Electrolyte problems in the heat are directly proportional to sweat loss so it makes perfect sense that horses working long periods are at greatest risk. This puts the spotlight on endurance horses.
Australian endurance horse champion Omani Mr. Squiggle owned by Carol Layton
Equine sweat is a concentrated electrolyte solution. Chloride is the most abundant electrolyte in sweat, followed by sodium then potassium with much smaller amounts of calcium and magnesium. The daily requirement for sodium can double with just one hour of low level sweating.
Even at low rates of sweating the horse will lose over a gallon of fluid per hour – and up to 4 gallons per hour with heavy sweating. That’s a lot of fluid! The first consequence of this is dehydration. Since sodium lost in the sweat is needed to hold water in the body tissues, drinking water alone is not enough to correct the dehydration. Even mild dehydration has a major impact on the ability to perform.
Electrolyte losses triggered by exercise and sweating can produce a variety of temporary signs which respond to hydration and electrolyte replacement. Normal levels are required for regular heart rhythm, intestinal motility, coordinated movements of involuntary muscles like the diaphragm, skeletal muscle contraction and relaxation and the regulation of nerve firing.
Endurance horses show some typical changes in their blood electrolyte profiles. Low chloride is common. As above, chloride is very high in sweat. They do not have good stores of chloride in the tissues to replace what is lost in sweat. Low potassium is more common than low sodium. This is because they can pull sodium from the tissues surrounding the body’s cells to keep blood levels up. The kidney also conserves sodium by reducing sodium in the urine and replacing it with potassium.
Low potassium interferes with normal contraction of intestinal muscles, skeletal muscles and the heart. The loss of chloride also worsens these changes. In the body, negatively charged chloride and bicarbonate normally balance out positive charges. When chloride becomes too low, more bicarbonate is produced. The bicarbonate then binds up “free”/charged calcium and magnesium ions which in turn disrupts muscle and nerve activity.
The bottom line of course is that successful endurance activity requires careful attention to electrolyte intake. Hay/grass is an excellent source of potassium but chloride levels vary and sodium is extremely low. Plain salt (sodium chloride) is the first consideration, feeding 2 oz/day along with generous forage.
If the horse is on a ride or working more than 1 to 2 hours/day, add a balanced electrolyte replacement with roughly a 4:2:1 ratio of chloride:sodium:potassium. In other words, twice as much sodium as potassium and four times as much chloride as potassium. Once you find a balanced product, calculate a dose that provides 10 to 12 grams of sodium. You need one such dose for every hour worked over the 1 to 2 hour mark.
The above program is designed to prevent significant electrolyte losses. If the horse has already been working heavily without electrolyte support, a different formula could be beneficial in targeting the existing situation first. Look for salt (sodium chloride) to be about double the level of potassium with magnesium about 1.5% and calcium 3%.
Correctly supplementing to provide optimal electrolyte support takes a little effort but is well worth it.
Eleanor Kellon, VMD