The use of cover crops in agriculture has many agronomic and economic benefits as well as challenges. In some agricultural areas, cover crop use is supported by state-funded programs to help increase water quality by reducing nutrient runoff and leaching during non-cropping months. According to the last Census of Agriculture taken in 2017, cover crop acreage increased around 50% from 2012 (10 million acres) to 2017 (over 15 million acres). Estimates by the Center of Regenerative Agriculture for 2020, based on growth rates, documented by the Sustainable Agriculture Research and Education/Conservation Technology Information Center (SARE/CTIC) estimates the number of cover crop acres on U.S. farmland at 20 million acres.1 The United States has indicated that it aims to double the 2017 cover crop census numbers to 30 million acres by 2030.2
Cover crops can help:
- Reduce wind and water erosion.
- Mitigate soil compaction.
- Improve water infiltration, soil aggregation, and soil water holding capacity.
- Improve nutrient cycling including nitrogen(N), phosphorus(P), potassium(K), and sulfur(S).
- Retain residual fertilizer in the field.
- Add organic matter and nutrients to soil.
- Dry wet soils through transpiration.
- Provide a food source for livestock and wildlife.
- Provide sustenance for pollinators.
- Convert atmospheric nitrogen into plant-available forms.
- Break pest cycles and suppress nematodes.
- Suppress weeds.
- Reduce soil crusting.
Potential cover crop challenges:
- Seeding a cover crop into a standing crop can prevent or reduce stand establishment if the soil surface is too wet, too dry, or too highly shaded.
- In no-till seedings of cover crops, good seed-to-soil contact can be difficult because crop residue may be pinched into the seed slot, or the seeds may be laying in crop residue instead of the seed slot.
- Issues of seeding cash crops into cover crops include:
- Cover crops can slow soil surface drying and residues of dead cover crops can trap excess soil moisture.
- Problems with seed slot closure.
- Difficulty cleaning cover crops out of the row at planting.
- Wrapping of cover crop residue on planter parts.
- Emergence of cash crop through cover crop residues.
- “Termination timing” can be challenging for non-winter killed species. This can be complicated by how the cover crop will be terminated, concerns over nutrient release (N tie up), concerns about soil moisture depletion by the cover crop prior to seeding the cash crop and issues with termination occurring during less-than-ideal weather conditions.
- Cover crop species selection may be limited by a previous herbicide application. The seeding window for cover crops after harvesting the primary crop may be too short to allow for successful establishment prior to freezing temperatures and early spring planting dates may requires early cover crop termination, limiting cover crop function and utility.
Cover Crop Selection
The desired end benefit of a cover crop should be determined prior to selecting which cover crop species to plant, different cover crops have different benefits. Grass species are more likely to hold soil in place from wind and water erosion; fast growing grasses or Brassica (cruciferous) species are a good choice to help nutrient retention and weed suppression; and nitrogen (N)-fixing legume cover crops (if properly inoculated with rhizobia bacteria) can potentially help reduce the amount of N fertilizer needed for the following cash crop. Feed for animals through grazing or harvesting may best be accomplished with fall or winter grasses, Brassica species and legumes. Soil compaction is best addressed with cover crops with penetrating root systems, like radish and deep-rooted annual ryegrass (Table 1).
Table 1. Common cover crops characteristics and recommendations for Missouri (planting rates may differ for other areas of the country).
Table 2. Warm and cool season cover crops.
Seeding Rates and Methods
Seeding rates vary depending on the seeding method and if a cover crop species is planted as one crop or as a combination of two or more species. Aerial, slurry-seeded, and broadcast rates should be seeded at rates of 20% or higher than if seeded with a row crop planter or drill because seed-to-soil contact is lower resulting in lower germination success. Smaller-seeded species such as brassicas, flax and clovers, are best suited for broadcast or aerial seeding. Larger seeded species such as peas, cereal rye, barley and sunflowers should be drilled or planted with a row crop planter.2 The mix ratio may vary based on the cover crop objective. A proportional rate for a two-way mix may be 55 to 60% of the normal rate of a species by itself. A three-way mix may be 35 to 40% of an individual species.3
Cover crops increases the amount of time a field has for actively growing plants. This increases sunlight interception that captures carbon through photosynthesis. Actively growing roots in the soil act as a carbon pump that helps feed soil microbes and the larger soil food web, which is the foundation of soil health. Cover crops can help fields rebound from intensive farming and tillage practices that have led to compaction, erosion, and soil structure damage. Some cover crops grown in the fall and spring have deep tap roots that can help reduce compaction, which helps the roots of a cash crop penetrate these layers by following old root channels.4
Reducing soil erosion depends on how much the cover crop reduces the forces of soil detachment and transport. Soils are more susceptible to erosion in fields without a crop canopy and crop residue. Above-ground biomass from cash crops may only last four months but cover crop growth extends the period of biomass production and can build soil organic matter (SOM). Climate and vegetation are two of the most important factors influencing the amount of SOM near the soil surface. Increasing SOM creates larger, more stable soil aggregates near the soil surface which reduces soil crusting and decreases the potential for soil detachment and erosion.
