If you have any experience with exercising horses, you know horses sweat a lot. And this is a good thing because sweating is the major method that horses use for cooling themselves in hot and humid weather, and after exercising. Dr. Peter Huntington from Kentucky Equine Research in Australia indicates that other than horses, the only mammalian species that sweat enough to cool themselves are humans and one species of monkey. However, human sweat is high in protein and low in electrolytes while equine sweat is low in protein and very high in electrolytes, so horses lose a large amount of electrolytes when they sweat. That's why working horses need to be supplemented with electrolytes all year round, and especially in hot weather.
Sometimes horse sweat seems thin and watery while other times it seems thick and foamy; the thick and foamy sweat contains a protein called latherin. Latherin is a detergent-like protein that helps move sweat away from the skin to the surface of the coat, and once the coat gets damp, there is a greater opportunity for evaporative cooling. Latherin causes foaming of sweat in places of contact where tack rubs the coat such as under the saddle, and where body parts rub together such as between the upper thighs. Dr. Huntington indicates that to keep your horse's skin and hair coat in the best condition, all sweat should be removed from the coat as soon as possible. In warm weather, rinsing the horse with water removes the sweat and helps to cool him off. If the sweat is allowed to dry without rinsing, a curry comb and brush can remove dried sweat. Removing it is a good idea because an accumulation of dried sweat, even for only a few days, can be damaging to the coat.