Barley doesn't self replicate well. The seed head has been bread to not shatter and disperse seeds, so they tend to stay in the head, out of contact with the soil. If you let them be, the next year you will get a few, and by the year after that you would be hard pressed to find anything in my experience.
The other thing to note is that between the grasshoppers and field mice, any volunteer seed head doesn't really have a chance. It's hard enough growing the stuff intentionally.