Cover Crop Species Selection

Identify Your "Givens"

Before you choose a cover crop, consider your current crop rotation, climate, desired planting and termination date and method, and soil conditions. Planning well and treating your cover crop as more than an afterthought will lead to better success.

Climate

Pennsylvania ranges from USDA plant hardiness zone 5a to 7b. Average rainfall varies from about 35 to 45 inches. Your local combination of growing season length and rainfall impact which plant species grow well, your optimum planting window, how much dry matter a given species can produce, and whether certain species overwinter or winter kill. 

Seeding method and date

Cover crops can be seeded by drilling or by broadcasting from the ground or aerially with or without incorporation. Each method has its strengths and weaknesses.

Drilling allows seeds to be placed into the soil and generally improves cover crop "catch" and the speed of emergence. Seed to soil contact is critical, especially during sub-optimal weather conditions like drought. This results in this method requiring the lowest seeding rate to obtain a good stand, so lower seed cost is a benefit. However, drilling is slower, so it requires more staff-hours for seeding; it uses more fuel than other methods; it causes more wear and tear to equipment; and it is therefore more expensive to complete a drill operation than other methods.

Broadcasting seed is the flip-side of the drill-seeding coin. It is significantly faster and easier on equipment, but the lack of seed-to-soil contact makes for slow stand establishment that takes more seeds per acre to grow an equivalent stand compared to drill-seeded cover. Broadcast seed establishment can be significantly improved by following up with a cultipacker or vertical tillage to help incorporate seed. Of course, this is not an option for broadcast interseeding, described below. 

Your current crop rotation will determine your cover cropping window. For example, in a corn-soybean rotation, harvest ranges from mid-October through November (see figure below). 

Key Observations
Most Active Periods: Many crops, such as Corn (Grain) and Soybeans, have their most active harvesting period in late autumn (October and November).
Continuous Harvesting: Hay (Alfalfa and Other) maintains a broad harvesting window spanning from late spring through early autumn.
Double Windows: Some crops like Cabbage have distinct summer and fall planting and harvesting cycles to provide fresh produce across multiple seasons.

Therefore, if you are constrained to seeding after grain harvest, you must find a cover crop that can thrive in an October to November planting window or wait to plant early in the spring. 

If you have the ability to interseed, or seed cover crops into a standing crop, then your planting window can expand by several weeks. Interseeding can require special equipment and management for optimum results.  

Cover crops can be drill-interseeded into corn around the V4 stage, with best results in central Pennsylvania and north. Certain species are better adapted to the shading and heat that coincides with this practice, in addition to herbicide plant-back restrictions. Find more details in a digital download of  Interseeding Cover Crops in Corn Production: Best Practices for Drill-Interseeding in Mid-Atlantic Crop Systems.  

Broadcast interseeding into corn can be accomplished with a high-clearance ground rig, airplane, helicopter, or drone around the black layer (R6). This practice is gaining popularity with the increase in custom drone applicators, as other aerial applicators are difficult to find in Pennsylvania, and our small, irregularly shaped fields make ground applications undesirable from a yield-loss perspective.  

Broadcast interseeding into soybeans from the ground or air in September can be a viable option to expand the cover cropping window. The practice works best when soybeans will be harvested late, and when paired with delayed spring cover crop termination. Read a summary of recent research on this practice in Pennsylvania, "Cover Cropping into Standing Soybean". 

You can also broadcast interseed red clover into wheat in late winter, also known as frost seeding. Frost seeding results in a clover stand that is already established by the time wheat is harvested. This provides a work-around for red clover's slow establishment. Read more about Frost Seeding.

Even if frost seeding is not a practice you plan to use on your farm, small grains in the rotation offer an opportunity for planting after harvest in July. In the southeast, double-crop soybeans typically fill this window. However, double-crop soybeans generally become less profitable the farther north you are located in Pennsylvania, so cover crops become an attractive option. Planting in mid-July after small grain harvest, compared to October or November after corn or soybeans, drastically expands the number of viable cover crop options.

