Importance of water
Horses require a clean, fresh supply of water at all times. Adequate quantities of water are necessary for the horse's normal metabolism and propulsion of feedstuffs through the gastrointestinal tract. If the horse does not consume sufficient quantities of water, the results can range from impaction of feedstuffs in the intestine to dehydration.
A 1,000 pound horse, at rest in a cool climate, eating a normal diet of good quality dried roughage will normally drink from 8 - 10 gallons of water a day. If the horse is turned out on pasture the water content of the fresh forage will meet some of the horse's requirements, but not all.
Young horses, pregnant or nursing mares need additional quantities of water. With increased temperature, humidity and/or exercise, voluntary water consumption can increase 2 - 4 fold.
A horse that is working hard in a hot environment can lose up to 2 - 4 gallons of sweat/hour with a total loss of as much as 10 gallons of sweat.
Because of the composition of horse's sweat, even though there has been a loss of a lot of water, the horse may not necessarily be thirsty i.e., the basis for the phrase: "You can lead a horse to water but you can't make him drink." You should, however, encourage water consumption by a horse that is sweating extensively. Voluntary water consumption is enhanced when the water offered is clean and between 45 - 64° F. A horse that is working should be allowed sufficient opportunity to drink every couple of hours. Although it is a commonly held belief that a hot horse should have water withheld until it is cool, there is no scientific basis to support that belief.