Cover Crops and Planting Strategies

The use of cover crops is a long-term investment in increasing soil health. Cover crops can reduce wind and water erosion, increase water infiltration, choke out weeds, break insect and disease cycles, and provide habitat and forage for pollinators and other beneficial insects. Importantly, the use of cover crops can also provide a yield increase in the primary crop. (1)

First Steps

• Clarify what is needed most from a cover crop and set one or two primary goals. Examples of goals would include provide nitrogen, improve soil structure, add organic matter, etc. Focusing the use or goal of the cover crop will simplify selection of an appropriate cover crop species or mix.
• Define the time to establish the cover crop. Look for open periods in each field that correspond with good conditions for cover crop establishment. Depending on cropping system and rotation this may be fall, spring, or overlap with cropping windows.
• Establish the production parameters for the cover crop such as when to plant, terminate, and seed the primary crop in the cover crop residue.
• Consider the impacts of the cover crop on the following cash crop. While cover crops can offer many benefits, the wrong cover crop, or the right cover crop that grows too long or produces too much biomass, can negatively impact the following cash crop.
• Select the cover crop or the mix of species to accomplish the primary goal and fits with the cropping system.

Establishment of cover crops is varied and depends on the species, cropping system, and equipment availability. Broadcasting, small grain drill, slurry seeding, and aerial seeding can all be used. In general, seeding depth is related to seed size: larger seeds should be planted deeper. As a rule of thumb, the seeding depth should be 4 to 6 times the size of diameter of the smallest seed size used. Select high quality seed and if including a legume, ensure that the proper rhizobium inoculant is used.

Broadcasting

Broadcast seeding is the least intensive seeding method but can allow for seeding quickly prior to harvest of corn and soybeans, thus extending the growing season of the cover crop. This relies on rain, freeze/thaw cycles, or snow to incorporate the seed. Heavier seed such as cereal grains are more adapted to this method when seeding into freshly harvested crop residues. Broadcasting can result in unsuccessful establishment or lower than desired stands when compared to drilling. (2) In addition, accurate calibration can be challenging especially when broadcasting with a spinner spreader. Increase seeding rates to compensate for poor seed-to-soil contact and poor uniformity of seed distribution.

Generally, the seeding rate for broadcasting should be 15 to 25% higher than when incorporating the seed. Broadcasting is not recommended when planting large-seeded cover crops, such as peas, or mixtures of species that require different planting depths. Additionally, some species, such as crimson clover, overseed poorly, despite their small seed size, while vetch is less sensitive to seeding method. (3) In areas with shorter fall growing seasons after harvest, broadcast seeding either by air or high clearance equipment into the standing crop can allow for additional establishment time.

Drill

A small grain drill is perhaps the most reliable method of establishment for most cover crops. The drill provides good placement and seed-to-soil contact. Drilling can be especially successful in no-till management systems. This method is used after the primary cash crop has been removed. Most drills are equipped with a legume/grass seed box for smaller seeds while the standard drill box for can be used for larger seeds. Use the large box if possible as it provides the best seed placement and coverage. Lower seeding rates may need to use the small box as the narrow gap between the seed opening gates and seed meters in large boxes may crack seeds. Most medium size seeds can go in the large box, while small-seeded Brassica crops and clovers may need to go in the small one. For mixes, use the large box and slightly reduce seeding depth of large seeds or use both boxes, placing shallow germinating seeds in the small box. Generally, it is best if the small box seed tube is in front of the press wheel.

An alternative to drill seeding is using an air seeder combined with a vertical tillage (VT) tool. In these combinations, seed is typically blown ahead of the coulters, allowing the VT tool to improve seed-to-soil contact. While there is little research available on the technique, many farmers consistently obtain satisfactory stands from the practice and cite increased acres per hour productivity.

Split Row or Narrow Row Planting

Using a split row planter with a narrow row spacing (15-inch row width or less) can provide an excellent stand for cover crops. (4) Consult the planter manual to determine which seed plates will work best for the desired cover crop species. If using a planter to establish cover crops, the seed disks may need to be modified within the planter meters. Specialty disks are available for most planter types for medium size (small grains) and small size (clovers, Brassica crops) seeds. Alternatively, soybean disks can be used for peas and sugarbeet disks can be used for Brassica crops and some clovers.

