I do some version of this post every year for good reason. At the least it will safeguard your horse’s well being and performance in the heat. At best it could save his life.
Sweating is the horse’s major mechanism for cooling off. Human sweat has the same concentration of electrolytes as plasma but equine sweat is much higher. Sodium and chloride, which together make salt, are by far the major electrolytes lost in sweat. Even a lightly sweating horse will double his sodium requirement with one hour of work. When sweating heavily for an hour, the sodium requirement goes up 500%.
Every cell in the horse’s body works like a mini battery. It performs its functions by maintaining a gradient between the concentration of sodium outside versus inside the cell. Electrolyte fluxes are involved with everything from absorption of nutrients in the gut to brain, heart and muscle function. Sodium and chloride are also the major electrolytes in blood and the fluid around cells. Without sufficient sodium, the body tissues cannot hold normal levels of water and the kidney is unable to conserve water. Dehydration rapidly ensues.
The consequences of salt and water depletion begin before you can even tell that the horse is dehydrated by common tests like the skin pinch. Reduced performance is the first sign. Muscle cramping is common. Humans report experiencing nausea (? colic or gut motility changes in a horse). This is followed by more obvious weakness and “hitting the wall”.
Exercising a horse with suboptimal salt and water levels greatly increases the risk of heat stroke. To prevent this, and make sure your horse performs comfortably and up to his best potential in the heat, you must understand what your horse’s needs are and make sure you meet them.
Before even worrying about sweat losses, the average horse has a basic salt (sodium chloride) requirement of approximately 1 oz/day. An average size horse weighs about 450 kg. You can estimate sweat losses for moderate sweating at about 10 mL/kg/hour so 4500 mL each hour. This is the equivalent of about 1 1/4 gallons of water and another 1 oz of salt. This takes care of the sodium. There is also a large loss of chloride, which will be replaced partially by the hay/grass and partially by the salt. Potassium losses also occur but are covered easily by the hay/grass as long as sodium needs are being met.
Adding only more salt will work for horses exercising up to 2 hours. Beyond that you will also want to use a balanced electrolyte supplement to make sure potassium is covered as well. A supplement balanced to sweat will have twice as much chloride as sodium and about twice as much sodium as potassium. When deciding how much you need to feed, don’t rely on product instructions. Find out how much sodium is in the product and figure on giving 11 to 12 grams of sodium for each hour. For example, if the product is 30% sodium, to get 12 g you need to feed 12/0.3 = 40 grams (1.4 ounces) of the product. This is on top of the 1 oz basic salt requirement and 1 oz/hour for the first 2 hours of exercise.
Timing is important. Water should be freely available at all times. Horses that have water restricted after work do not drink sufficient amounts. It does no good to give extra salt/electrolytes several hours before work because they will end up in the urine. You can give the first hour’s dose within 30 minutes of starting exercise and the rest during or after exercise. Mix in food or syringe in after the horse eats and drinks.
The hardest part about supplementing electrolytes correctly is ignoring all the bad information out there. If you follow these guidelines you will help protect your horse from the heat and will likely be pleasantly surprised at how much better he performs.
Eleanor Kellon, VMD
Dr. K's Horse Sense 