A cover crop is a beneficial plant seeded into an agricultural field during or outside of the regular growing season. Its primary purpose is to improve or maintain soil health and ecosystem quality. A cover crop is considered an in-field practice.
Figure 36. An SWCD technician consulting with a farmer in a field of rye grass. Credit: Edwin Remsberg and USDA-SARE, covercropsimages.sare.org.
A cover crop can be used no matter the location, operation, or resource concern. Cover crops have been used for centuries to feed and protect the soil, suppress weeds, and manage nutrients. They have been used in:
- production fields (grain, vegetable, or livestock) that are subject to soil erosion. A living plant will stabilize the soil and prevent soil movement.
- fields with high nitrogen and phosphorus levels. Cover crops will absorb excess soluble nutrients and prevent movement off-site.
- fields with low nutrient levels. Cover crops can enhance soil organic matter and soil structure, and they add nutrients naturally back to the soil.
- combination with diverse crop rotations, no-tillage systems, and managed grazing systems.
A cover crop is used to save soil and support soil processes with living roots during the nongrowing season. Those who learn to manage the initial challenges of establishing cover crops will eventually experience the many benefits of cover crops in their farm operations.
Studies on cover crops are numerous and wide-ranging. The efficacy of cover crops depends on a variety of factors and conditions. Recent studies on cover crop effectiveness—including both field measurement and longterm, larger-scale simulations—are summarized below.
- Soil and on-farm benefits: A continuous in-field study of a central Iowa field with a 13-year corn-soybean rotation with and without a winter rye cover crop found that cover crop treatment had significantly higher soil water storage at a 0- to 30-cm depth; greater soil water content on individual days during the cash crop growing season; increased field capacity water content by 10–11 percent and plant available water by 21–22 percent; and did not reduce yields as compared to the noncover crop condition (Basche et al., 2016).
- Environmental benefits:
- An Ohio Study of cover crop impacts on water quality showed. The most consistent and significant impacts were reductions of Nitrate-N loss. When cover crops were grown the average monthly tile Nitrate-N loads were reduced by 50%. The amount of monthly nitrate-N load reduction was 0.9 to 2.3 pounds per acre with an annual reduction of 18 pounds. Total Phosphorous (TP) and Dissolved Reactive Phosphorous (DRP) tile loading did increase with a cover crop. The losses of TP and DRP from surface runoff show decrease. For both tile and surface runoff, the amount of change measured as pounds per acre is small and not statistically different based on annual average monthly losses. Table below shows summary of variables compared in Ohio study of cover crop impacts at edge of field.
- An in-field study in Iowa evaluated the effectiveness of oat and cereal rye cover crops in reducing nitrate concentrations and loads in subsurface drainage water. The rye winter cover crop significantly reduced drainage water nitrate concentrations by 48 percent over five years. The oat fall cover crop reduced nitrate concentrations by 26 percent. The research team also simulated a 45-year, corn-soybean rotation with and without a winter rye cover crop. They found an 11–29 percent reduction in erosion, up to a 34 percent decrease in nitrous oxide emissions, and a 2–18 percent reduction in soil evaporation due to the cover crop use (Basche et al., 2016).
- Impacts across the Midwest: An analysis of potential cover crop adoption and relative benefits to water quality in 10 counties across the five-state region of Ohio, Indiana, Illinois, Iowa, and Minnesota evaluated the potential for a fall-planted cover crop adoption based on a cash crop rotation and tillage systems. In those 10 counties, an estimated 34–81 percent of the agricultural land could have cover crops integrated into their corn and soybean cropping systems, which could have the potential to reduce nitrate loadings to the Mississippi River by approximately 20 percent (Kladivko et al., 2014).
- Another study simulated the adoption of cereal rye as a winter cover crop at 41 sites across the Midwest from 1961 to 2005. Results suggested that, on average, winter rye can reduce nitrogen loss in drainage water by 42.5 percent across the Midwest (Malone et al., 2014).
Type of seeding, method of seeding, method of terminating the cover crop, growth management, insects, nitrogen management, and roots and debris should be considered.
Type of Seeding
Many cover crop species exist. Species selection depends on when the species can be planted and what is the goal for its use. Soil erosion reduction is a primary goal attained. Legume species fix atmospheric nitrogen into a form that plants and microorganisms can use. Nonlegume species recycle existing soil nitrogen and other nutrients and can reduce leaching losses. A combination of two or more types of cover crops might be beneficial for establishment and improved nutrient utilization. The Midwest Cover Crops Council provides a tool to assist in determining species options based on the current field crop and expected planting date. Always use seed from reliable sources that have been cleaned, to prevent the introduction of unwanted weed species.
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Method of Seeding
Drills—along with broadcast seeding with or without tillage—are options for seeding. Larger seeds such as hairy vetch and winter peas can be planted through the normal hopper of a drill. Clovers might require a small-seeder attachment. Broadcasting cover crop seed can also be done with a fertilizer cart or by airplane. Light incorporation will increase seed soil contact where tillage is possible. If a legume is planted that has not been grown before, the proper inoculant should be added when seeding.
