Electrolytes can be confusing for horse owners, but in reality their use should be quite simple. ‘Electrolyte’ is a term used for the salts that control fluid balance in the body, amongst other things. The principal electrolytes are sodium, chloride (we use this term instead of the elemental chlorine, since chloride is the form found in the body) and potassium.
Electrolytes are essential minerals, so they are part of a balanced diet. Horse owners need to understand more about them than other essential nutrients because their horse’s requirements for them varies so much from day to day, unlike for other essential nutrients. Electrolytes are lost in sweat, so requirements depend on how much the horse has sweated. Horses in hard work – and especially in hot weather – have much increased requirements for electrolytes compared to horses in no work, or light work.
Other minerals including magnesium and calcium are also lost in sweat, but in small enough quantities that they will be replenished from a normal balanced diet. But large amounts of sodium, chloride and potassium can be lost by hard working horses, and there is rarely enough sodium or chloride in normal horse diets to replace these, even when the diet is balanced for all other nutrients. Neither forages, compound feeds, balancers nor vitamin and mineral supplements supply enough salt (sodium chloride) for working horses. Potassium is a little different because it occurs in such large amounts in forages, so extra supplementation is much less frequently required.
All working horses need to be fed extra sodium and chloride (on top of a balanced diet) and horses in endurance training who are working for long periods of time (might include eventers as well as endurance horses) ideally should have potassium added as well as sodium and chloride.
You need to add electrolytes to the feed to replenish salts lost in sweat. Electrolytes are added to water only to encourage rehydration in a dehydrated horse who has lost a substantial amount of fluid via sweat. These are two distinct uses of electrolytes and should not be confused. Obviously you will replenish small amounts of salt via a rehydration solution, but never enough for a hard working horse. Electrolyte solutions for rehydration must be isotonic i.e. at a similar concentration to body fluids if they are to be effective. It is the amount of water the salts are mixed into, not the proportion of salts in a commercial electrolyte, that makes a solution isotonic.
To conclude, all horses that sweat during work (training and competing) should have electrolyte salts added to their diets daily. Hard working horses who are dehydrated after work, whether that be during training or after competition, should also be offered oral rehydration solutions made from isotonic salt solutions.