Winter Brassica Mix:
- Daikon Radish
- Winter Canola
- Hard Red Winter Wheat
One of the more common “newer” mixing ideas is to add a brassica or non-legume broadleaf to the cover crop mix, either in place or with legumes. While adding legumes provided the potential of N directly to the production system, the benefit of legumes is often nutrient scavenging and bio-tillage. Most brassica species have deep tap roots that can grow throughout the soil system, and they have the potential to use these expansive and deeper rooting systems to find and take up nutrients (N, P, K, and other micronutrients) from deeper in the soil profile. These are redistributed onto the soil surface and can be more accessible to future cash crops upon decomposition. In doing so, growers can potentially limit nutrients entering groundwater from overfertilization or under-recovery of previous crops. The other benefits of these rooting systems are biotilling. This results from more extensive rooting systems, creating an increased number of macro and mesopores in the soil system. This can increase water infiltration in future cash crop systems. For both benefits, adequate growth must be achieved.
While these crops can scavenge to find nutrients, if a major deficiency is present, limited root growth will limit the ability of the crop to reach these lower depths. The other major issue is the time needed for adequate growth. Additional discussion is below on the potential for winter kill in these brassica species, but if too much or too little fall growth is achieved before a major killing frost/freeze, significant winter kill can be observed. Winter kill could limit the benefits of these crops.
Planting date is a significant factor for covers that integrate brassica. Some brassica species (such as Daikon Radish) have some winter survivability but typically much less than other species. However, others like canola can have as good winter survival as the winter small grains. This can be good as this will typically maintain solid soil coverage throughout the winter and spring months. However, canola can be challenging to terminate once the crop has transitioned from vegetative to reproductive growth. Therefore, winter killing this cover crop may be the desired outcome. Growers will then have to tend to maintain soil coverage. Therefore, mixing these crops with a small grain is always a good option. Brassica has also been documented to provide other benefits, such as aiding in nematode control. This occurs when the glucosinolates from the plants go through the breakdown process. An intermediate step of this breakdown is a volatile compound that can act as a fumigant. This means these cover crops could provide a biodegradable fumigant to help in insect and nematode control.