The most important nutrient in the horse’s diet is one that is rarely added to feeds: water. Although it is often overlooked in discussions involving equine nutrition, water could be considered the first limiting nutrient of all horses, as they cannot survive for as many days without water as they can without feed.

The amount of water required by the horse is determined by the magnitude of water losses from its body. These losses occur through feces, urine, respiratory gases, and sweat and, in the case of lactating mares, milk.

These losses are affected by the amount, type, and quality of the feed consumed, environmental conditions and the health, physiological state, and physical activity of the horse. Horses will generally consume as much water as they need if given access to a palatable water source.

Horses at rest in a moderate climate will generally consume between three and seven liters of water per 220 lb (100 kg) of bodyweight. This translates to around 4-9 gallons for an 1,100-lb (500-kg) horse