How should you choose cover crops? First, determine a goal for using the cover crops. What do you want to do? Once the goal is identified, then it's time to think about the current crop and next crop in rotation. This is where it can get tricky, so I'll run though some ideas for different rotations.
Corn - soybean: One option is to interseed corn with cereal rye (and radish) any time after the five leaf stage. Cereal rye with establish under corn, grow after harvest and in the following spring prior to planting soybean. Do not use a chopping head for corn harvest - leave the stalks standing and intact in the soil.
Monitor cereal rye in the spring to make sure it doesn't dry out the seedbed before planting soybean. Soybean can be seeded or planted into living or terminated cereal rye. Do not use tillage to terminate cereal rye (instead use herbicide) - tillage will commit you to five more tillage passes because of the root ball on cereal rye.
Corn - soybean - wheat: Follow the above steps for corn-soybean. After soybean, you can rely on the remaining corn and cereal rye residue and let that be your "soil cover." If you want to try a cover crop, flying on oats and radish before leaf drop on soybean is one option. Wait until just before leaf drop because the canopy can limit seed reaching the soil and shade can inhibit establishment. Use oats before wheat so you have herbicide options for volunteer oats in wheat.
At wheat harvest, cut the wheat high to leave residue standing or use a stripper head. Run a drag (if needed) to spread out residue. Then seed radish (and maybe some flax and faba bean) in bio-strips on the spacing of next year's corn crop. Use the residue managers on the planter to push aside some of the wheat stubble and residue from the cover crop to help create dark strips. The volunteer wheat will also help manage moisture.
Corn gets planted into the cover crop strips the following spring. If there is getting to be too much fall growth and you get nervous, this can always be terminated with herbicide before winter, but the strips should winter kill.
Corn - soybean - wheat - sugar beets: Same as above. I'll admit that I haven't worked much in sugar beets, so I'm not sure if the biostrips will work well going to sugar beets. This would be a "try on 20 acres and not 200 acres" practice for getting started. Flying on cover crops into sugar beets before topping is an option or seeding in pre-harvest areas can help (use barley/cereal rye mix in pre-harvest areas).
If using cereal rye (which is what I would recommend with sugar beets), then plan on terminating the cereal rye 14 days in advance of planting corn. Or skip the corn in rotation and plant soybean directly into the cereal rye (again monitoring moisture in fall and spring).
Sunflowers in rotation: I'm not sure how it will go planting sunflowers into cereal rye - we saw stunted growth when we did it, but didn't measure yield. If you are going to strip till into the cereal rye before sunflower, make sure to put in the strips in the fall before the root structure gets established.
Edible beans in rotation: I haven't worked much in this area, but am getting a lot of questions. Flying on oats, barley or cereal rye pre-harvest is an option. Just remember that if you use barley or cereal rye, you will not be planting wheat the following year.
There is still so much to learn when it comes to cover crops in rotation. This is just a start to what we have tried at our research and demonstration sites and what we are learning.
I really appreciate information that farmers share with me about their experiences. Follow me on Twitter at @NDSUsoilhealth, or email me at email@example.com.
Be sure to check out our fall field tours for more information. Go to ndsu.edu/soilhealth under the "events" or catch me at the Big Iron Farm Show in West Fargo at the North Dakota Corn and North Dakota Soybean booths.