Soil structure, soil fertility, soil water holding capacity, and soil health are associated with SOM as are many physical and chemical characteristics of soils. No-till practices help preserve the SOM gained from cover crop growth. Conversely, tillage breaks down SOM as aeration of the soil through tillage results in a flush of aerobic bacteria that quickly consume available organic matter. Research shows that SOM can increase substantially within a short time frame with the use of cover crops in conjunction with a no-till system. Organic matter can help provide protection for the soil during periods of extreme drought conditions.5 SOM results reported in most soil tests are stated as the percent of total soil which contains about 95 percent of all soil nitrogen (N). About 30 pounds of N per acre can be released (mineralized to nitrate) during the cropping season from each 1 percent SOM present.6 Agricultural topsoil has SOM levels that generally range from 1% to greater than 5%.
Additional soil fertility benefits of cover crop use include increased rates of infiltration, nutrient cycling, and residue decomposition. Cover crops can reduce surface nutrient transport and loss by improving soil water infiltration and absorbing excess soil moisture. By keeping precipitation in fields, sediment detachment and transport can be reduced, which is important for the retention of soil phosphorus (P).
Grazing or Harvesting
Establishing cover crops for livestock feed provides a dual advantage – conservation and economic return through meat, milk, and manure. Residual herbicide labels should be read and followed as grazing and hay harvest restrictions may apply.
Depending on the cover crop, grazing may occur in the fall, winter, or spring. For fall or early winter grazing fast-growing species should be used such as oats, rye, or Brassica species. However, rye or other overwinter species should not be overgrazed in the fall because of lower overwinter plant survival if in-spring grazing is desired. If spring grazing is desired, winter-hardy species such as winter rye, triticale, barley, or wheat are good options. Brassica species should not be grazed alone because of their ability to take up excess N that may result in bloating of ruminants. Grazing should include a mix of species for rumen function to maintain rumen health. Cover crops harvested as hay may require species that can dry during the spring months (Brassica species usually are not good for this). Ensiling can be used if the area or the crop is not conducive to dry hay production. Nitrate issues can be a problem with sorghum and sudangrass especially if grazed or harvested after drought conditions. If the grass cover crop is to be harvested for feed it should be cut by boot stage for best forage quality and water preservation (water uptake increases during reproductive stages). Grazing helps keep some of the nutrients in the field while harvesting can remove nutrients but still provides some soil health benefits. Sweet clover should not be harvested for hay as it contains a substance called coumarin, which is converted to dicoumarol by mold in the hay. This poisoning can cause several health issues and ultimately death.
Crop insurance programs should always be reviewed to determine any potential effect on insurance payments.
Impact on Weed Management
Cover crops can be used as a physical barrier to help suppress weeds after harvest and into early spring. Management decisions that can help contribute to a cover crop necessary for weed suppression are:
- Both planting and termination dates can impact weed suppression. Often the termination date will need to be delayed, maximizing weed suppression, which can delay corn and soybean planting dates beyond the typical planting dates for the area.
- When a mixture of cover crop species is planted, the species that produces the most biomass is often the most important for weed suppression. Cereal rye can often produce enough biomass needed for weed suppression.
- Increasing the seeding rate can be an important consideration especially when the planting date of the cover crop is delayed. It is less of a concern when the primary cover crop is cereal rye as the tillering ability of this species can compensate for a late planting date. However, very late planting dates or an early winter can reduce tillering and shorten the growing season.
- Several cover crop species, including cereal rye, produce compounds (allelochemicals) that can suppress small-seeded weeds growing in the cover crop residue. While the effect of these compounds is known, it is difficult to determine how important this effect is compared to the importance of cover crop biomass.
- If the cover crop is grazed, the timing and intensity of the grazing can have a large effect on biomass and termination date. Cover crops that winterkill or are terminated with herbicides in the spring can still provide a mat that prevents weeds from germinating.
- Summer annual weed suppression is dependent on the growth stage of the cover crop at termination. More mature cover crops can provide a longer duration of weed control. Following termination, cover crop residue will dissipate over time. When cereal rye is terminated before stem elongation, the plant residues degrade quickly and weed suppression is short-lived.7
Figure 3. Lifecycle of an annual weed. Photo courtesy of Penn State College of Agricultural Sciences.8
Cover Crop Termination
Terminating cover crops prior to planting in the spring can be challenging. Termination of cover crops is influenced by the species used (grasses, Brassica species, cereals, broadleaves) their growth stage, temperature, and the crop to be planted. Methods used to terminate cover crops include winterkill, tilling, crimping, mowing, and herbicides. All these termination methods have advantages and disadvantages but, in many situations, an herbicide application may still be necessary.
