As we move through the height of our summer competition season, it is not uncommon to have multiple days of severe heat and humidity—you know, the days when you sweat standing still.

But heat and humidity can be harder on your horse than it is on you. Most of us have worked to motivate a sluggish horse through the final jump off or Day 3 of a competition, but why does this happen? Was our training off the mark? Perhaps they need a different energy source in their feed? Or was it that energy-zapping heat and humidity? Let's take a close look at the role of sweating in the horses' ability to cool and how extreme heat and humidity can affect their ability to perform.

What happens to horses exercising in the heat?

As the horse begins to work, heat is produced as a by-product of muscle contraction at a 4:1 ratio. As the body temperature climbs and adrenaline levels increase, sweat glands respond by producing a hypertonic (highly concentrated) salt solution that coats the hair. Under normal circumstances horses cool by evaporative cooling (when the sweat coats the hairs and as air flows over them it pulls the moisture and the heat off the horse) and convection (when blood vessels near the skin dilate and allow the transfer of heat from the blood into the air). The movement of air over their body is paramount to both of these mechanisms.

During intense exercise, horses can lose up to 10 to 15 liters (about 2.6 to 3.9 gallons) of sweat per hour. Once their body temperature reaches greater than 42°C (107.6°F), the respiratory system kicks in to help "blow off" some of the extra body heat (appr