Hilchos Shemittah v’Yoveil 5:11

A major principle of shemittah produce is that whatever is designated as food for people may not be used as a medicinal plaster or a bandage, not even for a person. This includes such things as wheat, figs, grapes, etc. This principle is derived from Leviticus 25:6, which says “yours for food,” meaning that whatever is designated as “yours” is for use as food, not for medical use. Anything that is not designated as food for people, such as thorns and brambles, may be used as a medical plaster for a human being, but not for an animal. Something that is not designated as human or animal food – here the Rambam names a few herbs – depends on one’s intentions. If one intended to use it as fuel for kindling, it’s treated as lumber; if he intended that it be eaten by people, it’s treated as produce. If something is designated as both food for people and animal feed, it is treated according to the stringencies of both categories: like food for people, one may not make a medical plaster of it, and like animal feed, one may not cook it.

Hilchos Shemittah v’Yoveil 5:12

One may sell human food and animal feed and then use the proceeds to buy human food, but one may not sell animal feed to buy animal feed. It goes without saying that we may not sell human food to buy animal feed. If animal feed was bought using human food, or if human food was traded for animal feed, the feed is considered like human food and may not be used to make a medicinal plaster, not even for a person.