Water and Electrolyte Balance in the Exercising Horse
The following paper was presented to the Horse Industry Breeders Conference: Sponsered by the Horse Section of Alberta Agriculture.
Last Revised/Reviewed May 8, 1996
Laurie Lawrence, Ph.D. Department of Animal Sciences, University of Kentucky, Lexington, KY
Exercise generates a great deal of heat that must either be dissipated from the body or stored. The dissipation of heat is very important, because if a large amount of heat is stored, body temperature will rise to dangerous levels. In the horse, the processes that contribute to the dissipation of heat during exercise include radiation, convection, conduction and evaporation. Of these, evaporation may be the most important; particularly when horses are exercised in warm conditions. To facilitate evaporative cooling, horses sweat. Sweating is certainly desirable as a means to maintain body temperature, but high sweating rates result in high rates of water and electrolyte loss. The following discussion will address the effects of water and electrolyte losses on exercising horses as well as some strategies for helping horses maintain fluid and electrolyte balance during competition.
In a practical setting, it is difficult to accurately measure the total amount of sweat that a horse loses during an exercise bout. However an estimate of sweat losses can be obtained by weighing horses before exercise and after exercise.
We have recently been involved in weighing horses before and after workouts, races or other events to estimate sweat losses. At this time we have not weighed enough horses in different environmental conditions to have solid estimates of sweat losses in all situations but some examples of weight losses that we have observed in field conditions are shown below.
Range of weight losses when horses were weighed before and after various events**
Type of Horse
| Range of Weight Loss |
| Standardbred Harness Horses |
(before and after a 1 mile race)
|12 to 33|
| Field Hunters |
(before and after 3 hours of fox hunting)
|24 to 100|
| Thoroughbred Horses in Race Training |
(before and after galloping or breezing)
|10 to 16|
| Endurance horses |
(from the night before a race to the end of 54 miles)
|22 to 88|
** These values may not apply to all horses performing these types of events
Almost without exception, the owners, trainers or riders/drivers of the horses are amazed at the weight differences of their horses before and after work. Although there are often fecal losses during exercise, weight loss during most types of exercise is generally thought to be about 90% water, so clearly water losses during some activities are quite large.
Where does the water in sweat come from? The horse's body is about 65% water. Most of the water is contained in cells (intracellular water) but some is outside of individual cells (extracellular water). Blood plasma, which constitutes a large portion of the horse's blood volume, is an important component of the extracellular water pool. When horses (or humans) sweat, some of the water in sweat is obtained from the plasma volume. Consequently, if sweat losses are large, the plasma volume may decrease. A reduction in plasma volume (and thus total blood volume) may affect the ability of the horse to maintain adequate blood flow to muscles during work. Progressive dehydration may also result in a reduction in sweating rate and thus an increase in body temperature.
Horse sweat contains many things besides water. Horse sweat contains calcium, magnesium, some trace minerals and protein. However, most notably, equine sweat is relatively high in sodium, chloride and potassium. When horses lose large volumes of sweat, they lose considerable quantities of these electrolytes. Although we have not measured the amount of electrolytes actually lost during various events, it is possible to roughly estimate electrolyte losses if the approximate sodium, potassium and chloride content of sweat is known. Many researchers have measured the amount of the various electrolytes in sweat. Chloride is present in the highest concentration, but sodium and potassium concentrations are also reasonably high. The following table provides estimates of the sodium, chloride, and potassium losses that might be experienced by horses in various activities, given the weight losses listed earlier.
Range of Estimated Electrolyte Losses of Horses Performing Different Activities**
Type of Horse
| Sodium |
| Potassium |
| Chloride |
| Standardbred |
(during a race)
| Field Hunters |
(3 h foxhunt)
| Thoroughbred |
(during a work)
| Endurance Horse |
(54 mile ride)
** These values are estimated from weight loss during exercise and approximate composition of equine sweat. Many factors could cause the actual values for a particular horse to be higher or lower.
It is apparent that heavily sweating horses experience large electrolyte losses as well as large fluid losses. Electrolytes have many functions including maintenance of acid-base balance in the body fluids and nerve and muscle function. Large losses of electrolytes can result in several neuromuscular and systemic disturbances including muscle cramping, tieing up, synchronous diaphragmatic flutter (thumps) and systemic alkalosis. Horses with large electrolyte losses may also have reduced sweating rates and therefore a reduced ability to manage body temperature. Electrolyte concentrations in the blood may play a role in the horse's thirst response or desire to drink. Ironically, when horses sweat a lot and lose a considerable amount of sodium, their thirst response may be depressed and the horse will not drink adequate fluids to maintain a desirable state of hydration.
Maintaining Fluid Balance During Work
When humans work for long periods in hot environments, frequent fluid ingestion is recommended as a means of preventing dehydration. For example, an intake of 200 to 300 ml every 2 to 3 km has been suggested for humans running in the heat. A similar suggestion might be applied to horses except for the all too common "you can lead a horse to water, but you can't make it drink" scenario. Some horses will willingly drink during endurance rides or other long term activities; but many horses do not exhibit a strong thirst response. There are a number of reasons that horses may not drink during an event, such as excitement or fatigue, or because the water is not palatable to the horse. A poor thirst response may also be attributed to the loss of sodium that accompanies water loss in sweating horses. In humans, the sodium content of sweat is quite low, so sweating results in a proportionally greater water loss than sodium loss. The loss of water without a proportional loss in sodium results in an increased concentration of sodium the plasma. Importantly, the increased concentration of sodium in plasma may be a signal for the thirst response in humans. In horses, sweat is quite high in electrolytes and as a result sodium is lost in a proportional amount to water. Therefore, in the heavily sweating horse, plasma sodium concentrations may not increase and the "signal" for drinking is not produced.
