What factor reduces the efficiency of cooling in horses?

The factor that reduces the efficiency of cooling in horses is the insulating effect of hair and subcutaneous fat. Horses have a layer of hair and fat that provides insulation, which can be beneficial in cold weather by keeping body heat from escaping. However, this same insulation can hinder the horse's ability to dissipate heat when the temperature rises. When a horse becomes hot during exercise or hot weather, its body needs to cool down effectively through sweating and evaporation. If the insulating layer is thick, heat can be trapped, and the natural cooling mechanisms become less effective, leading to overheating.

Humidity, high activity levels, and exposure to sunlight can exacerbate the heat stress on a horse, but they do not inherently reduce the efficiency of the horse's cooling mechanisms in the same way that the insulating effect of hair and fat does. These factors can contribute to a challenging environment for cooling, but insulation remains the primary factor in decreasing cooling efficiency.