Adapted from Extension Article “Responsible Horse Care for Winter and Summer”
By Steven M. Jones, Extension Horse Specialist
Proper year-round horse care is important to maintaining a healthy horse. Although you have no control over seasonal changes, they have an effect on horses that you must recognize. Extreme variances in temperature require adjusting your management programs for the benefit of the horse.
Prevention of Thermal Stress
In the hot and humid weather, horses generate a significant amount of metabolic heat during exercise that must be dissipated to prevent thermal injury. During hot, humid conditions sweat will not evaporate; therefore, evaporative cooling becomes ineffective, quickly leading to debilitating and potentially life-threatening situations.
We often use the phrase, “sweating like a horse.” Horses and men are the only athletic mammals that cool themselves primarily by sweating. Some heat dissipation occurs by means of radiation, conduction, convection and respiratory evaporation. Evaporative cooling through sweating is the most important route for release of heat from the body to the environment.
Thermoregulation for exercising horses requires ample blood flow to carry heat from the body core to peripheral blood vessels in the skin, where dissipation of heat can occur. Simultaneously, the heart begins delivering blood to working muscles, essential organs, tissues and the brain. At the onset of exercise, blood pressure is preferentially maintained at the expense of thermoregulation, resulting in an increase in body temperature. As heat accumulates, blood flow from the body core to the surface of the skin is increased to transport the heat from the body core to the surface. As exercise continues, sweat, made up of water and minerals, carries the heat through the sweat glands to the body surface, resulting in heat loss to the environment. Continued exercise and sweating lead to progressive dehydration and loss of plasma water from the bloodstream. The greater the exercise intensity of an event, the greater the heat load generated, resulting in a greater need for heat dissipation.
Thermal injury is caused by animal dehydration. With prolonged exercise, water intake may increase 300 percent. Research in humans and a recent series of equine studies show a positive correlation between fluid losses, inability to maintain temperature and onset of fatigue during endurance exercise. The consequences of excess dehydration can be severe: electrolyte and pH disturbances, fatigue, gait incoordination, increased risk of orthopedic injury and death. Under normal conditions, dehydration can be minimized through the provision of adequate water, salt and mineral supplementation, and balanced diet. Horses rehydrate within a 24-hour recovery period between exercise programs.
Another measure used in the prevention of thermal injury is monitoring of weather conditions to determine the potential risk to the horse. Several inexpensive devices are available for quick measurement of temperature and humidity. These are used to calculate the "comfort index," which is the sum of the temperature in degrees Fahrenheit and the relative humidity as a percentage. If the sum is below 130, thermoregulation should not be a concern. When the comfort index is between 130 and 150, horses will sweat, but they should be able to exercise without major problems if normal fluid replacement is allowed. When the comfort index exceeds 150 and the humidity is greater than 75 percent, heat dissipation can be a problem. Horsemen should monitor their horses very carefully during strenuous workouts under these conditions. When the comfort index exceeds 180, normal routes of heat dissipation fail to work and workouts should be discontinued.
Under normal conditions, a balanced ration and a salt-mineral supplementation program should be sufficient to maintain electrolyte balance. However, with intensive exercises, substantial sweating occurs, leading to water and electrolyte deficiency that results in weakness, muscle cramps, acid-base imbalance and decreased performance. Mechanisms for the conservation of sodium and potassium improve with the horse’s acclimation to temperature and humidity. A sodium and potassium deficiency occurs additionally in untrained and non heat-acclimated horses. It is critical to monitor and, when appropriate, to provide electrolyte supplementation to horses beginning a vigorous training schedule or adjusting to elevated environmental temperatures.
Thermal stress resulting from exercise-induced dehydration can affect performance, causing serious problems or even death for your horse. However, thermal stress is preventable with provision of adequate water and minerals, along with monitoring environmental conditions and using common sense.
eXtension Article: Respnsible Horse Care for Winter and Summer
eXtension Article: Horses and Water
eXtension Article: Equine Thermoregulation