Fluid Balance The horse’s body contains 65-75% water. Horses maintain their body fluid balance by drinking water or eating moist feeds like pasture as well as through the metabolism of protein, carbohydrates, and fats. Horses lose water through urine, feces and sweat excretion as well as expiration while breathing. A lactating mare also loses fluids through milk secretion. Water can also be lost through illness--diarrhea, for example, can be extremely dangerous to foals; they can lose water too quickly and upset their body fluid balance.
Diet Diet plays a role in both water retention and excretion. Horses that eat a high forage diet will have wetter feces than horses on a grain-based diet. Horses on pasture will have increased water in their feces versus horses fed hay. The kidney plays a major role in maintaining body water balance: it has the ability to concentrate urine (decrease volume) when water intake is low and dilute urine (increase volume) when water intake is high.
Sweat Loss Sweating is the primary cooling mechanism for a horse. A horse performing moderate to intense work can lose up to 10 liters/hour and 10% of its body weight in sweat. Sweat loss is 50 – 100% greater in hot, humid weather. A 50-mile endurance ride will last approximately five hours. At a loss rate of 10 liters/hour, a horse could lose up to 50 liters (~13 gallons) of sweat in five hours! With this sweat, significant amounts of sodium, chloride, and potassium are also lost. Loss of too much sweat and electrolytes causes a thermoregulatory challenge, making it difficult for the horse to cool itself. Overheating then causes a competition between blood flow going to the muscles for work and to the skin for cooling.
Heavily sweating horses generally need 1 to 3 oz of supplemental salt. An easy recipe is 1 to 2 oz of an equal mix of salt and lite salt—½ sodium (NaCl) and ½ potassium (KCl) added to grain twice a day in addition to having free access to a trace mineral salt block.
Climate and Exercise Both environmental conditions and the length and rate of work a horse performs influence the amount of sweat produced and the rate of respiration. A horse will increase both sweat production and respiration rate in response to increased environmental temperatures. In hot temperatures, high humidity can further challenge a horse by decreasing its ability to lose heat through skin evaporation. At this point, heat lost through respiration will become even more important to maintain the body’s core temperature. Both training and climate acclimation can help increase a horse’s exercise tolerance and response to change in climate. However, a horse exposed to high heat, especially combined with a humid climate, should be monitored closely to prevent dehydration.
Dehydration Dehydration is one of the most common causes of impaction colic. An idle horse will drink 10 to 12 gallons of water a day in a mild climate. This amount can greatly increase with hot weather, exercise or lactation. A dehydrated horse will appear depressed, refuse feed and may have a rapid respiration rate and elevated temperature. The horse’s flanks may also appear sunken and tucked up tight, and the inside of its mouth may be dry. In severe cases, the horse will have an increased heart and respiration rate.
When assessing dehydration, pinch a large fold of skin along the middle of the neck. Let go of the skin and observe how quickly it snaps back. If the skin stays in a folded position for a second or more, the horse is dehydrated. Be sure to practice skin tenting during normal conditions, so you will have a good basis on what is normal for your horse.
If the horse is dehydrated, immediately offer the horse clean, fresh water. If the horse won’t drink, call the vet!
Assess why the horse is dehydrated:
Has the water source been cut off or contaminated?
Is the horse unable to get to the water source due to illness or a bossy horse?
Water Requirements As mentioned before, an idle adult horse weighing 1,000 pounds in a mild climate can drink 10 to 12 gallons of water per day. Fresh water, at a comfortable temperature, should be offered free-choice. Water requirements vary based on activity level and production. A lactating broodmare may need 50 to 70% more water than when she is not lactating. An exercising horse may need an increase of 20 to 300% beyond its normal water intake! Remember that a horse can lose 5 to 10% of its bodyweight through sweat, and that amount must be replaced by water. Hot weather can cause the requirement for water to quadruple. Beware of combined factors such extreme heat, humidity, exercise and lactation that may increase the water requirements of your horse.
During long performance bouts like endurance rides, horses should be offered water frequently. After completing an exhaustive performance like a cross country course, horses should continue to be offered water during the cool down period. This is in contrast to the traditional belief that a hot horse should not be offered water - probably due to the storybook of Black Beauty. However, Michigan State University researcher Dr. Hal Schott has repeatedly found in his studies that horses do best when offered water frequently, even when continuing on with strenuous exercise.
Special care should be taken when hauling horses long distances. Water should be offered every few hours, more frequently in a hot climate. Some horses will not drink water that tastes or smells different than their normal water. Eventually, they will overcome their pickiness, and finally quench their thirst. However, a horse that is being hauled to a competition may not have ample time to adapt to the new water source before competing. If this horse is asked to perform in a dehydrated condition, it could result in heat exhaustion. Many people will haul their own water source to avoid this scenario.
Water Supply With the consequences of water deprivation being so high, it is clear that a horse’s water supply should be checked daily for manure and rodent contamination. The level should also be checked to ensure there is plenty of fresh drinking water. Water heaters and automatic systems need to be checked daily to ensure they are working properly.
Ideally, a horse will have access to free-choice, clean, fresh water. In a group pasture setting, make sure the water troughs are located where even the most timid horse will have access – keeping in mind the rules of herd hierarchy.
Dehydration and Electrolyte Losses in the Sport Horse – Tuffs Cummings School of Veterinarian Medicine
Do Electrolytes Really Help My Endurance Horse? – Hal Schott, Michigan State University College of Veterinary Medicine Website
Equine Thermoregulation – eXtension HorseQuest Website
Hot Weather Care for Horses – Judy Marteniuk, Michigan State University College of Veterinary Medicine Website
Hyperthermia – Michigan State University eXtension HorseQuest Website
Nutrient Requirements of Horses. Sixth Revised Edition. 2007. National Research Council. Academic Press.