Beating the heat involves water—both on the inside of your horse via drinking and on the outside of your horse via hosing down.

Summer’s heat and humidity can be much more than just uncomfortable. They can be deadly. Horses lose their lives every year to heat stroke. Countless others struggle through anything from weakness to colic as a result of inadequate care in hot weather. Don’t let this happen to your horse!

How Your Horse’s Body Cools Itself
The business of simply being alive, breathing, digesting, producing manure, processing foods, etc., keeps your horse’s body temperature in a range between 98.5 and 101 degrees Fahrenheit. When your horse begins to work, an inevitable consequence of increased energy generation and movement is for the body temperature to increase. To avoid reaching temperatures that can damage the brain and organs by interfering with enzymes, your horse must have a way to get rid of that heat.

Some of the heat is transferred to air exiting the lungs, but this is not enough for efficient cooling in an animal this size. The remainder of the extra heat is carried from the interior of the horse to the skin surface by the blood stream. Blood vessels very close to the surface of the skin dilate and heat is lost from the skin’s surface by several mechanisms:

• Conduction is the transfer of heat from the body to the cooler air. The larger the difference between the air temperature and the body temperature, the more efficiently conduction removes heat.

• Convection is the movement of hot air away from the surface of the body, replacing it with cooler air. Since hot air rises and cool air falls, this will occur naturally but the addition of a breeze makes it much more efficient.

• Evaporation of sweat is the final method of removing heat and is absolutely necessary for efficient heat removal. Even in subzero temperatures, a hard working horse will still sweat.

Methods for Cooling Your Horse
When you understand conduction, convection, and evaporation, all the commonly used cooling methods make sense. Slow walking creates a bit of a breeze over your horse’s body surface, increasing convection. A fan works even better! Conduction increases when your horse is in the shade, where air temperatures are cooler. In extreme cases, air conditioning is used for conduction. Sweat (and hot fluid losses off the lungs) is nature’s way of cooling by evaporation. You take advantage of the same principle of evaporation by hosing off your horse. During hosing, heat is also efficiently lost by conduction as long as the temperature of the water is cooler than the surface of the horse’s body. The most efficient method of all—which has been adopted for Olympic horses performing under dangerous summer conditions—is to use misting fans. The water from the fans causes cooling by conduction and evaporation, while the breeze from the fans improves both evaporation and convection.

Allowing your hot horse to drink also has cooling effects. What happens when you add milk or an ice cube to your hot coffee? It cools off, of course. The temperatures of the two liquids equalize. The same thing happens inside your horse when he drinks. The temperature of the consumed water and the interior temperature of your horse will equalize. Allowing him to drink also serves another very important function. Sweat loss means water loss. Even at low levels of sweating, a horse can lose up to 4 liters of water (over a gallon) an hour. As exercise intensity increases, it can climb to 3 or even 4 times as much. This can result in sweat losses in only one hour that are equal to half a day’s total water consumption. That’s a lot of water, and losing it quickly causes dehydration.

Dehydration severely cripples your horse’s ability to reduce his own body temperature by moving hot blood to the skin surface. If the circulating blood volume is not normal because of dehydration, blood will be diverted away from the skin and preserved for the organs.

Last, but far from least, is the fact that an exercising horse loses electrolytes along with water in his sweat. The cells in your horse’s body function like small batteries with different concentrations of electrolytes inside versus outside the cell—there are even differences in concentrations between the structures inside the cells themselves. Another important function of electrolytes, especially sodium, is to "hold" water in the body. Sodium is so important to maintaining enough water in the body that the brain reads the concentration of sodium constantly, with thirst being triggered if the concentration of sodium gets too high and salt hunger triggered if sodium gets too low.

Replacing Electrolytes
Electrolyte losses in sweat cause dehydration, overheating, muscle problems, and poor intestinal tract movement. Replacing water is easy, but to keep it in the body, the electrolytes also have to be replaced.

Table I shows losses of the three major sweat electrolytes over an hour of work, compared to the horse’s baseline need. As you can see, it’s quite a lot. To fix this problem, you just start feeding a scoop of electrolytes, right? Wrong!

To figure out what your horse needs, you have to know what is already in his diet. Except for a small amount that may be added to commercial grains, there is basically no sodium in the diet. Sodium and chloride are what make up plain white table salt. An average-size horse needs 10 grams of sodium a day, not counting sweat losses. That amounts to just under an ounce (2 tablespoons) of table salt. Hay does contain chloride, although not quite enough to meet requirements. When you add in the chloride that comes from the plain salt, the horse will then have enough with even a bit left over. Potassium is plentiful in hay. Just five pounds of most hays will give the horse around twice as much potassium as is his baseline need.

Electrolyte supplements are meant to replace sweat losses, and good ones will provide close to the amounts listed in Table I in the low sweating column, per dose. However, as noted above, you don’t need that potassium. There is a place for electrolyte supplements, but they have to be used correctly.

Checklist for Proper Use of Electrolytes
• Start by meeting your horse’s baseline sodium and chloride needs with plain salt at 1 ounce/day in winter, 2 ounces/day in summer.

• If your horse is working two hours or less at low sweating rates, or one hour or less at moderate sweating rates, add 1 extra ounce of salt for each hour of low sweating work, 2 ounces for each hour of moderate sweating.

• If working longer than the times above, feed the extra salt only to meet the needs of the first two hours (or the one hour of moderate sweating), then use an electrolyte replacement for any additional work above that level.

Last but not least—and this is crucial while also being easy to do—give all horses as much water as they want, as often as they want it!