To increase the likelihood that a horse will drink during an event, the horse should be allowed to drink during training bouts. Adapting the horse to flavored water for several days before an event and then flavoring the water available at an event may encourage the horse to drink water that is otherwise unpalatable. Allowing the horse access to forage may also increase water consumption. Some horses may drink more if a bucket is held for them; other horses may be stimulated to drink if they see another horse drinking.
We have been researching another way to assist horses in maintaining fluid balance during exercise. In particular, we have been interested in how the pre-event feeding practices can affect the ability of horses to deal with fluid and electrolyte losses during exercise. We hypothesized that diets that maximize water intake and retention would be best for horses that are going to compete in events where fluid and electrolyte losses may impair performance. Nutritionists have long observed that water intake is correlated with dry feed intake; that is, the more dry feed a horse eats, the more water it consumes. The table below shows the water intake of horses (from one meal to the next) eating two different diets -that contained about the same number of calories, but different amounts of total feed.
Effect of amount of feed intake on water intake in horses
| Total kg feed |
| Water intake (L) |
| diet C |
(2.1 kg hay + 2.5 kg grain)
| diet R |
(6.1 kg hay + 1 kg grain)
From; Danielsen et al, 1995
Meeting Electrolyte Needs
Electrolyte availability can become a problem when the rate of loss exceeds the rate of replacement. If a horse is not involved in regular strenuous exercise and has access to good hay or pasture and a salt block, it will probably receive adequate amounts of the major electrolytes: sodium, chloride and potassium. When horses engage in regular exercise, it is generally believed that they will increase their salt intake to compensate for their increased losses. However, during an endurance ride or when a horse is worked very hard several days in a row, intake of electrolytes from a salt block and the regular diet may not be able to match the losses in sweat. Thus, in these situations, a horse may benefit from electrolyte supplementation.
When considering the use of an electrolyte supplement, it should be remembered that horses do not "store" sodium, potassium or chloride from one day to the next. Therefore, a high level of daily electrolyte supplementation is necessary only when horses sustain high sweat losses every day. For horses that are not sweating very heavily every day, adding high levels of electrolytes daily will probably only increase water intake, urine losses and the owner's sweat losses when cleaning the stall!
While high rates of electrolyte supplementation are probably not necessary on a daily basis, relatively high levels of electrolyte administration may be appropriate during an event. It may be advisable to provide the electrolytes as a paste or in feed, rather than in the water to eliminate any possible negative effects on water intake. Electrolyte administration will be most effective if the horse is actively drinking, and should be given only under the supervision of a veterinarian if the horse is already very dehydrated or experiencing metabolic problems. While administration of electrolytes without water can create problems, administration of water without electrolytes may also be detrimental to dehydrated horses. The veterinarian will be most qualified to determine what combination of fluid and electrolyte is most beneficial for a very dehydrated or exhausted horse.
Many electrolyte products are commercially available and recipes for home made mixes can be found in textbooks or magazine articles. The most appropriate way to chose a product is to compare the electrolyte content of the supplement to the electrolyte content of sweat. If economy is a concern, the supplement should be evaluated on the basis of $/units of actual electrolytes, not $/unit of supplement, since some supplements may contain "filler" ingredients. Some "electrolyte" supplements actually contain very low levels of electrolytes, or are designed. for horses that have electrolyte disturbances from disease, rather than exercise.
There are lots of opinions about appropriate administration schedules and dose rates. As noted above, electrolytes should not be given to a horse that is already dehydrated, except under the supervision of a veterinarian. However, small amounts of electrolytes can be safely given to most horses before they reach a critical depletion point. The ideal situation would be to make enough electrolytes available to simultaneously match losses, but this would be hard to do in real events. Reasonable strategies for horses that are competing in situations that will result in large sweat losses include the following:
Give small doses of electrolytes at rest periods, before ,the horse gets dehydrated. A reasonable dose might contain around 3-7 g sodium, 6-15 g chloride, 1-3 g potassium, .4-.8 g calcium and .1-.3 g magnesium. Another way that riders judge the appropriate dose amount is to estimate losses and then administer an amount of electrolyte supplement during the event that replaces 1/3 to 1/2 of the total losses.
Make sure water is available and encourage the horse to drink. If possible, keep track of about how much water the horse is consuming.
Like water, electrolytes can be retained in and then absorbed from the large intestine, so adding some electrolytes to the diet just prior to the event may be helpful.Once the event is over, monitor the horse carefully. Although it is not always convenient, it may be best to wait several hours after finishing a long ride or event before transporting the horse home. Transportation can be a dehydrating experience on its own, and transporting an already dehydrated horse may increase the potential for more serious problems such as colic.