What is the main factor in a horse's shedding of their hair?

The main factor influencing a horse's shedding of hair is lighting, particularly the changes in daylight hours that occur during different seasons. Horses typically molt their coats in response to the amount of natural light they receive, as this signals their bodies to prepare for temperature changes associated with different seasons. In the spring, as days lengthen, horses will shed their thicker winter coats to make way for a lighter summer coat. In the fall, the decreasing light influences them to grow a thicker coat for winter.

While temperature, exercise, and diet can have impacts on a horse's health and overall coat condition, they do not directly trigger the shedding process like changes in lighting do. Temperature changes do play a role in how quickly or slowly a horse sheds depending on whether it’s warm or cold, but the primary cue for the shedding response is the duration of daylight.