Summarize:

Since your horse's natural foragegrassis unavailable, stick with good quality hay as the bulk feed source. In addition you can supplement this with high fiber foods such as sugar beet or alfalfa. These foods take a lot of chewing, which helps keep the horse occupied during the long dark hours of winter. An older horse with poor teeth may struggle with certain food sources. Try feeding older horses on haylage, or you can soak hay cubes to make them easier to chew. Horses normally burn some reserves of body fat in order to keep warm over winter. If you try to avoid this by feeding energy-dense foods (such as nuts or pellets), they may become over-stimulated and buzzy. This can lead to pent up energy and possible accidents in the stable. With hay as the main food source for your horse over the winter, you may want to add vitamin or mineral supplements in order to keep the horse healthy without overfeeding. This will provide the horse with everything it needs nutritionally without adding the extra calories or too many nuts or mixes. Consult your veterinarian and the recommended servings for supplements based on what you add to your horse's diet.
Use good quality hay as the bulk of the horse's feed. Don't overfeed your horse. Add mineral supplements to your horse's feed.