I think in southern Iowa turnips after soybeans should work well. You may want to select an earlier maturing variety of soybean for your area to give the turnips a little more time to bulk up. The deer by me go crazy for turnip foliage once it is the only greens left (about Nov 1), and they will dig through the snow until all of the bulbs are eaten. Maybe try a smaller area this year to see if they will grow, and if the deer in your area like turnips.