According to the veterinarians interviewed in this article: http://www.equisearch.com/article/when-your-horse-needs-electrolytes The average pleasure or show horse doesn’t need a daily electrolyte supplement, despite what manufacturers and current feeding trends may have you believe. Horses can replace what electrolytes are lost during normal sweating by eating hay, drinking water, and having access to a salt block. Supplementing a horse that isn’t sweating excessively with electrolytes is simply a waste of money – the horse will urinate out all of the extra minerals.
However, horses that sweat excessively for a prolonged period of time can benefit from electrolyte supplementation. For example, trail horses who are ridden for hours at a stretch in hot, humid conditions, endurance horses, and horses being shipped long distances in the summer heat are all at risk of losing significant amounts of electrolytes through sweat. Horses like these that are producing a lot of sweat can benefit from a powdered, pelleted, paste, or liquid electrolyte supplement to help replace the necessary minerals.
So before you go adding electrolytes to your horse’s feed, take a good look at how much they’re sweating to determine if your horse really needs them!