If a horse is an easy keeper (does well on hay and pasture) adding 1/2 cup ground flax when no green grass is available, along with a little soaked alfalfa and beet pulp is good. It also makes a good base to mix supplements with.
For hard keepers (don't keep their weight, either just in the winter or year round) the flax, larger amounts of beet pulp and alfalfa, black oil sunflower seeds (BOSS) and rice bran are good choices.
As a kind of warning, some horses seem to be "allergic" to alfalfa. Some go bonkers when fed more than a pound or so and others hoof quality decreases. The hotter breeds may react more with too much energy/ spookiness while fat horses getting too much alfalfa have the hoof issues more often. Just have to increase slowly and watch for issues.
Just so you guys know, I'm a "Natural Hoofcare Practitioner", a natural barefoot trimmer or barefoot farrier if you will. I do hoof boot fitting, nutrition work and mineral balancing.