Is a cereal rye cover crop enough?
I am often asked this question: "Is using only cereal rye as a cover crop enough or do I need more diversity?" My answer is, "It depends on what you are trying to do with that cover crop."
One goal of using cover crops is to build diversity in the cropping system, but there are plenty of other goals associated with using cover crops, such as reducing erosion or managing moisture or weeds to name a few. The most important step in using cover crops successfully is to determine what your goal is. You can then pick cover crops around those goals and also crops in rotation.
If the goal is to build diversity in a cropping system, then adding more species could be beneficial. I always fall back on the "five food group" concept that Lee Briese (Independent Crop Consultant with Centrol Ag) developed because it makes using cover crops doable, economical and somewhat more universal across systems.
Under this concept, you need a cool season grass, cool season broadleaf, warm season grass, warm season broadleaf and a legume within a couple of years of your rotation. In corn-soybean, add cereal rye (a cool season grass) by interseeding corn to achieve four of the five "food groups." If radish (a cool season broadleaf) is interseeded with the rye, then all five groups are covered. If your goal is diversity, is this approach as good as including several species in a cover crop mix every year? I don't know, but it is reasonable, economical and applicable.
For erosion control or moisture management, the timing of seeding and terminating a cover crop are more important than the number of species in the mix. In this case, seeding cereal rye into a standing crop provides more growth in the fall for better erosion control over-winter. If cereal rye is seeded too late in the fall, desired erosion control in the winter may not be achieved. As a result of having more growth on rye in the fall, it could use more moisture in spring. But don't let the cover crop use too much moisture in the spring — terminate cereal rye when the soil is just about perfect to plant a cash crop (typically soybean).
To hedge your bets on moisture usage, cereal rye can be seeded at a variable rate in the fall. Lower rates on sandier soils (example: 10 pounds per acre) that are more droughty and higher rates on higher clay soils (example: 60 pounds per acre) that hold more water. This provides a little more flexibility and still achieves the intended goals of erosion control and moisture management.
For weed management, cereal rye is also a great choice because of timing, cost and plant characteristics. It will compete with weeds by growing in the fall and then again in spring. Cereal rye is cheap and it breaks up other expensive modes of action for weed management. It also has an allelopathic effect, which is a chemical released from the roots, that stunts weed growth. You don't necessarily need diversity to achieve the goal of weed management.
The big picture: there isn't a perfect approach for using cover crops. Do what works for your farming operation and comfort level. My point is, do not get discouraged or feel like you're not fully achieving soil health by having one species in your cover crop mix. If that's what helps you achieve your goals, then I say go with it.