Horses prefer, and are better off, outdoors even in cold weather, and will acclimate to cold temperatures if given the opportunity.
However, horses should have access to shelter from wind, sleet and storms. Easy access to a stable or open-sided shed works well, as do trees if a building is not available. In the absence of wind and moisture, horses, acclimated to the extreme cold can tolerate temperatures near 5 degrees. If horses have access to a shelter, they can tolerate temperatures as low as 40 degrees below zero.
However, horses are most comfortable at temperatures between 18 degrees if the horse has a winter coat and 59 degrees if the horse has a summer or wet coat. Keep in mind that a horse's winter coat can be an excellent insulator, but its insulating value is lost if it gets wet, so it is important to keep the horse dry and sheltered from moisture.
Blanketing the horse is beneficial when the wind-chill and/or temperature is less than comfortable. Blanketing is not beneficial when the temperature is comfortable for your horse. If you house your horse in a closed and heated stable, make sure it is properly ventilated. Poorly ventilated barns can result in respiratory problems.
In terms of feeding, energy needs for maintenance of horses increase 1.4 percent for each degree of temperature below 18 degrees. It is best to provide the extra energy as forage (hay). For example, if the temperature is zero, a 1,000-pound idle, adult horse would need an additional 4 pounds of forage each day. Ideally, forage would be provided on a free choice basis so the horse could eat what it needed. Much more heat is produced when forages are fermented, which helps heat the horse from the inside, compared to the digestion and absorption of grains.
Most data suggest that the need for other nutrients do not change during cold weather. Water should be kept at 45 to 65 degrees (use of a tank heater, etc.) to maximize water consumption. Waters should be cleaned regularly (even in the winter), and clean, fresh water should always be available, regardless of temperature.