I've heard I should be adding salt to his feed. It is a common way for many animals to cool themselves. The horse generates a tremendous amount of heat when working. The production of energy by the muscles releases heat with every contraction. This heat is released to the air via the heavy breathing that the horse demonstrates during exercise and by the evaporation of the sweat that the horse produces when hot.
Sweat is rich in electrolytes.
Exercise and high ambient temperature greatly increase the daily water and electrolyte intake required by the horse. Horses at rest require on average, about 4 - 5 gallons of water daily. This need will dramatically increase during periods of exercise and hot weather. The horse will regulate its water needs if abundant amounts of fresh water are available at times of need. Horses that are in use can lose substantial amounts of water through evaporation of sweat and through the respiratory tract.