Perspiration cools a horse through evaporation---as water is converted from a liquid to a gas, it absorbs energy from its surroundings. In this case, that energy is in the form of heat from the skin and the air just above. (Interestingly, only horses and primates cool themselves primarily through sweating.)
• When working in warm weather, horses produce about one gallon of sweat every 15 minutes. Sweating begins on areas covered by tack, then spreads to the chest, neck and between the hind legs. After a workout, it’s normal for a horse to sweat profusely, but a horse who sweats even when standing still may need some help staying cool and will appreciate being hosed down. Sweat appears on the head, flanks and top of the rump when a horse is extremely hot and may be at risk of heat stress.
• A horse who does not sweat as much as his herdmates in the same environment---or who doesn’t sweat at all---may have anhidrosis, a dangerous failure of his thermoregulatory system. This condition increases the risk of heat stress and stroke, even in weather that doesn’t seem that hot. The cause of anhidrosis isn’t fully understood, but it is thought to be related to prolonged stimulation of the sweat glands in hot and humid conditions.
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An anhidrotic horse needs a lot of help staying cool. This means no work in hot weather and confinement indoors, near fans, with repeated cool baths during the day. Some owners and veterinarians report that nutritional supplements can help, but often the only solution is moving the horse to a cooler region. Many horses with anhidrosis function well in more moderate climates and may even begin sweating again after several years.
• As a horse sweats, he loses electrolytes, which are minerals necessary for most of the body’s electrochemical processes. Electrolytes also play a key role in the movement of fluid in and out of cells, the absorption of nutrients and the regulation of the body’s fluid balance.
Forages and commercial feeds typically contain ample electrolytes, so a horse can replenish his stores through a regular diet. If a horse has been sweating for several hours, however, an electrolyte supplement can help speed his recovery. Remember, the cause of the sweat is irrelevant: A horse who sweats on the local trails is losing just as many electrolytes as one who sweats while running barrels.
Electrolytes are available as oral pastes or powders to top-dress on grain or add to water. Whichever form you choose, follow the manufacturer’s directions and make sure fresh water is available to the horse after administration.