I assume you are meaning cover crops, in our area the big ones are rye, raddishes, and turnips. They are either harvested or grazed and are supposed to help out with soil fertility and erosion control.
We use oats but I think that would be a poor choice for you. You'll start getting winter Kill in oats at around 20 degrees. Wheat is more cold tolerance and grain rye even more so. I agree turnips or radishes mixed in with the small grains may be a good choice. Again I don't think oats are going to be cold tolerant enough for your area
