors. Bear in mind that not all of this weight loss is due to sweating--evaporation of water through the respiratory tract contributes to the total water loss.

The sweat of horses is rich in electrolytes, particularly sodium, chloride, and potassium, although smaller quantities of calcium and magnesium also are lost. Figure 3 on page 92 provides an illustration of the huge quantity of sweat fluid and electrolytes that can be lost during exercise (see also Kingston et al., 1999). Horses completed 45 km (28 miles) of low intensity (steady trot) exercise over a three-hour period. On average, body weight loss was approximately 33 kg--of this loss, about 25 kg was sweat fluid that contained about 250 grams (9 ounces) of electrolytes. Importantly, the ambient conditions for these experiments were moderate (about 70°F, 45% relative humidity). Much greater sweat fluid and electrolyte losses can be anticipated in hotter conditions.

Dehydration, Electrolyte Imbalances, & Performance

Without replacement of these losses, the horse will become dehydrated and develop electrolyte imbalances, both of which can adversely affect exercise performance. Decreases in body electrolyte content also suppress the horse's thirst response, thus worsening the level of dehydration, particularly with continued exercise and sweat loss. When body weight deficits exceed 4%-5% (a net water loss of 23-28 liters), horses are less able to lose body heat during exercise--in other words, body temperature will increase at a faster rate (see Geor and McCutcheon, 1998). In hot weather, the combination of dehydration and exercise can be dangerous because of the risk of overheating and development of heatstroke.

Electrolyte imbalances also are of concern. "Thumps" (synchronous diaphragmatic flutter), muscle cramping, and "tying-up" (exertional myopathy) are potential consequences of heavy sweating and associated electrolyte losses. For horses required to perform sustained exercise (e.g., endurance racing, three-day eventing, or hunting), dehydration and electrolyte loss contribute to the development of premature fatigue. The worst-case scenario involves development of the "exhausted horse syndrome," a life-threatening medical disorder associated with severe fluid and electrolyte loss, glycogen depletion, and overheating.

Fluid & Electrolyte Replacement Strategies

Given the dire consequences of dehydration and electrolyte imbalances, our task is to ensure that the horse is provided with adequate fluid and electrolyte replacement. Of prime importance is daily electrolyte supplementation. For horses on a predominantly hay or pasture diet, potassium intake usually is in excess of daily requirements. On the other hand, sodium and chloride intake often is marginal, even in horses which are relatively inactive and therefore not losing a great deal of electrolytes in sweat. Therefore, some form of salt supplementation is a must.

A commonly used strategy for sodium supplementation is provision of a salt block. For non-exercised horses, voluntary salt intake from such blocks probably is adequate to meet sodium and chloride requirements. However, recent research in athletic horses has demonstrated that voluntary sodium intake from a salt block is highly variable (see Jansson and Dahlborn, 1999). In that study, the sodium intake of four of six horses was well below even the maintenance requirement. For that reason, a salt block is not the ideal source of salt for athletic horses. By all means, leave a salt block in the horse's stall or feed bucket. However, salt also must be provided by other means.

An alternative strategy is to "top-dress" a salt supplement on the horse's grain ration. This supplement can be in the form of common salt, a combination of common salt and "Lite salt" (potassium chloride), or commercially available preparations. There are numerous electrolyte supplements on the market, some designed for addition to the horse's feed and others for administration to horses before and during athletic activities. Read the product labels carefully and take note of the actual quantities of sodium, chloride, and potassium in the supplement. Avoid supplements that contain a lot of sugar and not much else.

How much salt should be added to the ration? For horses in regular training, but in cool to moderate ambient conditions, I have added 2 ounces (56 grams) of salt to a horse's diet, usually split between the morning and evening feedings. During the summer months, when sweat losses are likely to be much higher, I recommend 3 to 5 ounces (75-125 grams), either straight salt or a 3:1 ratio of common salt and Lite salt. In reality, the horse will receive ample potassium providing there is adequate good-quality hay in the diet. Buy a measuring spoon (1 ounce size), and keep it in the barn. The table on page 90 provides more details on daily electrolyte requirements for working horses.

Another important consideration is electrolyte supplementation before and during prolonged exercise, such as endurance racing. Experienced endurance riders know that provision of adequate water and electrolytes is critical for prevention of medical problems during a race. Typically, electrolytes are mixed as a paste (often with applesauce or blueberry yogurt) and administered to the horse via a dosing syringe. These pastes are given before and at regular intervals during the race.

