The crop that has been talked about most for use as a cover crop is turnips and/or radish. Both are good choices and have benefits like breaking up compacted soils, retrieving deep nutrients and after the tuber dies, it leaves a wonderful depression within the soil surface that facilitates water infiltration. One of the major drawbacks of these crops are that they are non mycorrhizal. A mycorrhiza is a symbiotic association between these natural soil fungi and many crop plants. This association helps the crop assimilate phosphorus and can be very important to crops like corn when phosphorus becomes less soluble – like in soils that are wet and cool. A bare soil, or one that has a crop that does not support mycorrhizal associations will have a greater chance of showing ‘fallow syndrome’ symptoms the follow planting season. Being non mycorrhizal is a big deal. For this reason, planting a turnip or radish cover crop by itself is probably not the best choice but a good choice as a base for a cover.
This is my suggestions: use the turnip/radish crop a small grain such as oats. This will be one of your most economical choices and you get the advantage of having the turnip/radish as well as a crop that supports mycorrhizal associations. Adding a legume to the mix (if planting in lost bean acres) will give some recovery of the lost legume credit for your next corn crop. There are also mixes available that create a great deal of biomass that can be chopped or baled if needed for forage as well.
A cover crop can be as simple as you need it to be; don’t get caught up in the idea that you mix needs to be some exotic blend of many different species. Just remember that if you are using a small grain or other patented protected crop that bin running the oats, wheat or other as use in the case could be violated the Plant Variety Protection laws.
One of the suppliers I talked with today told me that if there is a place available, he will supply some seed to create a cover crop plot in order to get some local exposure to the cover crops. I would like to take advantage of a drowned out spot to do so if anyone is interested allowing so.