Frequently Asked Questions (FAQ)
Why should I plant cover crops? What is the purpose, what are my goals in the end?
It is critical to first consider what you want to achieve with cover crops in your crop rotation. Typically, the main goal for growing cover crops is enhancing soil health, by reducing erosion, increasing organic matter content, improving soil structure (aggregation, porosity), enhancing soil microbial diversity and abundance, enhancing nutrient cycling and water infiltration.
Cover crops can also be used as a weed control tool, by suppressing weeds before and/or after main crop planting. Certain cover crops may help to slow down the spread of herbicide-resistant weeds.
In some areas, cover crops are grown for the main purpose of preventing nutrient (mostly nitrate) leaching to groundwater and/or nutrient run-off.
Leguminous cover crops are often used as “green manures”, because they fix N which in turn can provide some or all of the N needed by the next crop.
Cover crops add diversity to the cropping system. They can create habitat for beneficial animals, provide a food source for pollinators (think flowering cover crop species), and support wildlife.
http://mccc.msu.edu/what-are-cover-crops/
https://www.sare.org/Learning-Center/Topic-Rooms/Cover-Crops
What should I consider when using cover crops as forage?
When and what types of cover crops should I plant?
When to plant cover crops depends on the crops you grow, where you live (climate and soils), and what your goals are.
The Midwest Cover Crop Council developed a program which lets you enter your goals and shows which cover crop species best achieve these goals.
Should I plant one species or a mix of species? How many species should there be in a mix? What are benefits of selecting winter-hardy versus winter-killed species?
Whether to mix species or plant a single species as a cover crop depends on your goals, time of the year, and costs. Planting more than one species can help fulfill multiple goals that producers often want the cover crop to achieve. Grass cover crops are most often planted to control erosion and prevent nutrient loss. Legumes, on the other hand, can supply N to a subsequent crop. Legume seed is more expensive and legumes are slower growing than other cover crop families. Brassicas are fast-growing, scavenge nitrogen and can be grazed. Brassicas mature quickly, and winterkill in our environment. Combining grass and legumes in a cover crop mix may reduce erosion and provide N, although there is often somewhat of a trade-off. If you focus on one goal, for example preventing erosion, it makes more sense and is cheaper to plant a single species suited for that goal, such as cereal rye.
Click on the link below for a more detailed discussion on mixes versus single species cover crops. The article also contains tips on how to select species for a mix.
https://cropwatch.unl.edu/2020/what-are-benefits-cover-crop-mix-versus-single-species-cover-crop
How does cover crop selection differ between corn and soybean fields? When and how (seeding depth, seeding rate, drill settings) should I plant the cover crop?
When planting after corn, going into soybean, a cereal rye cover crop is usually the best choice. Cereal rye can be planted later in the fall than most other cover crops and can still be productive even when planted after corn harvest.
This Nebraska Cover Crop Recipe provides more information on planting and termination when introducing cereal rye cover crops into the corn-soybean rotation.
https://cropwatch.unl.edu/2019-CW-News/2019-images/cover-crops/Nebraska107ADA.pdf
When planting after soybean, going into corn, there are more cover crop options. With soybean harvest usually done earlier in the fall, cover crop species that require earlier planting dates may be grown, either alone or as mixes.
Find more details on planting and terminating cover crops grown after soybean, before corn, in this Nebraska Cover Crop Recipe.
https://cropwatch.unl.edu/2019-CW-News/2019-images/cover-crops/Nebraska108ADA.pdf
Broadcasting cover crops into corn and soybean stands before harvest extends the cover crop growing period. It often results in more productive cover crops. Click on this link to learn more about seeding rates for broadcasting.
https://cropwatch.unl.edu/2020/seeding-rates-broadcasting-cover-crops-late-season-corn-and-soybean
Is it better to fly the cover crop on or to drill after harvest?
Drilling provides better seed-soil contact, improving seedling emergence compared to crops that are flown on or broadcast with high-clearance equipment. On the other hand, broadcasting can be done earlier in the season than drilling. For example, broadcasting by plane or high-clearance equipment in early to mid-September gets cover crop seeds into the fields 4 to 6 weeks sooner than drilling after harvest (especially when broadcasting into corn stands). This provides broadcast cover crops with a head start which can result in greater biomass production.