Identifying your seeding method and date options with equipment you have on hand as well as work you could hire out in mind, helps narrow down which species can realistically fit into your crop rotation.

Termination method and timing 

How and when you choose to terminate cover crops depends on your access to equipment, equipment capabilities, and cropping system. Cover crops can be terminated (killed) either mechanically or chemically, or you may choose to capitalize on the natural life cycle of the cover crop and intentionally winterkill.

Mechanical termination can include tilling, or roll-crimping. Mowing tall cover crops before tillage can help manage residue, while mowing should not be done before roll-crimping. While disturbing the soil, tillage does help cover crop residues breakdown more quickly, which results in faster nutrient release than roll-crimping in a no-till system.

Roll-crimping works best when cover crops are fairly far along in their maturity. Data is not available for all species, but for cereal rye this looks like anthesis (pollen shed), while for hairy vetch, roll-crimping is most effective at the early pod stage. Roll-crimping should usually be followed by chemical termination for complete cover crop kill.  

Chemical termination is done by spraying with herbicides. Chemical termination is commonly used in no-till cropping systems. Always follow herbicide labels closely regarding rates, timing, and other use requirements to get an effective cover crop kill without harming the following crop.

Cover crops should either be terminated two or more weeks before cash crop planting, or at or shortly after cash crop planting. Early termination, so that the cover crop is well-desiccated before cash crop planting, usually results in lower biomass accumulation and carbon-to-nitrogen ratio material. The benefits of early termination can result in faster nitrogen availability to the cash crop and easier residue management. Drawbacks to early termination include slow soil drying and slug issues in no-till systems.  

Planting green termination at or after cash crop planting can maximize cover crop biomass and benefits, but also comes with intensive management and equipment requirements. The practice works best with soybeans, but can be done successfully with corn when enough early-season nitrogen is supplied. Planting green is often, but not always, accompanied by roll-crimping at, before, or after planting. Read about planting green in detail in this research summary. Make sure to adhere to NRCS cover crop termination guidelines for your region to be eligible for crop insurance.

Special equipment is necessary to successfully plant green in most circumstances, so identify whether you have the correct and properly maintained equipment, or if you have access to someone who could custom farm this way. 

Soil conditions 

Knowing your soil chemical and physical limitations can help you choose the best cover crops for each field.

It is important to keep your soil fertility in mind when choosing a cover crop. Do you have low fertility, degraded soils? Or do you apply manure in four out of five years? Different cover crop species perform differently in different fertility situations. For example, grasses and brassicas thrive in high-nitrogen environments, while legumes can grow well and provide much-needed organic nitrogen into non-manured environments.

Soil texture and how it impacts the growing environment is important, too. Some species tolerate poorly drained, heavy soils less well than others. For example, annual ryegrass does much better under flood conditions than crimson clover.

Your soil may also have limitations such as compaction or degraded soil structure. Keep this in mind when choosing a cover crop species. While trying to build soil structure, for example, high carbon species with fibrous roots, such as grasses, can be beneficial. Meanwhile, some data support the use of forage radish, with its robust taproot, as a way to "bio-till" compacted soils.

Identify Your Goals

Once you identify your environmental, timing, and equipment constraints, identify the main benefits you want to get out of your cover crop. Keep in mind that some goals work against one another—like nitrogen tie-up and nitrogen provision. So, expecting one species to provide maximum benefits of both of those functions is unrealistic. Some of the most common cover crop benefits, also known as "ecosystem services," are briefly detailed below. 

Prevent soil erosion

Cover crops help prevent soil erosion by physically shielding the soil surface from raindrop impact, while living roots hold the soil and improve aggregate stability. Reducing erosion improves water quality and keeps high-value topsoil in place. A cover cropped field always reduces erosion compared to no cover crop, but some functional groups outperform others. Grasses provide the highest average benefit, followed by broadleaves, and legumes provide the lowest, yet still significant erosion protection. 

