By Dr. Kim Rose
B.Sc (BVMS), Equine Veterinary Surgeon

Water is the cheapest nutrient and the one needed in the largest quantity and yet is often neglected. A horse or most animals will survive longer without feed than without water.

A 500KG horse can drink anything between 20-80 liters of water, depending on the type of feed it is fed, the speed and duration of work and the climate.

Water should be freely available at all times and frequent inspection of water supply and cleaning of automatic waters and buckets is essential. Horses will tend to drink the largest amount of water at the time of feeding.

Under hot tropical conditions, horses need to sweat heavily to keep cool and are prone to dehydration. Some horses can develop 'anhydrosis' or 'dry coat' in hot tropical climates.

The underlying cause is thought to be environmental stress and failure to acclimatize but there is no singular treatment or management procedure that can reduce the risk of anhydrosis.

Over the years, confining affected horses to air conditioned boxes, and working horses in the cool of very early mornings have been the only part viable solutions to the problem of anhydrosis.

Under dry conditions, sweat output increases as the environmental temperature increases, with sweat output of 10-12 liters per hour in hot conditions. In humid conditions, the efficiency of heat exchange between sweat and moist air is reduced, with less heat transfer resulting in heat overload and subsequent increased stress.

Electrolytes are salts that play an important role in maintaining osmotic pressure, fluid balance and normal nerve and muscle activity.

The most important electrolytes are Sodium (Na+), Potassium (K+), Chloride (Cl) and Magnesium (Mg++).

Exercise electrolytes loss causes fatigue, muscle weakness and decreases the thirst response to dehydration.

As most of the electrolyte loss in the horse occurs through sweating, one method of calculating electrolyte requirements can be based on the amount of fluid loss, where 1KG body weight loss equals to 1 lit re of fluid.

Under conditions of extreme fluid and electrolyte loss, a diet of 4KG of chaff and hay and 6KG of grain supplies all the Potassium and Magnesium needed by the horse. This is because chaff and hay is a rich source of Potassium and the amount of Magnesium lost in sweat is small. This means that if the horse is being plenty of good quality chaff and hay, the only electrolytes theoretically needed to be supplemented are Sodium and Chloride.

However, if trainers cut back on roughage intake prior to racing or if horses are poor eaters, Potassium and Magnesium deficiencies may occur so an electrolyte supplement should contain some Potassium and Magnesium. Also poor quality chaff and hay may have a lower electrolyte content.

In summary, electrolyte replacement must be matched with sweat loss and the horse must have a readily available clean source of water.

Working horses in hot and tropical environments would be best during cooler temperatures (night time).