A Nebraska study that compared the productivity of cover crops (cereal rye and mixes) that were broadcast into corn in mid-September versus drilled after corn harvest found that the response to those different planting times was site specific. In eastern and north-eastern Nebraska, cover crops were more productive when broadcast mid-September than when they were drilled after harvest, despite the fact that broadcast stands were sparse in the fall. In south-central Nebraska, drilling after harvest in most years resulted in more productive cover crops than broadcasting mid-September.
Read more about this study and its findings on how different planting times and species selection affect cover crop productivity:
https://cropwatch.unl.edu/2019/research-optimum-planting-times-cover-crops
When and how should I terminate? What are the benefits of planting green versus planting brown?
https://cropwatch.unl.edu/2020/considerations-when-planting-green
What is the effect of a cover crop on germination of corn and soybean?
There are several ways that cover crops might impact germination of corn and soybean, for example by being a host to seedling diseases or by allelopathic effects.
Emerging research suggests that cover crops such as cereal rye, when grown before corn, can provide a “green bridge”, increasing the amount of seedling pathogens, because both cereal rye and corn are grass species. You can learn more about that work here.
https://crops.extension.iastate.edu/cropnews/2016/12/reducing-risk-corn-seedling-disease-yield-loss-after-cereal-rye-cover-crop
Many producers have heard of the term “allelopathy” used to describe certain effects of cover crops on corn and soybean. What does “allelopathy” mean and do I really have to worry about it? Cover crops, like all other plants, produce chemicals to defend themselves against pests, including competition from other plants. Cereal rye is an example for a cover crop that has a well-known track record of suppressing weeds through allelopathic chemicals. The allelochemicals work mostly be inhibiting the germination and early growth of weeds. In contrast to weeds, corn and soybean have much larger seeds, making them less susceptible to allelopathic chemicals. Studies have shown that at least in the lab, they are rarely affected by allelochemicals. Allelochemicals quickly degrade in the soil, usually within a week or two of terminating the cover crop, so the following corn or soybean crop is unlikely to be impacted.
A recent CropWatch story by UNL researchers further explores cover crop allelopathy when planting corn and soybean green. It lists potential risk factors that could make allelopathic effects more likely to occur.
https://cropwatch.unl.edu/2020/considerations-when-planting-green
For a detailed description of allelopathic chemicals produced by different cover crop species, and their interactions with row crops, see the 2020 peer-reviewed open-access study below.
Koehler-Cole, K., S.E. Everhart, Y. Gu, C.A. Proctor, M. Marroquin-Guzman, D.D. Redfearn, R.W. Elmore. 2020. Is allelopathy from winter cover crops impacting row crops? Agricultural & Environmental Letters. doi: 10.1002/ael2.20015
https://acsess.onlinelibrary.wiley.com/doi/full/10.1002/ael2.20015
How much nitrogen does my cover crop take up and when do I get it back? Do I have to adjust my corn fertilization?
Cover crops influence soil fertility through their uptake of nutrients, especially nitrogen (N). Cover crop N uptake can prevent N loss, for example through leaching, during times when no crops are growing in the field. When cover crops decompose, N is recycled back to the soil and can be used by the subsequent crops. The following article discusses differences between cover crops in terms of N uptake and when this N may become available to the crop.
https://cropwatch.unl.edu/2020/how-much-nitrogen-does-my-cover-crop-take-and-when-do-i-get-it-back
How do you adjust your herbicide program to account for carryover effects that might impact cover crop germination and/or grazing opportunities?
Planning for a cover crop should start early, when making decisions about which herbicides to apply to your main crop. Certain cover crop species (brassicas, some legumes) are susceptible to carry-over from herbicides. Check the herbicide label for the plant-back interval of the cover crop you are planning on planting in this field.
https://cropwatch.unl.edu/2019/check-herbicide-restrictions-planting-and-using-cover-crops