All higher forms of life require the six basic nutrients water, protein, carbohydrates, fats, minerals, and vitamins.
Horses will actively seek out water to meet their needs. Water makes up about 70% of all mammalian bodies and has the following functions:
The quantity of water needed by the body is determined by many factors, some of which include environmental humidity and temperature, dry feed vs. green feed, lactation, and work. In general, a horse will drink one-half gallon of water per 100 lbs. of body weight in cool weather and may drink more than 1.5 gallons per 100 lbs. if under excessive heat, work, and/or heavy lactation. With such a wide range in needs, the rule for feeding water is to give free access to all the clean, fresh, not frozen water the horse wants.
An exception to this rule is a horse that has been worked and is very hot. As long as a hot horse continues to work it can drink its fill, but a hot horse should never drink its fill and cease working. A hot horse that has ceased work needs to be cooled out (3090 minutes) before you give it all the water it wants. The hot horse can be given two or three swallows of water every three to five minutes while cooling out if you are walking the horse to aid in the cooling process. Where these recommendations have not been followed, some hot horses have suffered from colic and/or laminitis (founder).
If water is deficient, the first thing that happens is the horse stops eating or decreases food intake. The most common reasons for deficiency are a dirty water container, lack of water supply, or frozen water. Horses without water are also likely candidates for colic. If a horse does not get sufficient water (most common in winter with dry hay and frozen water), the feedstuffs cannot be sufficiently mixed with water to allow normal passage through the gut, and impaction colic results.
Another problem that may occur is that when you are away from home and offer the horse water that may smell and/or taste different than what it is used to, the horse will refuse to drink. Although most horses would drink before they would die, they will become dehydrated and lack stamina. A way to handle this problem is to add a little brown sugar or other harmless tasty material to the water at home sometimes, and then when away from home, adding the sugar will mask the taste of the different water. If the water smells bad, a little mentholated ointment in the horses nose will mask the odor; however, as with the sugar, use it at home some so the horse is used to it.