The advantages of using this precision placement method are less seed is required and better stand establishment. This method should not be used if weed control is the primary purpose. If at least two cover crop species are used it can meet the USDA criteria for soil erosion prevention and increase in soil quality. One strategy is to plant winter rye (winter survivor species) in one set of seed hoppers and tillage radish (or other winter-killed species) in the other set of seed hoppers into soybean stubble. The following season, the soil warms more quickly in the rows with the winter killed tillage radish and the corn planted into those rows emerges faster and more consistently. This emerging method has been shown to be comparable in yield to fall strip till, when cover crops are planted following winter wheat. (5)

Interseeding

In areas where there is not enough time after harvest for successful establishment of cover crops, interseeding into the existing crop can be successful. To interseed into standing corn, the early interseeding recommendation is to begin at V4. (6) Cover crop species that can survive are limited when interseeding early, and early interseeding success may be more challenging under higher corn populations, longer season corn products and high yield environments, and the practice generally shows more success north of the I-80 corridor, although exceptions do apply. Additionally, early interseeding should be limited to drilling with a high clearance interseeder to ensure that cover crops find adequate moisture. (7) Annual ryegrass, forage radish and crimson or red clover have showed consistent success in establishment in early interseeding.

In soybean, interseeding should occur about the time soybean leaves begin to turn yellow. Late season interseeding is usually broadcast. For late season broadcasting into soybeans, the small grains and annual ryegrass have performed consistently and Brassica crops, such as radish, may also establish well. Broadcasting clovers has been met with limited success, while hairy vetch has proved to be more consistent. However, due to the relatively higher seed cost of vetch, broadcast interseeding of this species may be more risky than other species.

One challenge with the use of interseeding, especially when performed early, is herbicide compatibility with the primary crop. Two resources that list potential injury to selected cover crops of herbicides used in corn include: Cover Crop Interseeder: Potential for Injury from Corn Herbicides and Cover Crop Interseeder: Improving the Success in Corn. Most of this research has been limited to a few locations and herbicide risks may differ between different geographies and soil types.

Sites for Additional Local and Regional Information

• Michigan State University - https://www.canr.msu.edu/cover_crops/index
• Pennsylvania State University - https://northeastcovercrops.com/states/pennsylvania/
• University of Nebraska - https://cropwatch.unl.edu/cover-crops
• Mississippi State University - https://southerncovercrops.org/cover-crop-resource-guide/local-experts/mississippi/
• Sustainable Agriculture Research and Extension - https://www.sare.org/resources/cover-crops/

Sources: 1 Myers, R., Weber, A., and Tellatin, S. 2019. How do cover crops impact yield over time? SARE Outreach. https://www.sare.org/publications/cover-crop-economics/how-to-get-a-faster-return-from-cover-crops/how-do-cover-crops-impact-yield-over-time/

2 Fisher, K., Momem, B., and Kratochvil, J. 2011. Is broadcasting seed an effective winter cover crop planting method? Agronomy Journal, 10(2), 472-478. 3 St. Aime, R., Noh, E., Bridges Jr., W., and Narayanan, S. 2021. A comparison of drill and broadcast planting methods for biomass production of two legume cover crops. Agronomy, 12 (79). https://doi.org/10.3390/agronomy12010079

4 2014. Recommended cover crop seeding methods and tools. NRCS-USDA. https://efotg.sc.egov.usda.gov/references/public/IN/Technical_Note_3_Agronomy_Cover_Crop_Seeding.pdf 5 Yasmin, F., Nasielski, J., Schneider, K., and Van Eerd, L. November 2022. Effect of overwintering cover crop mixtures and bio-strip tillage on the yield of grain corn. Poster presentation at ASA-CSSA-SSSA International Annual Meeting. Baltimore, MD. 6 Rees, J., Proctor, C., and Melvin, S. 2021. Interseeding cover crops into corn and soybean: What we’ve learned. University of Nebraska Extension. https://cropwatch.unl.edu/2021/interseeding-cover-crops-corn-and-soybean 7 Noland, R. Wells, S., Sheaffer, C., Baker, J., Martinson, K. and Coulter, J. 2018. Establishment and function of cover crops interseeded into corn. Crop Science. https://acsess.onlinelibrary.wiley.com/doi/10.2135/cropsci2017.06.0375