It is easy to confuse what the horse needs with what we think the horse needs to be comfortable. The two things horse owners have the most control over is housing and nutrition. Horses do not have to be kept in their stalls covered with layers of blankets to be comfortable. Horses have a thick winter coat of hair to keep them warm during the cool seasons of the year. This thick hair coat lets the horse remain warm without much human assistance.
Body heat is trapped between the hair fibers providing a layer of insulation from the environment.
Placing a blanket on a horse actually compresses the hair fibers, reducing their ability to trap heat. Horses begin to put on their winter coat when daylight hours begin to shorten. The length of day, not temperature, stimulates hair growth. Cold weather by itself will not make healthy horses ill. Soaking rains, snow and wind can make horses sick.