Horses actually do quite well even in fairly cold weather if a few conditions are met. They need plenty to eat, water to drink, and shelter from wetness and wind.
Food is first on the list. Forage is very important for horses – both for gut health and for warmth. Eating forage increases body heat. In cold weather, it would be ideal if your horse could have free choice hay. If that is not possible, consider adding some alfalfa pellets or cubes to your horse’s diet. Additional forage is better than simply adding another scoop of grain. A cup of oil mixed in with your horse’s grain will add some fat and also provide extra calories for warmth.
Water is an essential nutrient. It is very difficult for a horse to eat enough snow to meet the daily requirement for fluid. Also keep in mind that extra energy is lost melting the snow internally. There are many ways to provide warm water. Most barns do not have running hot water, but you can purchase tank or bucket heaters that can warm up water for your horse. Providing some warm water twice a day is very helpful in cold weather.
Shelter means a way for your horse to stay dry and out of the rain or snow. It also means protection from chilly gusts of wind. A wet horse standing out in cold wind is a recipe for disaster. Even a three sided shed can be adequate if it has a southern exposure and your horse has plenty of feed, but a barn is the best choice.
Whether or not to blanket your horse in cold weather can depend on many things. If your horse is not being worked, its coat can be allowed to grow out and will insulate nicely. If you need to blanket your horse, you should check on the horse at least twice daily. Take the blanket off once daily and check for any areas that are chafing. Some horses are “rough” on their blankets – ripping them, chewing them, getting caught up in straps, etc. These horses are better off without a blanket. If your horse has been wearing a blanket and it gets wet, you’ll need to remove it, clean it, and replace it. A wet blanket is worse than no blanket at all for your horse.