Harvest flax seed when 90 percent of the seeds are brown. Pull the plants up at the roots, tie them together and hang them upside down to dry. Remove the seeds from the plant when dry.
Grind the seeds before feeding them. Use a mortar and pestle or coffee grinder to grind the seeds. Ground seeds are easier for horses to digest, but they do not have a long shelf life because the oil in them turns rancid. Store ground seeds in the freezer.
Feed 1 tsp. of flax seed meal to each horse once a day. Add the flax seed meal to any other grains the horse is consuming.