How does the farmer benefit from planting cover crops?
Many farmers ask why they should consider planting a cover crop; they see it as extra expense and work. Many benefits of cover crops are more long term while some responses, like nitrogen sequestration, can provide more short-term benefits. Individual field response to cover crops will vary based on field slope, existing wind protection, soil type, species of cover crop planted, preceding crop, and the crop to follow the cover crop.
Do all fields suffer equally from water erosion? The answer is no - a relatively flat field would obviously have less risk. If a field has any areas of slope, water erosion prevention may provide a substantial benefit. There is a high risk of water erosion from late fall or early spring rains in fields previously in corn silage, soybeans, or with fall tillage. An actively growing cover crop in late fall and early spring, such as winter cereal rye, can provide water erosion protection during that critical time.
How about wind erosion? Have you ever observed soil blowing in the wind? Soil losses from wind erosion can be substantial. Based on her research, University of Minnesota Extension Professor Jodi DeJong-Hughes estimates that in an open winter, accumulation of nitrogen (N), phosphorus (P) and potassium (K) averages about $55 in each ditch-acre in western Minnesota; this ditch dirt was originally topsoil in the adjoining fields. A growing cover crop like winter cereal rye will help minimize these wind erosion losses.
What is the toughest nutrient to manage in corn and small grains? Most farmers indicate that it is nitrogen. So anything we can do to minimize nitrogen losses should save us money, right? The nitrogen in manure or commercial fertilizer applied in late fall should mineralize to the stable ammonium form in the fall. However, during spring soil warm-up, this nitrogen will quickly nitrify to the nitrate form and be susceptible to leaching during spring rains. Winter cereal rye has the capability to sequester, or take up, a lot of nitrogen, preventing that nitrogen from leaching. If a farmer terminates and incorporates the winter cereal rye before it reaches 10 inches, my research, as well as the research of others, indicates that most of the nitrogen in the rye should be available to the growing crop in the summer. Obviously, corn’s big need for nitrogen is in the summer - not early spring.
Accumulating soil organic matter is a long-term benefit of having something growing in the soil as much of the year as possible. Some plant species are better at building soil organic matter than others; many rate winter cereal rye among the best for building organic matter. What is the benefit of increasing organic matter? Organic matter contains nitrogen, some will be available to the growing crop; therefore, higher levels of organic matter will reduce commercial nitrogen fertilizer needs. Higher level of soil organic matter also increases the soil’s cation exchange capacity (CEC) and water holding capacity, which are additional benefits.
Another key component is soil health. Healthy soil microflora is essential for all the soil processes necessary for plant growth. Soil microflora needs actively growing plant roots to remain healthy. Therefore, anytime there is barren soil, we have an unhealthy soil condition. Winter cereal rye can bridge the gap from corn silage harvest to next spring’s planting in providing a healthy soil environment.
We are in the second year of our trial in which we plant winter cereal rye immediately after harvesting either corn silage or soybeans. We then inject liquid manure into the growing winter cereal rye later in the fall when soil temperatures are below 50 degrees Fahrenheit, minimizing fall nitrogen volatilization losses. In the spring, we measure the 24-inch soil nitrate levels under the winter cereal rye versus the bare ground just before rye termination. Then we plant corn and measure yields in the fall.
The 24-inch soil nitrate tests demonstrate that the winter cereal rye is capable of sequestering a considerable amount of nitrates in the spring, minimizing the potential of nitrogen leaching. As long as we terminate and incorporate the winter cereal rye before it reaches a height of 10 inches, it appears we are able to get most of the nitrogen back for the growing corn crop in the summer. Winter cereal rye that is terminated at taller than 10 inches provides a lot of plant material to incorporate and has a more lignified plant structure, making rapid decomposition in the soil difficult.
Another option for dairy farms is to harvest the rye as a forage in mid-May, delaying corn planting. Winter cereal rye harvested as a spring forage will contain a fair amount of nitrogen. If the farmer plants corn after harvesting the rye forage crop, replenishment of this nitrogen by additional application of manure or commercial fertilizer would probably be necessary.