Consideration should be given to herbicide programs used in row crop production and their influence on cover crop establishment. Rotational crop restrictions provide some guidance on success in establishing cover crops and are summarized in the Ohio, Indiana, and Illinois Weed Control Guide at u.osu.edu/osuweeds/files/2016/12/2017-Weed-Control-Guide-tnkpfi.pdf and in Penn State Extension’s Herbicides Persistence and Rotation to Cover Crops at extension.psu.edu/herbicides-persistence-and-rotation-to-cover-crops.
Method of Termination
Cover crop termination should be planned in accordance with the next cash crop. Ohio is in NRCS Zone 4, which recommends for nonirrigated crop termination at or within five days after planting but prior to the crop emerging. Winter-killed cover crops are those that will not survive cold conditions in a given region. Overwintering cover crops continue to grow after conditions warm and must be terminated with tillage or herbicides. Weather and growth of the cover canopy should be considered in timing termination. Crop insurance guidelines should be reviewed to determine if termination conflicts with coverage.
Termination will generally be through nonselective contact or translocated herbicides. Follow the herbicide label for rate and timing. Purdue Extension’s Successful Cover Crop Termination With Herbicides is a useful guide.
Cover crops require timing management to control growth. Lack of moisture and late establishment in the fall can affect cover crop success. When terminating a spring-growing cover crop, the crop should be allowed to grow as long as possible to add additional nutrients to the soil and suppress weeds. But it can also use up soil moisture and hurt the following cash crop if dry conditions exist. Hard seed that did not germinate might also require control later in the growing season.
Insects and Other Pests
Cover crops can attract into a field insects that might feed on the planted cash crop once the cover crop is terminated. Armyworm, soybean seed maggot, grubs, or cutworms can attack the cash crop. Scouting is recommended to identify potential problems so proper treatments can be used. The presence of cover crops can also support the reproduction of soil nematodes and other pests such as slugs. For example, some winter annual plants increase soybean cyst nematode populations. Voles (mammals that eat the seed of establishing crops) have increased in some field situations. The extended green cover or species mix might also change the ecosystem to support beneficial soil-dwelling insects or foliage-dwelling insects. New cover crop users should inquire about the effects seen on insects or other pests in their region so that they can use management practices that avoid crop injury.
Cover crops can be good scavengers of nitrogen from the soil, but predicting when that nitrogen will become available to a subsequent crop is complex. If cereal rye, annual ryegrass, triticale, or wheat is used, termination when these covers are in vegetative stages is recommended because more nitrogen is immobilized. Certain leguminous species can fix nitrogen as well as potentially add to soil-nitrogen cycling. The Purdue University fact sheet Cover Crops for Soil Nitrogen Cycling provides a more complete discussion of nitrogen management and cover crops.
Roots and Debris
Observations of roots, debris, and sediment clogging tile systems have been associated with cover crop growth during warmer-than-normal winters. Debris might be entering tile systems where vertical tillage results in detached residue that moves in surface water flows toward tile risers or catch basins. Connection points between mains and laterals might also collect debris. This topic is further explored in the Purdue University fact sheet Agricultural Tile Drains Clogged With Cover Crop Roots?
Estimating the costs and benefits of a cover crop system is field-specific and depends on a variety of factors. For example, some farmers are able to grow their own cover crop seed to save on seed costs.
In a recent study determining the cost-effectiveness of various BMPs, researchers found that across Iowa, Illinois, Indiana, and Ohio, the average annual cost of cereal rye cover crops was $61 per acre per year. Iowa had the highest costs, and Indiana had the lowest. Establishment of cover crops required 69 percent of the total cost, while management, including termination, accounted for the remaining 31 percent of the cost (Roley et al., 2016).
The NRCS provides a free Cover Crop Economics Tool to help farmers and others determine the immediate costs and benefits of cover crops. The tool uses cost inputs such as seed type and rate, planting, termination, and management-and-benefit inputs such as chemical-inputs reduction, yield increase, reduced erosion, long-term soil fertility improvement, and long-term water storage and infiltration capacity of soils. The tool puts a dollar value on difficult-to-quantify costs and benefits such as soil erosion. It also uses net-present value to allow for long-term investment scenarios.
Cover Crops and the Evironment
- Cover crops enhance biodiversity, create habitat, and attract beneficial insects.
- Improvements in soil health and quality can result from increases in:
- porosity (reduced compaction), water-holding capacity, and infiltration
- soil organic matter
- beneficial microbes
- micro- and macro-invertebrates
Cover Crops and Farming
- Reduce wind and water erosion on all types of soils throughout the year.
- Improve soil tilth when plant roots grow into compacted areas.
- Protect the soil surface from heat and erosion, and reduce sealing.
- Increase soil organisms, such as earthworms, that break down residue on the field.
- Increase organic matter and break disease cycles.
- Retain nutrients lost through soil erosion and runoff, especially phosphorus.
- Balance nutrients in the soil. Legumes can add available nitrogen to the soil. Nonlegumes can take up and recycle excess nitrogen, available phosphorus, and potassium to the following crop. This is important after manure application.
- Suppress weeds through soil-shading or releasing allelochemicals.
- Natural insect control, such as lady beetles or ground beetles, might be encouraged by planting cover crops.
Extensive design details are provided in NRCS Conservation Practice Standard 340.