Horses digest the fiber portion of their diet in the caecum and colons, called collectively the “hindgut”, where there is a huge population of bacteria which ferment the fiber and break it down. As they do so they give off as a waste product, short chain fatty acids (SCAs), acetic, butyric and propionic, they also generate a considerable amount of heat as they ferment the fiber. This benefits the horse twofold: the SCAs are absorbed by the gut walls and contribute to the horse’s energy needs and the heat helps to warm the horse.
Hay is the principle source of fiber in domestic animals, and provision should be made to obtain sufficient hay for them, as hay is a far better feed in very cold temps than any kind of concentrate feed, as well being far safer. This means that access to plenty of hay is very important to generate both the energy source and the warmth. Along with the hay, the horse needs a supply of liquid water, they can and sometimes do, eat snow, but they won’t eat as much as they need. Cold icy water hurts their stomachs, so when the temps drop the horses drink less, this puts them at risk of dehydration and colic. Providing liquid water is probably the hardest part of caring for horses in the cold temps.
The hay should be reasonably palatable, and preferably fed free choice, make sure there is enough room for all the horses to get access to it. If it can be fed in a dry area with some degree of wind protection so much the better. Horses usually have good winter coats which have excellent insulating properties, but a stiff wind will reduce the effectiveness of the coat. Horses with thin or poor coats might need some help in the form of a wind proof and waterproof blanket so they can retain body heat. Blankets should be checked twice daily in case they slip, bind and entangle the animal.
Place the hay in a sheltered area, either near a purpose built shelter or a clump of trees of even down in a hollow will help them. Offering 24/7 hay as well as access to liquid water and your horses should cope with these awful temps just fine.