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A 450 kg horse drinks 38–45 litres of water a day, and its stomach holds only about 15 litres — which is why several small feedings beat two large ones.
Water is the largest daily input by volume. An average 1,000-pound (450 kg) horse drinks 10 to 12 US gallons, or 38–45 litres, per day. In hot weather or on dry forage, intake rises above that baseline.
Under hard work or lactation, a horse's water requirement may be as much as four times greater than normal. That puts a working or nursing horse well above 100 litres on some days, and it means water should never be rationed.
Water makes up 62–68% of a horse's body weight. A horse becomes dangerously dehydrated if it loses 8–10% of its natural body water, so a clear drop in drinking is a serious signal rather than a minor variation.
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The stomach is small relative to the daily ration. The average horse has a stomach capacity of only 4 US gallons (15 L) and works best when it contains about 2 US gallons (7.6 L); the stomach begins to empty when two-thirds full.
That combination — a large daily water requirement and a small stomach — explains the feeding pattern. Horses prefer to eat small amounts of food steadily throughout the day, and dividing the ration into several small feedings keeps the stomach within its working range.
Protein is the other number to keep in view. Most adult horses only require 8–10% protein in their diet, so the daily ration should be built around forage with the protein share held in that band rather than pushed higher.
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