Suppress weeds

Weeds rob yield from field and forage crops. Cover crops can reduce weed quantity and can slow weed development so farmers have more time to control weeds. GROW reports that in the northeast US, at least 7,000 pounds per acre of cover crop biomass can provide more than 80 percent weed suppression. 

Small-seeded weeds are suppressed by cover crops better than large-seeded weeds. In Pennsylvania, cover crops have shown incredible potential to control marestail and pigweed species, specifically. 

Build organic matter

Planting cover crops increase soil organic matter by adding biomass to the soil (both above and below ground). Additionally cover crops provide habitat for microorganisms that break down plant residues, some of which eventually turn into stable soil organic matter called humus.

Read more from SARE about how cover crops increase soil organic matter in this factsheet.

Nitrogen capture or release 

Cover crops can be used to manage nitrogen. You can harness a cover crop's characteristics to either take up and hold on to nitrogen from the environment that may otherwise be lost to leaching or denitrification, or volatilization. Conversely, you can plant species that naturally capture nitrogen from the atmosphere and fix it so that it can be converted into plant available nitrogen fertilizer for the next crop, once the plant tissues are processed by soil microbes.

Though historically an inexpensive input, inorganic nitrogen fertilizer can be costly, especially during uncertain economic times. Therefore, it can be valuable to focus on keeping as much existing nitrogen in the root zone as possible, and/or fixing additional nitrogen for next crops in the rotation. 

Compaction remediation

Soil compaction can cause poor crop emergence and reduce porosity and water infiltration. According to the USDA Natural Resources Conservation Service, compaction can reduce yields by 10 to 20 percent. 

Subsoil tillage is an option to help fix deep compaction problems, but comes with drawbacks. 

Cover crops can help remediate comp

action by forming root channels through which cash crops can grow, access, and explore deeper soil below a compacted layer. Additionally, increasing days of the year with living roots in the soil and biomass additions from cover crop residues can help increase soil organic matter over time, which can improve soil resistance to additional compaction. 

Habitat for beneficials

Natural enemies are beneficial invertebrates that help control crop pests. They include insects, arthropods, nematodes, and others. For example, in no-till systems that suffer from slug damage, ground beetles are highly sought after biocontrol agents. Planting cover crops can increase these natural enemy populations by providing habitat and attracting prey organisms for ground beetles to consume. 

In addition to these natural enemies, pollinators can also benefit from having additional food sources when flowering cover crop species are grown. 

Read more about the impact of cover crops on natural enemies and pests in this factsheet, and about cover cropping for pollinators and beneficial insects in this bulletin

Forage production

Some farms require options for emergency forage. Though by definition cover crops are not harvested, having the option to chop, bale, or graze cover crops can provide options for those with livestock to feed. 

In these situations, keep in mind the forage value of the species, alongside other ecosystem services it can provide.

Consider Species Characteristics

Cover crop characteristics

Cover crops can be broken up into various groups based on their characteristics. These characteristics influence which ecosystem service(s) a given cover crop species will provide. 

The main categories are vegetation type; grasses, legumes, or non-leguminous broadleaves and season of growth; summer (warm season) or fall/winter (cool season). Different cover crop species exist for each of these combinations, as displayed in the USDA Agricultural Research Service cover crop chart below. 

For example, I want to fill a fallow period I usually have after my soybeans; I want to tie up any nitrogen I can and have overwintering vegetation for weed suppression — in this case, we know we need a cool-season grass or broadleaf, which will meet these goals.

Examining some additional information can help narrow down which species is the best fit. 

You may consider the above and belowground architecture of the plant. Above-ground architecture can be described as erect or upright, viney, spreading, single or multi-stemmed, etc. Root architecture can be described as tap, fibrous, rhizomatous, deep, or shallow rooting, etc. Upright species may still provide excellent groundcover, and can be easier to manage than viney species. Tap and fibrous roots have been shown to assist with alleviating compaction. 

