The importance of water
Horses require a clean, fresh supply of water at all times. If a horse does not get enough water to drink, the results can range from impaction colic to dehydration to death. A 1,000 pound horse, at rest in a cool climate, eating lush pasture needs a minimum of 3 and up to 10 gallons of water to drink each day. If supplemental water isn't provided to horses, the incidence of colic is increased dramatically.
Young horses, pregnant or nursing mares need even more water. Horses can sweat large amounts (2 to 4 gallons of sweat each hour) in order to control their body temperature. A horse that is sweating, whether just from the heat of the summer sun or from working hard, will need more water. With high ambient temperatures, humidity and/or exercise, voluntary water consumption can increase 2 to 4 fold. An exercising 1,000 pound horse in the summer heat could easily need to drink 12 to 16 gallons of water if it is eating fresh forages and much more if it is being fed dry hay. Some water will be supplied by fresh forage, but not with dried hay.
Horses will drink more when the water offered is clean and between 45° and 64° F. The water trough should be clean, kept out of directed sunlight, and located near a shelter but away from feed sources.
Although it is a common belief that a hot horse should have water withheld until it is cool, there is no scientific basis to support that belief.
Even though there has been a loss of water and the horse is actually dehydrated, the horse may not necessarily be thirsty. This is the basis for the phrase: “You can lead a horse to water but you can't make him drink”! You should, however, encourage your horse to drink in order to prevent further dehydration. Ideally, dehydration should be avoided completely by allowing a horse that is working opportunities to drink every couple of hours. Although it is a common belief that a hot horse should have water withheld until it is cool, there is no scientific basis to support that belief.