Preventing Thermal Injury During Hot Weather

Now that we are almost into the “dog days of summer”, horse owners should pay attention to temperature and humidity while exercising their horses. We will likely experience the highest temperatures of the year over the next couple of months, and the humidity can be pretty high in the Southeast during the summer.

As the horse exercises in hot weather, heat builds up in the core.  Blood flow increases to transport that heat buildup to the surface (skin) where hopefully the heat is dissipated.  One way that horses dissipate heat is through sweating.  Sweat is composed of water and minerals, and helps dissipate heat through evaporation and provides evaporative cooling for the horse.  It goes to reason that the more intense the exercise, the greater the heat buildup, and the greater the loss of fluid to sweating is.

Thermal Injury is caused by animal dehydration.  A horse’s water intake can increase 300% with prolonged exercise, and there is a direct correlation between fluid loss, inability to maintain temperature, and onset of fatigue during endurance exercise.  Too much dehydration leads to electrolyte and pH imbalance, fatigue, uncoordinated gait, and increased risk of orthopedic injury or death.

To minimize the risk of thermal injury during exercise, provide plenty of water, adequate salt and minerals, and a good, balanced ration.  After strenuous exercise, allow horses a 24-hour period to rehydrate between exercise programs.

To further minimize the risk of thermal injury, pay attention to weather conditions.  Knowing the temperature and relative humidity allows us to calculate a “comfort index”.  The comfort index is the sum of the temperature in degrees Fahrenheit and the percent relative humidity.  A total less than 130 indicates that thermal injury should not be a concern.  If the score is between 130 and 150, a horse will sweat, but should not incur serious thermal injury.  If the comfort index is over 150 and the relative humidity is over 75%, heat dissipation can be a problem and horses should be monitored closely during exercise.  If the comfort index is over 180, heat dissipation will likely be a problem and the workout should be discontinued or rescheduled.

Under normal conditions, diet and mineral supplementation, along with adequate water, should be enough to maintain electrolyte balance.  However, with intense exercise programs, the horse will sweat a lot, which can lead to water and electrolyte deficiency.  Symptoms include weakness, muscle cramps, acid-base imbalance, and decreased performance. 

As the horse becomes more acclimated to the exercise level, the heat, and humidity, its mechanisms to conserve sodium and potassium improve.  Horses that are not acclimated to heat and humidity, and that are starting a vigorous exercise program should be monitored closely, and may require electrolyte supplementation to avoid thermal injury.

Much of the information in this article and additional information on the topic of thermal injury is available at http://articles.extension.org/pages/25671/responsible-horse-care-for-winter-and-summer by Steven M. Jones, Extension Horse Specialist.