Managing Horses in Hot Temperatures

Introduction

Horses are always producing some level of body heat through metabolism. As horses eat, their body will break down that food into energy, and this creates heat. When horses exercise, they produce a lot of additional heat in their body. This happens as they use energy for muscle contractions.

Heat must be lost from the body, and to do so, heat must move to the surface (skin) of the animal through blood vessels. Once the heat reaches the body surface of the horse, there are several ways for horses to lose heat. This is considered heat transfer, and there are 4 ways that this can happen for horses (and all mammals). The four methods are radiation, conduction, convection, and evaporation. It is important to note that no matter which method of heat transfer, heat always moves from the area that is warmer to the area that is colder. In other words, we can say that heat moves down a gradient. Additionally, for the methods of radiation, conduction, and convection, horses can not only lose heat through these methods but also gain heat. This is important to consider when we think about warm humid environments and how horses could be feeling the heat of that environment. Evaporation is only used when animals lose heat to the environment.

Let's walk through the four ways that horses can gain or lose heat.

Methods of heat transfer

Radiation involves heat that is transferred between the body and the environment through infrared radiation. On a sunny day, the number one source of environmental radiation will be the sun. You can feel the effect of this when you stand in broad daylight versus standing under shade; it will feel warmer to stand out in the sunlight versus in the shade. There is also reflective radiation, and that can come from the animal's surrounding environment. For example, consider the footing that your horse is working on. Footing that is darker in color will radiate more heat than footing that is lighter in color. In terms of heat loss, radiation is a minor way in which horses lose heat.

Conduction involves the transfer of heat via direct contact of the animal with the environment. In other words, two objects must be directly touching for heat to transfer. For horses, this is not a common way to gain or lose heat. The one exception is water. Water has a higher conductivity rating compared to air, so a wet horse will lose heat more rapidly than a dry horse. Keep this in mind for later when we talk about cooling off horses in hot environments!

Convection involves the transfer of heat via moving air or water. This is also not a major way that horses will gain or lose heat. It can make some difference, though - for example, think about the effect of wind chill in the winter. In the summer, you may use fans in your barn to cool your horse – this is using convection. You should also consider convection when you haul your horse; a breeze through the trailer might feel good in the summer, but might make them feel colder in the winter.

Evaporation is the final method of heat transfer, and it involves the conversion of water to vapor. As water is converted to a vapor, it will pull heat along, removing the heat from the object. For horses, it is the sweat that is going to evaporate off the horse, and as that sweat becomes vapor, it is pulling heat molecules off the horse. The main way that a horse is going to lose body heat is through sweating. Horses are perhaps one of the sweatiest animals, similar to humans. In fact, horses and humans are the only species that cool primarily through the evaporation of sweat. A horse exercising hard in a hot and humid environment can lose up to 3 gallons of sweat an hour. This is very important to remember as we think about our horses staying comfortable in warm climates and replacing fluids and electrolytes.

Horse sweat has a lot of electrolytes in it; compared to human sweat, horse sweat is more concentrated in electrolytes. In scientific terms, we would call horse sweat hypertonic and human sweat hypotonic. From a practical standpoint, this means your horse is losing a lot more electrolytes from their sweat than you are. When we consider refueling our horse and getting them rehydrated, you should consider how sweaty they are and whether water alone is sufficient to fully hydrate them. Severe electrolyte loss can result in numerous health issues for horses, including muscle cramps, weakness, poor performance, and acid-base issues.

If you've ever noticed a foamy white substance on your horse's chest or between the hind legs after exercise, you are seeing latherin. Latherin is a protein that helps the sweat spread out evenly across the body hair of the horse and improves the evaporation rate of sweat. It does this by increasing the amount of sweat covering each strand of hair on the horse. This increases the surface area available for the sweat to evaporate from, thus cooling the horse more rapidly.

There are some horses that cannot sweat properly, this condition is known as anhidrosis. It tends to occur in horses that live in hot environments and can occur in either hot and dry conditions or hot and humid conditions. In the United States, it tends to occur more in the southern region of the country. It is still not understood why anhidrosis happens. Horses with anhidrosis exhibit patchy/lack of sweating across the body, elevated heart rate and respiratory rate, and exercise intolerance. Sometimes the hair coat can look quite poor. Unfortunately there are no cures for anhidrosis, and typically the best thing for the horse is to relocate it to a cooler environment.

Managing horses in hot climates

When temperatures start to get both warmer and more humid, the effectiveness of sweating to cool the body decreases. So, if your horse is working hard and it's a hot, muggy summer day, the horse's body will struggle to cool off. So, how do we keep our horses cool?