Considering the same example as above, let's also consider that I want a multi-stemmed, erect cover crop so I can get some weed suppression while still being able to manage the residue with my equipment. I am also interested in deep, fibrous roots for reducing erosion and breaking up some compaction. It seems that I am left with cool-season grasses as my options. 

Rate of growth is very important to consider, to make sure the cover crop is providing the desired ecosystem services--for example, I would not want a slow-growing cover crop if I were seeking fall weed suppression or nutrient uptake. Rate of growth is also important to make sure the cover crop doesn't "get ahead" of you in the spring. Cereal rye is notorious for early maturation and and rapid growth in the spring, so many farmers opt to grow winter wheat instead. Winter wheat provides many of the same ecosystem services, or fills the same niche, as cereal rye, but matures more slowly and does not grow as vigorously in the spring, nor does it reach as high overall biomass. 

Chemical composition is another factor to consider. This can include allelopathic potential, C:N ratio, and root exudates. Allelopathic properties can be desirable if weed control is a main goal for the cover crop, but can injure very small seeded crops. Some brassicas are known to possess biofumigant properties. Some species are known to associate with arbuscular mycorrhizal fungi (AMF), while others do not. 

While Carbon to nitrogen (C:N) ratio plays a large role in nutrient dynamics and residue management in the field. Briefly, if a plant tissue or root C:N is above 24:1, microbes that break down the residue will take up free nitrogen from the soil to help with the breakdown process. Meanwhile, if the C:N is below 20:1, soil microbes will release plant available nitrogen into the soil, as there is more than enough nitrogen to help process the carbon in the plant tissues.

As you can imagine, since legumes fix nitrogen, they are generally lower in C:N than brassicas and broadleaves, but C:N is also highly variable depending on crop growth stage. Young cover crops that are vegetative and have not built carbon-rich reproductive structures can have a C:N below 24:1, illustrated below.  

Considering C:N can help you decide not only which species is best for your situation, but when is the most opportune time to terminate the cover crop, or what is the best way to adjust N fertilization in the main crop following the cover crop. 

You may also want to consider a cover crop species' tolerance to stress, including drainage, pH, shade, and low fertility. 

Time to flowering is important to think about as you come up with a termination strategy. It can be especially important to consider this in organic systems, so that you don't end up with a cover crop that sets seed, which can become a weed in the next phase of the rotation. 

Pest resistance or susceptibility is also important to consider. Though the effort has improved in recent years, compared to cash crops, cover crops have benefited from very little selective breeding. So, we often count on inherent pest resistance traits. We do know that certain insects, nematodes, and diseases favor certain cover crop species as hosts, which should be avoided when possible as part of your integrated pest management (IPM) plan. 

As you select your cover crop, it is important to consider the full picture, including all the different species characteristics, including strengths and weaknesses.

Tools to Help

Valuable localized information about cover crops is included in the Penn State Agronomy Guide. Managing Cover Crops Profitably, published by SARE, is another excellent resource to read about cover crops from researchers and from real working farms. 

The Northeast Cover Crops Council has developed a species selection tool that takes all this information and distills it into a user friendly interface. The tool takes the guess work out of species selection, providing recommendations based on your "givens" and "goals" --all while providing detailed seeding, termination, and other information about your selected cover crop species. 

Read this article, which includes a computer screen recording, if you would like assistance learning how to use the tool. 

Lastly, your local Extension Educator, NRCS, and Conservation District staff are more than happy to help you work through cover cropping ideas and figure out what the best fit is for your unique site, cropping system, and goals. 

Summary

To get the most out of a cover crop, be intentional about the species you choose. Consider your environmental, cropping system, machinery, and management constraints before choosing one or more species to meet intentional goals. For farm profitability, the cost of cover crop seed, establishment, and termination should be scrutinized, too. Numerous government, private industry, and non-profit programs can help absorb the cost of cover cropping. Reach out to your local Conservation District and NRCS offices for more information.