What is a Cover Crop?
A cover crop is a plant that is used primarily to slow erosion, improve soil health, enhance water availability, smother weeds, help control pests and diseases, and increase biodiversity. Cover crops have also been shown to increase crop yields, break through a plow pan, add organic matter to the soil, improve crop diversity on farms and attract pollinators.
Many species of grasses, legumes, and other forbs can be used as cover and green manure crops, sometimes called living mulches. For quick cover and abundant roots, grasses are a good choice. To increase soil fertility, nitrogen fixing legume species are popular. For reducing nematodes in crop rotations or "flash grazing" applications, brassicai species can be helpful, either alone or in combination with annuali ryegrass.
To learn more about the individual species, choose the species of interest from the following drop-down menu or click on the species grouping of interest from the left-hand menu (Grasses, Legumes, or Other Forbs).
The UC-Davis Cover Crop Database also has links to additional species http://www.sarep.ucdavis.edu/ccrop/)
When establishing a cover crop, match the cover crop species not only with the job traits desired, but also with the soil type and conditions.ﾠ Some species grow better than others in low fertility and/or poorly drained soils. A review of a soils map and a soil test will help evaluate the soil.ﾠ
Once the cover crop species is selected, it is critical to always sow good live seed.
When planting leguminous cover crops such as clover, vetch, lespedeza, and soybeans, be sure to inoculate the seed before planting.
Cover crops usually can be planted using [reduced] tillage or conventional tillage systems.
What's your goal?
Scavenge N – cover crops can scavenge or “trap” residual soil nitrate to prevent it from leaching into drainage water. This protects water quality, and some of the scavenged N will be available to succeeding cash crops while the rest helps build soil organic matter.
Produce N – Legumes “fix” atmospheric nitrogen for their own use. After the legume cover crop is terminated, much of this N will be released as the residues decompose, providing available N to succeeding crops.
Prevent Erosion – The classic use of cover crops is to cover the soil surface to protect against both water and wind erosion, thus conserving the soil resource base.
Build Soil Quality – Cover crops improve soil physical properties, increase soil organic matter, and increase soil biological activity. Fibrous roots build soil aggregation and deep-rooted crops improve permeability. Some tap-rooted crops help break up compacted layers, improving water flow, aeration, and cash-crop rooting. Cover crops stimulate soil biological activity by providing additional food in the cover crop shoot and root residues. Cover crops left as a mulch at the soil surface can also conserve soil moisture for later use by the cash crop.
Suppress Weeds – Some cover crops can suppress weeds by competition, shading, or allelopathy. Unfortunately, some cover crops can also become weeds in subsequent cash crops if not carefully managed.
Recycle Nutrients – Although N is the usual focus, cover crops may also help recycle other nutrients by reducing erosion, building soil organic matter, and increasing soil biological activity.
Protect Water Quality – cover crops scavenge residual soil N, thereby reducing N losses to drainage water. They also reduce erosion and the losses of phosphorus and pesticides bound to the sediments.
Enhance Wildlife Habitat – cover crops can provide water, cover and food for birds and other wildlife and increase landscape diversity.
Selection of Cover Crops
In addition to deciding the particular benefits or objectives for the cover crop, producers need to consider the following when choosing cover crops:
Cropping/tillage system – what are the available time windows for cover crops in each system; what are the crop types and how compatible are different cover crop types during each window.
Climate – this affects the length of the growing season available for the cover crop.
Soil types/natural drainage class/tile drainage – some cover crops tolerate wet or droughty conditions while others do not.
Simple species vs. mixtures – simple species are often easier to manage, but a mix of species can provide multiple benefits and improve soil biological activity more quickly.
What is a good Mixture?
Often at least one grass and one legume species are planted together in late summer providing soil protection over the winter with an actively growing crop in spring. Brassica species can also be added to the grass and legume mixture for their fast growth and rapid mineralization. There are many premixed cover crop mixtures on the market. Some of them are listed below.