Again, either commercially available electrolyte supplements or homemade concoctions can be used. In addition to sodium, potassium, and chloride, the supplement should contain calcium and magnesium because disorders in calcium and magnesium balance contribute to development of medical disorders such as "thumps" and muscle cramping.

Recent research has shown that administration of electrolyte pastes before and during prolonged exercise is safe and efficacious. For example, in one study Arabian horses were given either water or an electrolyte paste before and during 60 km (38 miles) of treadmill exercise (D'sterdieck et al., 1999). The exercise was divided into four bouts of 15 km. Ninety minutes before exercise, the supplemented horses were given 75 grams of a 2:1 salt-Lite salt mixture, with further 38-50 gram doses given at rest beaks between exercise bouts. Horses given the electrolyte supplement 1) consumed twice as much water (24 liters) compared to the non-supplemented horses; 2) lost less weight during exercise (0.8% of bodyweight vs. 2.8% for the non-supplemented horses); and 3) maintained a higher blood sodium and chloride concentrations.

In practice, some endurance riders will administer larger doses of electrolytes, both before and during exercise. Others have had some success in training horses to drink mildly salty water (e.g., 1 ounce of salt in 1 gallon of water). Regardless of the strategy used, horses must be acclimated to these procedures during training rides. Another key point is ensuring that the horse has access to water after administration of the electrolyte pastes. To gain maximum benefit from the administered electrolytes, the horse must consume water. Likewise, at rest stops and after completion of exercise, the horse must be offered water.

Among horsemen, the practice of allowing a hot horse to drink often is frowned upon because of the risk of colic and "cold water founder." Although I do not recommend offering water to a horse immediately after very hard exercise, such as racing, my experience (and those of others) with endurance-type work has been that the first one to two minutes after exercise is the best opportunity for rehydration. When offered water during that period, horses will readily consume six to 10 liters of water. Importantly, water consumption is higher if the water bucket is offered right after stopping. Make sure water is available to the horse as soon as possible after entering a rest stop and at the completion of exercise.


D'sterdieck, K.F., Schott, H.C., Eberhart, S.W., Woody, K.A., Coenen, M. (1999) Electrolyte and glycerol supplementation improve water intake by horses performing a simulated 60 km endurance ride. Equine Veterinary Journal Suppl. 30: 418-424.

Geor, R.J., McCutcheon, L.J. (1998) Hydration effects on physiological strain of horses during exercise-heat stress. Journal of Applied Physiology 84: 2042-2051.

Kingston, J.K., McCutcheon, L.J., Geor, R.J. (1999) Comparison of three methods for estimation of exercise-related ion losses in sweat of horses. American Journal of Veterinary Research 60: 1248-1254.

Jansson, A., Dahlborn, K. (1999) Effects of feeding frequency and voluntary salt intake on fluid and electrolyte regulation in athletic horses. Journal of Applied Physiology 86: 1610-1616.

McCutcheon, L.J., Geor, R.J., Hare, M.J., Ecker, G.L., Lindinger, M.I. (1995) Sweating rate and sweat composition during exercise and recovery in ambient heat and humidity. Equine Veterinary Journal Suppl. 20: 153-157.

McCutcheon, L.J., Geor, R.J. (1998) Sweating: fluid and ion losses and replacement. Veterinary Clinics of North America: Equine Practice 14: 75-95.

Schott, H.C., McGlade, K.S., Molander, H.A., Leroux, A.J., Hines, M.T. (1997) Bodyweight, fluid, electrolyte, and hormonal changes in horses during and after recovery from 50- and 100-mile endurance races. American Journal of Veterinary Research 58: 303-309.

Schott, H.C., Hinchcliff, K.W. (1998) Treatments affecting fluid and electrolyte status during exercise. Veterinary Clinics of North America: Equine Practice 14: 175-204.

Sosa-LeÛn, L.A., Hodgson, D.R., Carlson, G.P., Rose, R.J. (1998) Effects of concentrated electrolytes administered via a paste on fluid, electrolyte, and acid-base balance in horses. American Journal of Veterinary Research 59: 898-903.


About the Author

Ray Geor, BVSc, PhD, Dipl. ACVIM

Ray Geor, BVSc, PhD, Dipl. ACVIM, is professor and chairperson of Large Animal Clinical Sciences at the College of Veterinary Medicine at Michigan State University

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