Lactating broodmares may drink 50-70 percent more water. But, the most important factor is the level of performance. Horses in intense performance in hot weather may lose as much as eight gallons of sweat daily. You should be concerned if your horse reduces the amount of water it normally drinks in hot weather, especially if it is in moderate or intense performance. You can measure water loss by weight loss in horses.
Horses can lose five to ten percent of their body weight during strenuous competition.
This weight loss is mostly sweat, but urine and feces also contribute to water loss. Foals need water in addition to what they receive from their dam's milk. Make sure your horse has an adequate supply of clean, fresh water at all times, especially in the hot summer, the height of the horse show season. At this time of year, check natural water supplies if your horse is on pasture. Water troughs or tanks used for outside horses should also be inspected and cleaned periodically and contain fresh, clean water at all times.