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Electrolyte Supplementation of Endurance HorsesBy Kentucky Equine Research Staff · February 28, 2012

Electrolytes are a critical component of an endurance horse’s nutritional program since they play an important role in maintaining osmotic pressure, fluid balance, and nerve and muscle activity. During exercise, sodium (Na+), potassium (K+), and chloride (Cl) are lost in large quantities through sweating. Loss of these electrolytes causes fatigue and muscle weakness and decreases the thirst response to dehydration. It is vitally important that performance horses begin competition with optimal levels of fluids and electrolytes in their bodies and that these important nutrients are replaced throughout prolonged exercise.

Sweat Losses

It is important to have some idea of the magnitude of electrolyte loss a horse incurs during exercise before a feeding program can be developed to replace these losses. Because most electrolyte losses in the horse occur through sweating, one method of calculating electrolyte requirements can be based on different amounts of sweat loss.

The amount of sweat loss will depend on a number of factors such as duration and intensity of exercise, temperature, and humidity. In general, horses exercising at low intensity (12-18 km/hr) will lose between 5 and 10 liters (1.3 and 2.6 gallons) of sweat per hour. During higher intensity exercise (30-35 km/hr), sweat loss levels reach as high as 15 liters per hour. At the 1996 Olympic Games in Atlanta, horses lost an average of 18.4 kg (41 lb) of body weight during the speed and endurance phase of the three-day event, which translates to a sweat loss of around 15 liters (4 gallons).

Electrolyte Requirements During Endurance Training

Daily electrolyte requirements can be estimated by calculating the total amount of mileage logged weekly by the horse, taking into account the environmental conditions under which the training occurs. For example, if an endurance horse were logging 50 km (31 miles) of work per week in a cool environment (20-25° C, 68-77o F), it would only require about 60-120 grams (2-4 ounces) of a well-formulated electrolyte supplement to meet its daily electrolyte requirements. The lower range of supplementation would be adequate if the horse were also receiving adequate forage and a grain mix that contained supplemental salt, as well as access to a salt block. Horses at rest will normally consume around 50 grams (0.75 ounce) of salt per day from a salt lick.

As training mileage and environmental temperature increase, so does the requirement for electrolyte supplementation. Horses that are training heavily (100 km or 62 miles/week) in a hotter environment (33-35° C, 92-95o F) may need 140-200 grams (5-7 ounces) of supplemental electrolytes daily.

The recommendations given above are based on supplementing electrolytes at the same rate daily even though the amount of exercise performed each day will vary. This is probably a reasonable approach to supplementation except for days when the training distance is especially long. For those days, additional supplementation may be warranted.

As a rule of thumb, 60 grams (2 ounces) of electrolyte supplementation are required for each hour of exercise in moderate climates. This rate of supplementation will double in hot environments when sweat loss is extensive. A long training ride of 60 km/38 miles (~4 hours) in moderate temperatures would therefore produce enough sweat loss to require 240 grams (8 ounces) of electrolyte supplementation. This level of supplementation would need to be partially provided during the ride (60 grams or 2 ounces at 20 and 40 km) using an oral electrolyte paste with the remainder of the electrolyte administered after the ride. If the horse will not consume this quantity of electrolyte (120 grams or 4 ounces) in a single meal, 60 grams (2 ounces) can be administered as a paste at the end of the ride.

When administering oral electrolyte pastes, it is absolutely essential that the horse have access to drinking water. If the horse refuses to drink, do not administer an electrolyte paste.

Supplementation During Endurance Competition

There is a great deal of controversy about how to administer electrolytes during competition. Competitors have used a number of different strategies successfully, and the recommendations given here are not necessarily the only way to achieve success.

During competition, sweat losses can be very large. Using the sweating rates described earlier, an endurance horse will lose between 45 and 60 liters (12 to 16 gallons) of sweat during a 160-km (96-mile) ride. This represents electrolyte losses of 460-690 grams (16-24 ounces). Additionally, 9-14 grams (0.3 -0.5 ounces) of calcium and 5-8 grams (0.2-0.3 ounces) of magnesium will be lost through sweating. It is debatable whether all of these losses can or need to be completely replaced during the competition. Research has shown that endurance horses participating in 80-160 km events often have a fluid deficit of 20 to 40 liters (5 to 11 gallons) despite having access to water and electrolytes during the ride. Canadian researchers have shown, however, that endurance horses with less pronounced fluid and electrolyte alterations during a competitive ride were more successful than those with greater changes. Therefore, it is absolutely essential that a large proportion of the electrolytes and water lost in sweat be replaced during the ride.

Pre-ride electrolyte loading. The endurance horse must start the competition with adequate stores of both water and electrolytes. This can be accomplished in two ways. First, the endurance horse should be on a high level of forage (hay or pasture) intake before a ride. When a horse is fed liberal quantities of forage, it can store extra water and electrolytes in its large intestine. These stores can be called on to replace sweat losses early in the ride. Second, extra electrolytes can be administered the night before and the morning of the ride. The horse’s system is finely tuned to balance the amount of electrolytes and water that it stores in its body at rest, so excessive pre-ride electrolyte supplementation should be avoided. Moderate supplementation (60 grams or 2 ounces the night before and 60 grams or 2 ounces the morning of competition) will insure that the horse has adequate electrolytes within its body and will provide additional electrolyte stores within the gastrointestinal tract.

Electrolyte supplementation during competition. Electrolytes should be supplemented throughout competition. The type of electrolyte supplement used during competition is slightly different than that which is used during training. This electrolyte should provide additional calcium and magnesium along with sodium, potassium, and chloride. If calcium and magnesium losses are not replaced by mobilization of skeletal stores or by supplementation, metabolic disturbances such as thumps may occur. Electrolytes should be administered to horses at each vet check and at water stops along the trail. The best way to administer electrolytes is in the form of a paste. Pastes are commercially available, or they can be made up fresh at the vet check by diluting an electrolyte powder in applesauce, water, or liquid antacid. A reasonable dose of electrolyte powder (or equivalent) is 60 grams or 2 ounces at each vet check. Thirty- to 60-gram (1- to 2-ounce) doses of electrolyte can be administered on the trail. It is worth reemphasizing that the horse must have access to drinking water when receiving concentrated electrolyte pastes. These pastes are hypertonic (a greater concentration of electrolytes) compared to blood and will effectively draw fluid out of the horse into the gut if they are not diluted by drinking water. Administering large doses of electrolytes without adequate water intake will result in serious problems including colic, dehydration, and possibly death.

Post-ride supplementation. Administering 120-240 grams (4-8 ounces) of electrolyte over the 24-hour post-ride period can eliminate most of the post-ride electrolyte deficit. A portion of this can be given as a paste shortly after the conclusion of the ride followed by top-dressing supplementation of electrolyte on the next two or three meals.