If you plant in the fall, the grain will get a head-start before the chilly winter hits. This may result in a stronger plant and therefore a better yield at harvest time (approx. 20%). You also might be able to harvest a few weeks earlier (e.g. July/August instead of August/September).
IMPORTANT: Note that some cereal grains simply won't survive cold winter temperatures (e.g. oats, quinoa, amaranth), so must be planted in the spring.
