Just like a human, a horse benefits from shade and a cool breeze. If these are not available in a pasture or paddock, then keeping your horse in during the day might be the best option, turning the horse out at night. One factor that contributes to the well-being of the horse during the hot weather is the weight of the horse. An over-weight horse has a harder time keeping cool because of the insulating layer of fat. The excess fat interferes with the natural cooling system and it increases the amount of food necessary to maintain the horse's energy level.
More food means more body heat production.
This causes heat stress in hot weather. It is important to keep your horse at a body weight that will meet its performance needs. Feed intake requirements change during hot, humid weather due to the metabolism of grains, protein and fat. Energy from the diet is needed to maintain body weight in both cold and hot weather. However, during hot, humid weather, the horse's appetite and therefore, feed intake, decreases.