Thomas Gore

“outdoors in winter in most parts of the country, as long as they have adequate shelter from wind, rain, and snow. A shed open on one side is an ideal shelter for all seasons. Horses grow a long winter coat and store a layer of fat beneath their skin, both of which provide excellent insulation. However, when a horse is kept in a heated stable, he does not adapt and is more likely to suffer from chilling and pneumonia when taken outside. Horse blankets can be used to prevent chilling. For a horse in winter with a dry coat, or one who is used to being inside, the blanket is beneficial when the wind-chill temperature drops below 20°F (-6.6°C). For a horse in summer with a wet coat, wind-chill discomfort becomes a factor at temperatures below 60°F (15.5°C). Use common sense. If your horse has adapted to outside conditions, he probably does not need a blanket. However, for old horses, who are unable to regulate their body temperature well, or show horses without their natural haircoat, a blanket is a thing of comfort. Digestible energy is the principal dietary concern in cold weather. Protein, vitamin, and mineral needs increase slightly. In winter, it is important to feed a ration that helps the horse create internal heat. High-quality hay is best for this, and is a better choice than grain. This is because roughage is digested by bacterial fermentation in the cecum and colon, which produces a great deal of heat.”
― Thomas Gore, Horse Owner's Veterinary Handbook