Almost, if not as important as feed, is a horse's access to clean, fresh water on a year round basis. Horses, unlike some animals, typically will not drink dirty, stagnant or muddy water and may actually become dehydrated very quickly, especially in very hot conditions where they are being worked or exercised. Most horses, just like people, have a natural thirst mechanism that kicks in whenever the body water levels are dropping, however if the horse does not have access to water this natural trigger can prematurely shut down, resulting in dehydration.
Water makes up a good part, almost 60%, of the total weight of the horse's blood, digestive system and muscle mass. Dehydration causes the body to rob moisture from the body muscles, blood and tissue to keep the brain and other essential organs functioning. On average a horse needs to drink about 35-40 liters or 10-12 gallons of water a day to maintain the proper water levels in their body. Water is lost through sweating, breathing and even in the urine and fecal material year round, not just in the hot summer.
Lack of access to clean drinking water year round can result in several symptoms or signs of dehydration. The first sign of chronic dehydration is that the horse will not eat as much as they would normally, since there is not enough water in the body or saliva to properly digest the food. With this lack of saliva comes problems with digestion and a greater likelihood of colic or impactions due to constipation. Usually these symptoms are most commonly seen in the winter months when natural water sources such as ponds and even waterers may be frozen over. While horses will eat snow, there is much less moisture in snow and it will not provide adequate amount of water for the horse.
Summer dehydration is most often noted in horses that are working or exercising heavily, especially in the hottest times of the day. Since sweating is the single greatest source of water loss in horses in the summer, it is important to provide regular, frequent water breaks for the working horse. In a working horse it is not uncommon for the horse to lose 5-10% of his or her body weight per day in water loss due to sweating. This translates into about 1-2 liters of water per hour needed just to replace the water loss from sweat.
To test for dehydration a simple skin test can be used. Use the finger and thumb to pinch a small amount of skin on the horse's neck. Release and notice if the skin smoothes or snaps back into the flat position or stays in the pinched state. Horses that are appropriately hydrated will have a great deal of elasticity in the skin that will immediately go back to its normal shape. Dehydrated horses will not have elasticity in the skin and it will stay in the pinched shape.
Horses need to have lots of fresh, clean water in the pasture as well as in the stable. Salt blocks and electrolyte supplements can be added in the pasture or stable and horses can consume free choice to help increase water consumption.