Heat Stress in Horses

Written By Dr. Kris Hiney

How to cope so that summer is great for both of you!

As we approach the hottest part of the summer, it is important to review some basic strategies that will help us avoid heat stress in horses.  Often it is the summer months where we get the most enjoyment from spending time with our horses, but it is our job to make sure that we don’t overdo it with them.

So what conditions might make our horses over heat? Obviously high environment temperatures are the key, but also prolonged or intense exercise, or inadequate hydration may all contribute to heat stress.  Horses, just like us, dissipate the majority of their excess body heat through sweating.  Horses have a tremendous ability to sweat, and can sweat as much as 10-12 liters per hour.  Depending on the environmental temperature and the work load, it is possible for horses to become dehydrated in as little as 2-3 hours.  Horses that have inadequate access to water will not be able to sustain the same sweating rate as a horse with proper hydration.  For tips on water intake in horses please see Optimization of Your Horse’s Water Intake.  Horses also physiologically don’t help themselves out when it comes to hydration.  When we sweat, our sweat is hypotonic, or has less electrolytes in it, than does our blood.  Horses on the other hand, have either isotonic (the same) or hypertonic (more electrolytes) than does their blood. This allows horses to sustain sweating rates longer than we can.  So what does that matter? It is  the increase in tonicity of the blood through fluid loss that drives thirst.  As horse’s blood does not increase in electrolyte concentration with sweat loss, they may not have the natural stimulus for thirst.  Therefore a dehydrated horse may not actually drink when offered water. 

So when is it important to back off from activity with your horse?  Always think about both the temperature and the humidity.  Adding these two values together provides the heat index.  Horses will cool themselves normally, providing a normal hydration state and avoiding fatigue, if the heat index is below 130.  Conditions above a heat index of 150, such as 90 degrees Fahrenheit with 60% humidity, require more assistance in cooling.  With a heat index above 170, you might want to consider doing something else instead!  These conditions could be dangerous for both your horse and you!  Maybe consider watching a training video instead and give your horse a break.  If you have to ride, consider setting your alarm clock for the early morning hours or late in the evening.  More importantly, if you have to haul a long distance, it may be better to drive at night.  Trailers may often have inadequate ventilation to keep your horse cool.  In addition, the muscular work of balancing puts an additional heat load on the horse.  If you are considering a night trip, make sure that you are capable of driving at night or consider a good audio book to keep you awake.  It is important that everyone arrives at their destination safely.

Now, let’s say that we are going to ride and there is a heat index of 145.  What can you do to provide assistance to the horse for cooling?  Obviously we need to carefully monitor our horse throughout activity.  But we can help actively cool our horse through the  four ways animals to exchange heat: through the process of radiation, conduction, convection and evaporation.  Sweating obviously employs evaporation as a major way for the horse to dissipate heat.  Clearly a well hydrated horse is necessary to maintain stable sweating rates to dissipate thermal load.  But the environment plays a great role in how effective evaporative cooling may be.  High humidity levels will limit evaporation, which is why paying attention to the heat index is so key.  Water applied to the horse can greatly aid in cooling as it evaporates off the horse’s body.  Applying cool (not cold) water to areas which have large blood vessels near the surface of the body is the most effective.  Blood will cool as it passes through these areas and then return to the trunk of the body to help dissipate the heat load.   These areas include the legs of the horse and the neck of the horse.  The major blood vessels in the horse’s leg lie to the inside, so pay more attention to applying water to these areas. Continual application of cool water will prevent the warming of the water on the surface of the horses’ skin.  Otherwise, use a scraper to remove the warmed water and increase the rate of evaporative cooling.

Convection is another major way that an animal loses heat.  Convection simply is the heat that is lost due to air movement.  If you think about wind chill factors in the winter you can easily see how effective wind is in cooling!  Supplying fans or keeping the horse in an area with wind flow is ideal.  Misters with fans are often used in dairies in aiding with cow comfort, combining these effective cooling techniques.  If humidity is not high, these are fantastic methods to keep horses cool.  Fans with higher velocities will also provide more effective cooling.  If you live in a hot climate and have access to electricity, putting a fan near the arena will aid in cooling during rest periods.  Always make sure that your horse’s rate and respiration rate have dropped before returning to work.

We often think of radiation as a way to add heat to a system, but radiation simply means heat transfer through space.  The sun adding heat to the horse is an example of radiant heating.  We can avoid additional heat load by keeping the horse in the shade or riding in shaded areas.  The horse can also transfer its heat through space to any object that is cooler that it is.  While not practical, horses standing next to ice blocks would be radiating heat to the block.  However, standing under trees allows the horse to radiate some heat up to the leaves of the tree which are continually cooled by their own evaporation. 

Finally, the last method of heat transfer is through conduction, or the direct transfer of heat between objects of differing temperatures.  An example of conductive cooling would be a dog lying on a cooling mat or digging into the cool earth.  Any surface that is cooler than the horse that its body is in direct contact with will aid in cooling.  This is why cool water applied to the horse’s body helps to cool it. Remember the key is that the water is cool, not cold.  Cold water can actually result in vasoconstriction which can limit blood flow to the horse’s skin.  If a continual supply of water isn’t available, placing cool wet towels on the horse’s body would be an example of conductive cooling.  However, continual reapplication of cool towels is necessary as the horse’s body heat is transferred to the towels.

Next month we will discuss conditioning programs to prepare our horses for work in the heat, as well as dietary adaptations that may keep them cool.

 

 

 

 

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