Annual ryegrass and crimson clover
Radish, crimson clover and annual ryegrass
Radish and annual ryegrass
Radish and crimson clover
Radish and oats; oats
Crimson clover and radish
Red, ladino, and sweet clover
Annual ryegrass, crimson clover, red clover, radish, sweet clover
Hairy vetch and oats
Cereal rye and hairy vetch
Triticale and annual ryegrass
Peas, oats and hairy vetch
Note: species information is available within the Species topic segment (https://forages.oregonstate.edu/oregon/topics/species).
Disadvantages of Cover Crops
There are additional costs above normal cropping practices that must be considered in systems that include cover crops. Extra expenditures include the cost of the cover crop seed and labor and time for planting. Also, special or alternative equipment may be needed to handle the greater amounts of residue present in no-till systems. Cover crops must be managed like any other crop produced in the farm operation.
Interference with the primary crop
Unmanaged cover crops can act as weeds by competing with the primary crop for light, moisture, nutrients, and space. In a dry year, cover crops can rob primary crops of valuable soil moisture. In other years, they may also compete for other resources such as nitrogen if not managed properly. For most cropping systems with cover crops, the use of starter fertilizer during planting of the primary crop should compensate for nutrients used by the cover crop. Immobilization of nitrogen by the cover crop generally is negligible, especially if manure has been applied. However, if large amounts of vegetation are being tilled under, producers raising corn should split the nitrogen requirements between planting (one-third) and side-dressing (two-thirds).
Cover crops also may be affected by the same chemical and physical factors that contribute to weed control. As with weeds, crop species can be hampered by chemicals released from cover crops, cooler soil temperatures, and the smothering effects of cover crop residues. Cooler temperatures may delay the germination and growth of the grain crop. Crop residues also may act as physical barriers, making uniform spray coverage more difficult. These residues also may bind herbicides, resulting in reduced activity. Preemergence herbicide programs may need to be adjusted to account for this interaction.
Cover crops, like weedy fields, may harbor insects, diseases, and nematodes that could be harmful to the cover crop and detrimental to future crops. Consider specific pest/crop interactions that may become a problem. For example, cereal rye or orchardgrass can attract armyworms. Clover root curculio, a pest common to red clover, also can attack alfalfa. Heavy populations of certain weeds such as chickweed attract black cutworm or slugs, while johnsongrass is a host to maize dwarf mosaic virus, which can infect corn. Understanding these potential interactions and the conditions that favor them helps producers make proper management decisions.
Termination of Cover Crops
Producers should plan in advance for how they will control and terminate the cover crop. Some cover crops have the potential to become weeds and therefore careful planning and management are required.
Winter-kill – some cover crops do not overwinter in colder portions of our state. In those cases, the producer does not need to plan for the termination in the spring. Oats and oilseed radish are two examples. However, if the radish is planted early and goes to seed, or if it is planted too late and some seed does not germinate before cold temperatures, then some seed may germinate in the spring. In these cases, the crop can be readily terminated with herbicides.
Herbicide – Producers should always consult herbicide labels and the state weed control guide for current recommendations on specific chemicals and crops (e.g. PNW Weed Management Handbook: https://pnwhandbooks.org/weed. In general, a nonselective contact or translocated herbicide will effectively terminate cover crops. Some cover crops may need a second herbicide application a few weeks later for improved control. The timing of application is very important for some tough-to-control cover crops, and producers should be prepared and have a back-up plan.
Tillage – tillage can effectively terminate some cover crops. Incorporating large amounts of above-ground growth can be challenging, however. Tillage also reduces some of the benefits of the cover crops such as erosion control and building of soil organic matter.
Mowing or roller-crimping – Mowing or using a roller-crimper can successfully terminate some cover crops at the flowering or heading stage, but that is often later than desired for typical row-crop systems. Note: to control the cover crop, the roller-crimper needs to snap the cover crop stem rather than simply bend them.
Frankenfield, A. (2019, May 10). What Cover Crop Should I Plant? Retrieved from https://extension.psu.edu/what-cover-crop-should-i-plant
Kladivko, E. Agronomy Department, Prudue University (August 2011) Cover Crops for Modern Cropping Systems. Retrieved from https://ag.purdue.edu/agry/extension/Documents/CoverCropsOverview.pdf.
Authors: Clare Sullivan, David Hannaway, Shixiao Yu, Kayleen Schreiber