Cover crops and dairy farms
There are many benefit claims for cover crops. It can become confusing because of the variety of plants and the various systems used. A farmer should ask “What am I attempting to accomplish with a cover crop on this field this year?” Do I want to reduce soil or wind erosion, increase organic matter, improve soil structure, sequester nitrogen, fix nitrogen, suppress weeds, reduce surface crusting, or break hardpan? Is there one magical plant that will accomplish all of these features? What are my forage needs? Additional pasture, fall forage harvest, spring forage harvest, or maintain my corn silage/alfalfa rotation? We are unable to discuss all of these options in a short article, so let us concentrate on options following harvest of early to mid-fall crops such as corn silage or soybeans.
Many people have been impressed by the massive roots forage radishes will grow in a few weeks. If you have a field with a hardpan to break up, radishes could be an option. However, radishes should be planted by mid-August to allow enough time to develop those roots; too early for corn silage or soybeans. Forage radishes could follow a small grains crop in a typical year. If you have a deep-rooted crop like alfalfa in your rotation, is hardpan your biggest problem?
If the field will be planted to corn or small grains next year, planting a crop that fixes nitrogen could reduce nitrogen fertilizer use, i.e. a legume which has nitrogen-fixing potential. Would there be enough time for a legume to grow and fix much nitrogen in the fall following corn silage harvest? Probably not. How about after small grain? Maybe some. If the cover crop is terminated early spring, the fixated nitrogen may also be minimal. The nitrogen economic return from planting a legume between two corn silage crops combined with early season corn planting is doubtful. One would expect a better economic nitrogen return if the legume could have an expanded time to grow. This may not fit the typical corn silage/alfalfa rotation on many dairy farms. Most dairy farmers are already utilizing the best nitrogen fixer available, alfalfa, and it usually covers the ground for 3 years or more. In most cases, corn following a 2-year plus good stand of alfalfa should have little need for additional nitrogen.
What can we grow after harvesting corn silage that offers some benefit when we want to plant next year’s crop in late April/early May? We are mostly limited to small grains which are cool season crops that can grow in a Minnesota fall. Annual small grains such as oats, barley, spring wheat, and spring triticale will have adequate growth in the fall, provide ample fall ground cover, and will usually winter kill eliminating the need to terminate them in the spring. However, since early spring rains can lead to erosion and nitrogen leaching, having an active growing cover crop in the spring can minimize these effects.
Other options following corn silage or soybeans are the winter cereal grains - winter rye, winter wheat, and winter triticale. The one most likely to survive Minnesota winters is winter rye. Winter wheat is the most sensitive to winter kill and winter triticale is somewhere in between. If spring termination is the plan allowing for late April/early May corn planting, then winter cereal rye would be the logical choice.
If the desire is to harvest spring forage in late May prior to planting next year’s crop, one could consider winter triticale since its digestibility is slightly higher than winter rye; but many people harvest winter rye as a spring forage. Timely cutting during the boot stage, when the head is still enclosed by the sheath of the uppermost leaf, is critical to any small grain species. Delaying harvest until after boot stage increases yield, but feed quality drops off rapidly. What is the yield and quality tradeoff between full season corn planted the first of May vs. harvesting a winter cereal as a spring forage combined with a shorter season corn variety? Also, consider that small grain harvested as a forage will consume some nitrogen.
Two benefits of having a winter cereal grain growing in both fall and early spring is increasing the soil organic matter and the grain’s ability to sequester or take up nitrogen. Sequestering the nitrogen will minimize nitrogen leaching. Will this nitrogen be available to the corn plant during the growing season since the plant material must be decomposed first? Maybe, depending on amount of residue, seasonal rainfall, and temperature. Over the long term, higher levels of organic matter should reduce supplemental nitrogen needs.