With the summer months approaching, it is time for a refresher on the impact of the warmer environmental conditions on your horse. Warmer temperatures and high relative humidity place added stress on horses during exercise because of their increased reliance on sweating for control of body temperature. A major consequence of sweating is the loss of large quantities of water and electrolytes. Without adequate replacement of those losses, a horse’s performance will suffer and, worse, there is potential for development of serious medical problems during or after exercise.

In this article, I will discuss fluid and electrolyte losses in the horse during exercise, the impact of those losses on performance and health, and various strategies that can be used to ensure adequate replacement of fluids and electrolytes.

Water And Electrolyte Balance

Water makes up slightly more than 65% of a horse’s body mass. For a 1,200-pound (550 kg) horse, this constitutes more than 350 liters of water! This water is distributed into two main fluid compartments. The first is the intercellular compartment and refers to water inside the body’s cells. Fluids surrounding cells represent the extracellular compartment. Extracellular fluid includes blood plasma, the interstitial fluids (the fluid that flows in the microscopic spaces between cells), and fluid within the gastrointestinal tract. Two-thirds of total body water is in the intracellular compartment (see Figure 1 on page 88). Up to 60 or so liters of fluid can be present in the large intestine, and this fluid reservoir is drawn upon during endurance-type exercise. Diets high in fiber increase the size of the gut fluid reservoir, which is a real benefit for the endurance horse.

How much water does a horse need on a daily basis? As a general rule, a sedentary (couch potato) horse requires a bare minimum of about 30-35 ml of water per kg body weight per day for maintenance of