Water is one of the most vital nutrients for horses because the equine body comprises approximately 70% water, much more than humans. It means that a horse weighing 1100lbs. would have 770lbs. water equivalent to 96 gallons. Small horses or foals consume even much more water, and it is around 80%. Inability to provide adequate water to horses can affect its health, and you must treat it with due importance just as you would do for its feed.
How much water a horse consumes daily depends on many factors. Age, the type, amount and quality of feed, body condition, activity level and fitness level are important factors that affect the water consumption of horses. The purity and quality of water and its temperature also influence consumption. Due to low fat in water, fat horses consume less water than horses that maintain optimal bodies like the ones you see on TVG horse racing shows.
Water is related to food intake
The quality and quantity of food (dry matter) intake has a direct relation to water consumption because how well the food horse digests the food also counts. Going by the rule, horses that have a larger intake of hay consumes more water than horses that feed on an entirely pelleted diet or a combination of hay and grain diet. Horses that feed of good quality grass that contains about 65% to 80% moisture will naturally consume less water as its requirement of water is met from the grass it eats.
Other factors that affect water consumption
The higher is the activity more will be water consumption, and seasonal requirements will vary because they drink more water in summer than in winter. Horses that consume more salt will feel thirstier and drink more water. Even high protein intake is responsible for higher water consumption. It also increases urine output because the excess nitrogen in blood comes out through urine.
The thumb rule to quantify water requirement
To know how much water a horse needs under normal circumstances you can depend on the thumb rule. 6 to 10 gallons of water per day is the requirement for 1100 lb. horse when it remains idle, and it can go up to 15 gallons on a hot summer day. For workhorses, the average daily requirement of water is between 10 to 18 gallons in average temperature and increases much during warm weather.
Nursing mares consume a much higher quantity of water to compensate for the fluid loss associated with milk production and for consuming excess feed necessary to produce milk. On an average 20 gallons of water for nursing, mares are standard. Foals would drink 85 gallons of water per day in typical weather. The temperature of water affects consumption. The preferred temperate of water for regular use by horses is 45oF to 65oF, but warmer water would discourage horses to consume more.
Keep water in clean outdoor troughs so that horses are attracted to it. Clean the trough regularly to remove debris and prevent algae growth.