Let's first consider the horse from a day-to-day perspective as it's living in a pasture or stall. Make sure that your horse has access to fresh, clean water at all times. Horses can drink anywhere from 5 to 15 gallons of water a day, depending on the weather and if they have been exercised. In addition, make sure your horse's diet is well-balanced and that they are getting enough salt in their diet. Two key electrolytes are sodium and chloride, and those are both found in salt. If your horse is on a forage-only diet, they should have access to a salt block or a mineral block. You can also top dress feed with loose salt; for a ~1,100-pound horse, you would want to add ~1 ounce or ~2 tablespoons of salt per day. Talk with an equine nutritionist if you have concerns about the amount of salt in your horse's diet. Make sure that your barns and trailers have good ventilation, whether it's open windows or using fans. If your horse is outside, make sure they have access to shade somewhere in their pasture at all times of the day.

Exercising horses in hot environments

When exercising your horse in the summer, check the weather forecast. Consider not just the temperature but also humidity, wind, and whether it's a sunny or a cloudy day. An 85°F day that is cloudy with some rain and wind will feel very different than an 85°F day that is muggy and sunny. Many weather apps on phones have a "real feel" forecast and that usually takes into consideration both the temperature and humidity. These apps often also include how cloudy and windy it is, which can all help to determine the thermal load that your horse may be experiencing. Remember that in very humid environments, sweating is less effective because it cannot evaporate as well. So your horse will be hotter, and extra care will need to be taken to manage your horse’s body temperature.

A tool that many high-level horse events (like the Olympics) and human events use is the wet bulb globe temperature (WBGT). This tool is considered the gold standard for assessing thermal load for both horses and humans. It measures the temperature, humidity, radiation, and wind of a given environment. There are models that can be purchased online that are not terribly expensive if that is something you'd like to install in an outdoor arena for example. 

When exercising your horse, there are a number of things you can do to reduce the thermal load for your horse. Consider the time of day when you exercise your horse in the summer; morning is the coolest time of day, followed by the evening. The hottest time will generally be midday-late afternoon, so try to avoid exercising at that time. Consider reducing the distance or effort of the training session - perhaps do fewer/shorter jumps or ride for a shorter distance than you normally do. On very hot, humid days, you could even decide not to exercise your horse. Consider the horse's fitness level, as an unfit horse will struggle more with the hotter temperatures than a horse that is in shape. Horses should be allowed to drink water before, during, and after exercise, particularly if the exercise is strenuous or long in duration. This will not increase the risk for colic, it is essential to keeping your horse hydrated.

After exercising your horse, there will be several signs to indicate that your horse needs to be cooled off quickly. Your horse will be sweaty, blood vessels will be prominently visible on the skin, the skin will be hot when touched, they may be blowing hard through their nostrils, and there may be behavioral changes (tired or unsteady on their feet). The normal body temperature of a horse is approximately 99 to 101°F. If a horse is experiencing heat stress (rectal temperature above 103°F, increased heart rate and respiratory rate, profusely sweating, exhaustion/lethargy), you need to rapidly cool the horse with water and contact your veterinarian.

How to cool off your horse

To rapidly cool off a horse, hose off the entire body for 5 to 15 minutes until symptoms subside. Thoroughly wet the entire body of the horse - do not focus on certain areas. There is no need to scrape water off the horse - just keep applying water! If you don't have a hose nearby, use a sponge to liberally apply water all over the body or even dump buckets of water on your horse. As long as the water is cooler than the horse’s body, it will work.

There have been multiple research trials looking at the most effective way to cool a horse after exercise, and all have come to the same conclusion – continuous application of cool water for 5-15 minutes will lower the body temperature of the horse the quickest. It does not have to be ice water – water from a hose is perfectly fine. When water is being applied to the horse, conduction is utilized to remove sweat from the horse. Research has also shown that scraping the water will not improve the recovery of the horse – in fact, some studies have shown that it slows down the rate of recovery. The water on the horse is still cooler than the horse itself, so heat is still being pulled away from the horse. If you want to scrap your horse at the end before you put them in a stall, that is fine. But during the active cooldown, just keep applying water.

Conclusion

Even in warm temperatures, it is possible to enjoy the outdoors with your horse. Know the signs of when a horse is hot, and if you need to rapidly cool off your horse, continuously apply water over the entire body until symptoms subside. Be sure that your horse is well hydrated and has sufficient electrolytes in its diet. Contact your veterinarian if you suspect your horse is suffering from heat stress or heat stroke.