Horses that are exercising often sweat to keep cool. That sweat is full of electrolytes (sodium, potassium, calcium, magnesium and chloride in particular), which are simply lost by accident.
'All known higher lifeforms require a subtle and complex electrolyte balance between the intracellular and extracellular environments. In particular, the maintenance of precise osmotic gradients of electrolytes is important. Such gradients affect and regulate the hydration of the body as well as blood pH, and are critical for nerve and muscle function.'*
Simply put - your horse will struggle to retain water in their body if they are electrolyte deficient.
A combination of exercise and the rise in temperature and humidity increases the loss of these electrolytes. Without these vital minerals your horse can suffer from problems such as: dehydration, listlessness, poor performance and more. However, before you rush out and buy your horse electrolyte supplements, make sure you ask yourself:
1) Is my horse in hard work which generates lots of sweat?
2) Is my horse competing regularly in warm weather?
3) Is my horse showing any signs that they are electrolyte deficient?
If your answer to these questions is 'No', then it may be that your horse does not necessarily need to have electrolytes added to their feed or water as their diet will normally replace what is lost. Simply providing your horse with a salt lick or adding a small amount of salt to their feed should be sufficient.
However, excess sweating (through strenuous exercise, hot weather or stress) removes considerable amounts of electrolytes from the body and at this point you will need to replace what is lost to maintain your horses performance and wellbeing.
Some electrolyte products contain just salt and some potassium (the most important electrolytes of those lost through sweat), however more comprehensive products also contain calcium and magnesium.