Location can limit or be advantageous for cover crop winterkill. When termination of a cover crop uses a method, other than winterkill, there are termination date guidelines set by the National Resources Conservation Service (NCRS) to assure a producer, that wants to insure their crop, their termination date follows good farming practices. Cover crop termination guidelines can be determined by the location of each county in the U.S. This termination date can range from 35 days or earlier before planting of the crop to before crop emergence.9 Termination can be accomplished with tillage, but some of the benefits provided by the cover crop may be reduced.10 Roller-crimpers can be used to flatten tall-growing cover crops by breaking or crimping of stem. Mowing can also be an effective termination method for some species, but care is needed to evenly distribute the residue.
When selecting herbicides to terminate cover crops, consider the cover crop species, the weed species present, the growth stage, weather conditions at application, the nature of herbicide activity (systemic or contact), and the cash crop to be planted. Actively growing crops under warm temperatures have higher metabolism rates that move systemic herbicides to their site of action more quickly. When glyphosate is used to terminate a cover crop it is important for the crop to be actively growing and ideally make an application after several sunny days with daytime temperatures above 55°F and nighttime temperatures greater than 40 °F, which generally increase the activity.11
When using herbicides, the product label must be read and followed for application guidelines and re-cropping restrictions concerning the following crop.
To maintain farm program eligibility for federal crop insurance on spring planted crops and other programs, check with your local Farm Service Agency (FSA) to make sure termination dates follow NCRS guidelines. The NRCS Cover Crop Termination Guidelines were jointly developed by NRCS, FSA, and RMA to promote a consistent, simple, and flexible policy across all USDA programs. 9
When considering using a cover crop for the first time it is important to understand that there is no one-size-fits-all cover crop answer for all situations or production areas. A producer is often encouraged to start small and then apply what is learned the first few years along with learning what other successful cover crop producers are doing in the area. This allows adjustments to the program to meet the needs and the desired soil health improvements that are important to each unique situation.
1 2022. Cover crops in the US: Current status and trends. Center for Regenerative Agriculture. University of Missouri. https://cra.missouri.edu/cover-crops-in-the-us-current-status-and-trends/.
2 2022. USDA offers expanded conservation program opportunities to support climate smart agriculture in 2022. United States Department of Agriculture. https://www.usda.gov/media/press-releases/2022/01/10/usda-offers-expanded-conservation-program-opportunities-support.
3 Myers, R., Ellis, C., Hoormann, R., Reinbott, T., Kitchen, N., and Reisner, J. 2015. Cover crops in Missouri: Putting them to work on your farm. G4161. University of Missouri. https://extension.missouri.edu/publications/g4161.
4 Cover crops – Keeping soil in place while providing other benefits. Natural Resources Conservation Service. USDA. https://www.nrcs.usda.gov/wps/portal/nrcs/detail/ny/technical/?cid=nrcs144p2_027252#:~:text=A%20cover%20crop%20slows%20the,holding%20capacity%20for%20plant%20growth.
5 Ethridge, K.R. Increasing organic matter by using cover crops. Natural Resources Conservation Service. USDA. Increasing Organic Matter by Using Cover Crops | NRCS Kansas (usda.gov)
6 Self, J.R. 2010. Soil test explanation. Colorado State University Extension. Fact Sheet No. 0.502. https://extension.colostate.edu/topic-areas/agriculture/soil-test-explanation-0-502/.
7 Hartzler, B. and Anderson, M. 2019. Impact of cover crops on weed management. Iowa State University Extension and Outreach. https://crops.extension.iastate.edu/blog/bob-hartzler-meaghan-anderson/impact-cover-crops-weed-management.
8 2010. Curran, W.S. Suppressing weeds using cover crops in Pennsylvania. Penn State College of Agricultural Sciences. https://extension.psu.edu/suppressing-weeds-using-cover-crops-in-pennsylvania#:~:text=Choose%20cover%20crops%20based%20on,late%20summer%20or%20early%20fall.
9 Cover Crops. 2021. Risk Management Agency USDA. NCRS Cover crop termination guidelines. https://www.rma.usda.gov/en/Topics/Cover-Crops.
10 Johnson, J. Termination time for cover crops. USDA. Natural Resources Conservation Service. Iowa. https://www.nrcs.usda.gov/wps/portal/nrcs/ia/newsroom/features/STELPRDB1086071/#:~:text=Many%20of%20Iowa’s%20popular%20winter,to%20minimize%20yield%20loss%20risk.
11 Wallace, J. and Lingenfelter, D. 2019. Cover crop termination tips. PennState Extension. https://extension.psu.edu/catalogsearch/result/?q=2019+Cover+crop+termination+tips.+PennState+Extension.
12 DeJong-Hughes, J. 2021. Guide to planting cover crops in Minnesota. https://extension.umn.edu/cover-crops/planting-cover-crops#how-should-cover-crops-be-seeded%3F--714212.
Web sites verified 8/22/22.