How should dehydration be treated? How long can a horse go without water?

Assuming previous efforts at encouraging the horse to drink by offering of fresh, potable water have failed, you can treat dehydration by administering fluids and electrolyte solutions. Fluids and electrolytes are vital in treating and stabilizing horses with dehydration. However, you should seek the advice of your veterinarian and have him or her administer the fluids. Excess fluid administration can lead to physiological problems.
Most of the time, electrolytes will be administered by mouth. In severe cases, they will be injected intravenously. The method used will depend on the attitude, temperament, and health of the dehydrated animal.
How long a horse can go without water depends on many factors, but after three to four days, the horse will eat very little and will have experienced rapid weight loss. The weight loss is primarily due to dehydration.
A horse needs clean, fresh water daily. A horse needs at least a gallon of water per 100 pounds of body weight. For an average horse, this equals 10 gallons a day. Water requirements vary greatly according to the weather and the level of work that the horse is doing. For instance, if your horse is exercising in hot, humid weather, he may need two to four times the minimum amount. If your horse is a lactating mare, she will require 15 to 20 gallons of water a day to replace that secreted in the milk.
To test a horse for dehydration, pinch a fold of skin on the horse’s neck or shoulder. Upon release, the skin should pop back flat immediately. If it does not or is sluggish about doing it, then the horse is at some level of dehydration. You can also check the gums for moisture; if they are tacky and dry, again some level of dehydration is being experienced. It is best never to let a horse become dehydrated.