Early season cover crop interseeding in corn
Cover crops can provide growers with many benefits, including erosion control, reducing offsite movement of nutrients, supporting the development of soil organic matter, and in some cases, providing additional revenue streams (e.g. forages, oilseeds). Even though cover crops offer several opportunities to growers who utilize them, historically, adoption has been marginal. It is estimated that less than two percent of the agricultural land in Minnesota utilizes cover crops at some point in the rotation. There are several reasons for low adoption rates, but the most obvious reason is associated with the shorter growing season when compared to neighbors in the South.
The reduced growing season offers many challenges to successful integration of cover crops. However, there are new technologies that have the potential to overcome these challenges. The most notable technology involves early or late season cover crop interseeding (i.e. relaying cover crops into cash crops). Interseeding cover crops during the corn or soybean growing season can address some of the potential challenges (e.g. soil moisture and light) associated with shorter growing seasons. Currently the University of Minnesota Cover Crop team and Soil Health Partnership are researching innovative techniques and equipment necessary to interseed cover crops in corn and soybean.
Competition from cover crops
When considering interseeding classical cover crops (e.g. rye, clover, radishes, etc.), it is a good idea to review or at least acknowledge some of the core ideas from the discipline of weed science. This is not to say that the vast majority of disciplines associated with crop production, most notably breeding, genetics, entomology, plant pathology, and soil fertility to name a few, are not central to the advancement of cover crops. Rather, for this discussion, the following example focuses on our knowledge of weeds and the similarities weeds can have with cover crops.
It is well known that weeds, when allowed to to compete with the cash crop, can reduce crop yields. An important concept in weed management is the idea of the critical weed-free period. This is the time in the growing season when weeds are most likely to impact crop yields (Figure 1). Weed control efforts outside of the critical period offer no yield advantages, as long as early season weeds are controlled before the critical period begins. These critical periods of weed control are unique for each crop. Unfortunately, the critical period varies widely depending on the specific conditions in each field. However, most agree that the critical period of weed control in corn is between the V1 and V10 leaf stages.
Since cover crop interseeding does involve additional field operations, economically speaking, it makes sense to combine cover crop interseeding with scheduled cash crop operations or tasks (e.g. planting cover crops at sidedressing). A natural question arises when considering the critical weed free period in corn and early season interseeding of cover crops: What are the impacts of cover crops on corn growth, development, and subsequent yield? In general, a weed is thought of as a plant out of place. Could cover crops be thought of as a weed? Like weeds, both crop and cover crop productivity directly relates to the ability to sequester nutrients, water, and light. In this context, an early planted cover crop could be considered as a weed. Given that cover crops, depending when they are planted, could exhibit weedy characteristics, when is it less risky to interseed cover crops in corn?
Optimum time for interseeding cover crops
Figure 2. Cereal rye (left) and medium red clover (right) cover crop planted with a 3-in-1 Interseeder into previous corn crop at growth stage V7 in Waseca, MN (Photo taken June 19, 2016 prior to no-tilling soybean).
Most of the literature suggests that early cover crop planting dates that align with the V5 – V7 corn leaf stage have negligible impacts on corn yield. The Minnesota Department of Agriculture funded cover crop interseeding project has demonstrated similar trends, where the no-cover-crop check plot yielded the same as the cover crop plots across the last two years and there have been no yield reductions associated with the following no-till soybean crop (Figure 2).
Targeting the V7 corn growth stage can position the cover crop planting date near the end of the critical weed-free period in corn (Figure 1). Still, the question remains, how early can cover crops be interseeded in corn? In other words, how much flexibility is there with early season interseeded cover crops?
Figure 3. Avenger high-clearance adjustable tractor outfitted with Gandy Orbital Air Seeder and soil disturbance units.
In addition to interseeding dates, the following cover crops and seeding rates were used:
- Cereal rye (aka. Winter rye): 120 lb. live seed per acre
- Medium red clover: 10 lb. live seed per acre
- Hairy vetch: 25 lb. live seed per acre
- Annual ryegrass: 25 lb. live seed per acre
After each seeding event, there was sufficient rain, which greatly impacted the germination and establishment of the cover crops. In early July, the corn across the three cover crop planting dates visually showed no stress. It is important to note that the site has high mineralization potential and coupled with regular rain events may mask any "weedy" interference associated with the V2 planting date. As plots progress through the season, there is a good chance that grain yields will be reduced in the V2 treatments. The photos in Figures 4 and 5 were taken June 10, 2016.
Much of the understanding concerning cover crop interseeding in the Upper Midwest is still in the research phase. More time and experiments are needed to fully predict across a wide range of environments how covers will impact cash crop performance. For those intrepid farmers who are interested in planting cover crops, the following is advised:
- Set expectations low and be conservative. With all systems, there is associated risk and cover crops are no different.
- Determine which services most interest you (e.g. erosion control, reduction of offsite movement of nutrients, grazing, etc.) Doing so will aid in the decision on which cover cropping system best fits your desired cropping system needs.
- Have a plan. For example, if cover crops are seeded into soybean and they are winter-hardy, what is the plan for planting corn, not only into high-residue, but also void of fall tillage? Strip tillage is a viable option for many corn and soybean growers.
- Be prepared to move quickly in the spring. Cereal rye can add on biomass quickly in the spring and legume cover crops that survive the winter can be tricky to terminate with herbicides.
- Start with the basics. Cereal rye is bulletproof and relatively inexpensive. It establishes well in summer and in the fall, is winter-hardy, and is easier to terminate in the spring than the legumes.
- If new to cover crops, or any new technology, pick a uniform spot in the field and plant both with and without cover crops. Keep everything else constant. With yield monitors and adequate replication, you can assess if cover crops are a good fit for your farm.
- Reach out for help and advice. There are several resources in your state that can address your questions: University of Minnesota Extension, Natural Resources Conservation Service (NRCS), Minnesota Department of Agriculture (MDA), and U.S. Department of Agriculture (USDA), to name a few.