Water Needs
Voluntary water intake by the horse at rest in a moderate or cool environment, eating dry forage, varies from 0.3 to 0.8 gal/100 lbs body weight/day (25 to 70 ml/kg/day). The amount actually required is near the lower end of this range. At rest water requirement in quarts or liters/day is approximately equal to digestible energy requirement in megacalories/day.

The amount of water needed varies primarily with the amount of water lost from the body, which is altered by the amount, type, and quality of the feed consumed, the ambient temperature and humidity, and the health, physiological state, and physical activity of the horse. The horse, like all animals, consumes more water than needed if palatable water is readily available. The amount of water consumed, however, will decrease to just meet needs if water is poorly accessible or poorly palatable. The amount of water drunk, but not consumed, also decreases with increasing moisture content of the feed. Feed containing 40% or more moisture supplies enough water to meet the idle horse’s needs in a moderate environment. Although hay, grain, and nongrowing forage contain less than 15% moisture, growing forage contains from at least 60% to over 80% moisture. Thus, the horse consuming growing forage does not need to drink any water, although most will if it’s available and palatable.

The amount of water drunk directly correlates with the amount of feed dry matter consumed. Although horses generally drink 1.5 to 2 quarts of water per lb. (3 to 4 L/kg) of hay or grain only about 1 quart/lb (2 L/kg) are normally needed. Donkeys generally drink 1.2 to 2.6 and Shetland Ponies 2.2 to 2.5 L/kg, which may reflect at least the donkey’s desert origin. Water intake decreases with increasing diet digestibility, because increasing diet digestibility decreases the amount of feces and, therefore, the amount of fecal water excreted. Diet digestibility increases, and therefore the amount of water per unit of feed dry matter consumed decreases, with increasing forage digestibility and as the amount of grain in the diet increases.

Water needs and intake also increase with increasing protein and salt intake. Increased protein intake increases nitrogenous waste products excreted in the urine, and this, like increased salt intake, increases urine volume. Lactating mares have increased water needs to compensate for increased water loss in the milk. Lactation may increase water needs, and also dietary energy needs 1.5 to 1.8 times that required for maintenance. There is also increased water needs during growth and the last trimester of pregnancy. Although a small amount of increased water need for growth is caused by increasing body size and, in pregnancy, placental fluids and the fetus, most of the increased water need during growth, pregnancy, and lactation is due to increased feed intake. One of the major factors affecting how much water the healthy horse needs is how much water the horse loses through sweat and expired air in order to prevent the body’s overheating from physical activity or the environment. The amount of water needed may increase as much as 3 to 4 times with work at high ambient temperatures. Moderate work alone may increase water needs 1.6 to 1.8 times and hard work 2.2 times that needed at rest. At an ambient temperature of 0F (18C), horses consume 1 qt water/lb (2 L/kg) dry feed eaten, whereas at 100F (38C) they will consume four times this amount. An increase in temperature from only 55F (13C) to 70F (21C) increases the horse’s water requirements by 15 to 20%.

When water is readily available, increased water consumption occurs as a result of increased drinking frequency, not increased drinking duration or the number of drinks taken during a drinking bout. For example, there is a direct correlation between drinking frequency and ambient temperature, with a large increase in frequency at temperatures above 85F (30C). When water is readily available, most horses drink once for only about 30 seconds or less every few hours. However, if water is not readily available such as if there is a long distance between preferred grazing areas and water, more and longer drinks may be taken during a drinking bout.

This article is from Feeding and Care of the Horse, second edition, by Lon D. Lewis, Lippincott Williams & Wilkins, 1995. Reprinted with permission from the publisher.

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