Even slight, 2%, dehydration lowers your horse’s endurance by causing early fatigue. This level of dehydration isn’t detectable by the skin-fold test, as it doesn’t become abnormal until the horse is at least 5% dehydrated. Horses that aren’t well-hydrated won’t perform well and are susceptible to heat stroke. You need to supplement salt and electrolytes, which are critical to the normal functioning of the horse’s body.
The major electrolytes lost in sweat are sodium, potassium and chloride. Since he consumes generous amounts of potassium and chloride in his forage, at least for light work, all you need to worry about is sodium, the major electrolyte that holds water in the blood and tissues.
Your goal is to ensure that the horse begins work in a well-hydrated state with correctly balanced electrolytes ”on board” and ready to go when needed.
The horse’s basic sodium need, not counting sweat losses, is 0.02 grams x weight in kilograms. A kilogram = 2.2 pounds, so an 1,100-lb. horse weighs 500 kg. That horse’s baseline sodium requirement is 500 x 0.02 = 10 grams of sodium.
Table salt, sodium chloride, is the most commonly used source and is about 40% sodium by weight. Therefore, the horse needs to eat 25 grams of salt (a little under 1 ounce or about two tablespoons) per day as a starting point, or baseline, to keep him adequately hydrated.
If a horse isn’t getting that basic ounce of salt per day, his body will reset at a point where there is a normal amount of sodium and fluid in blood, but the tissues will always be in a state of dehydration.
A horse that is already mildly dehydrated when starting work will more easily get into trouble with overheating and electrolyte disorders when exercised. Requirements go up significantly the harder the horse works and sweats (see chart).
You can give the horse an additional 1 ounce of salt for every hour worked at low rates of sweating. With heavy work in the extreme condition of high heat, the requirement can be as much as four times higher. Obviously, you should avoid working the horse under those conditions.
As a rule of thumb, you can work the horse for up to 2 hours and meet electrolyte needs by providing an extra 2 ounces of plain salt. If the horse won’t eat it, you can syringe it in mixed in some oil. Do this after the horse has eaten to avoid stomach irritation. For work longer than 2 hours, continue the same salt but add a balanced electrolyte product to replace sweat losses over subsequent hours. The levels of electrolytes in commercial products are highly variable. If your product isn’t listed in our Favorites, read the label to see if it contains roughly twice as much sodium as potassium, and twice as much chloride as sodium.
Make sure the horse consumes adequate salt every day and for his work level. If working longer than 2 hours per day, add a balanced electrolyte supplement to his salt.