SUMMARY

• Cover crops can perform a variety of agronomic and ecological functions, with certain species providing specific benefits.
• Cover crop function is generally at its greatest when plants are at flowering. Therefore, prioritizing cover crop growth can help offer the most return on your seed investment.
• When selecting cover crops, prioritize your needs, select species that do an excellent job of meeting those needs, and avoid species that don’t meet your goals.
• During the species selection process, understand the limitations of your farm equipment and labor and the potential issues that can arise with certain cover crop species. Planning for these issues results in better success in integrating cover crops into your farming system.

Cover crops provide a variety of benefits when added to a crop rotation, with different species bringing different agronomic benefits. Maximizing the benefits of cover crops are typically best achieved when they are deliberately selected for a given function, crop rotation and growing environment. While there are many aspects to consider, a clear identification of goals and limitations can easily guide the selection process.

Cover crops maintain living roots in the soil at a time when they would otherwise not be present. These living roots exude carbon compounds that serve as an energy source to beneficial soil microbes that are responsible for cementing soil particles together, breaking down residue, and improving plant nutrient availability. The roots also release CO2, acidifying the soil immediately around root, which also releases soil nutrients. Additionally, cover crops add organic matter through the addition of above and belowground residues and carbon exudates. Cover crops also provide physical cover for insects, which aid in the breakdown of residue or serve as predators of crop pests.

While there are many agronomic and ecological services that cover crops can provide, they should possess a few key traits. First, covers should be easily established and germinate quicky to outcompete weeds. Second, they should be tolerant of their growing environment, surviving the winter if desired, and should terminate easily. Finally, potential cover crop species should not negatively impact the following cash crop. Additionally, the unique growth characteristics of certain species mean that certain cover crops can provide more benefit than others for any given need.

Most Common Cover Crop Goals:

• Protecting the soil from wind and water erosion - Cover crops with fibrous root systems and a dense canopy such as cereal grains (cereal rye, wheat, triticale, barley), annual ryegrass, millets, sudangrass and sorghum X sudangrass hybrids (sudex) are well suited for armoring the soil against raindrop impact and preventing soil erosion.
• Scavenging soil nitrogen - Extensive and deep rooting systems as well as high biomass production generally result in effective scavenging of soil nutrients. Best suited are cereal grains, annual ryegrass, millets, sudangrasses and brassicas. However, some of these species will release nutrients more quickly than others, depending on their stage of growth at termination and the C:N ratio of the plant.
• Fixing atmospheric nitrogen - High nitrogen demand crops, such as corn, can benefit from the nitrogen provided by clovers, sweetclovers, vetches, field peas, cowpeas and sunn hemp, with nitrogen-fixing potential varying by species.
• Weed suppression - Ideal cover crops for suppressing weeds germinate quickly, produce high amounts of biomass and have lasting residue. Cover crops that winterkill late or grow throughout the winter such as cereal grains, annual ryegrass and brassicas outcompete weeds, while crops like oats, sudangrasses, sudex, millets and buckwheat, establish quickly and leave residue that lasts into the winter. Additionally, some cover crops, such as forage radish, release alleopathic compounds that are toxic to germinating seeds.
• Improving soil physical properties - Cover crops that produce large numbers of fine roots can improve soil aggregation and water holding capacity, reducing crusting and increasing infiltration. These include cereal grains, oats, annual ryegrass, teff, millets, sudangrasses, sudex and phacelia.
• Penetrating plow plans and compacted layers - Deep rooted cover crops can drill through compacted layers, allowing for earthworms and roots of successive crops to penetrate. Cover crops best suited for this practice include sudex, forage radish and annual ryegrass.
• Breaking up pest cycles and suppressing nematodes - For some crop pests, cover crops can have a suppressive effect. Brassicas can release glucosinolates, which has a soil fumigating effect that can help suppress nematodes and other soilborne pathogens. Additionally, rolled cover crops, such as cereal rye, can alter disease cycles, including white mold in soybean.
• Supporting beneficial insects - The soil coverage provided by most cover crop species can provide habitat for beneficial insects. These insects size residue, help to cycle nutrients, serve as predators of crop insect pests, and can consume certain weed seeds.
• Livestock feed - Cover crops can serve as a source for livestock feed as grazed or harvested forage. Cover crops with the highest production and best suited to grazing include small grains, oats, sudex and millets, while brassicas and legumes can help increase the protein content of cover crop stands. Cover crops utilized for feed should be selected based on livestock ration needs, while management tactics should favor biomass production, including increasing seeding rates and applying nutrients if needed. Special care should be taken for potential poisoning issues from sudex, sudangrass and sweetclovers.

Figure 1. Comparison of root architecture among Austrian winter pea (left), hairy vetch (middle) and crimson clover (right). (Source: Zachary Larson. Used with permission.)

In addition to considering cover crop function, different species have different growth characteristics. Some grass species, such as teff, produces less than two feet of vertical growth, while sudex can exceed six feet in height. Clovers have a bunch-type growth habit, which can help reduce wrapping issues on planters, while vining legumes like vetch can climb taller crops such as small grains. Root architecture is also important, with grasses having more fibrous roots, and many legumes and brassicas having fewer, but larger roots (Figure 1). Finally, seed viability can be a factor, with certain legumes and brassicas having hard seed that can become a weed in subsequent cash crops. Herbicide-resistant soybean traits can help broaden control options for self-seeded cover crops, while for other crops some cover crop species should be avoided.

Fitting Cover Crops to Your Operation

Cover crops typically provide the greatest level of function at flowering. At this point legumes are at or near maximum nitrogen fixation, root systems are the most extensive, and biomass is near its peak. Therefore, cover crops, especially those that carry a higher per-acre cost, should be planted within a window that allows for them to reach flowering before they are winterkilled or terminated prior to planting the subsequent cash crop.

In most growing scenarios, cover crop growth rates are highly affected by growing degree unit accumulation. In many parts of the U.S., growing degree unit accumulation rapidly diminishes in the fall, meaning that the planting date of fall-seeded cover crops has a large effect on growth and maturity prior to dormancy. For instance, in Gibson City, Illinois (Figure 2), the average base 40F growing degree unit accumulation for the week surrounding September 15th is approximately 180 GDUs, while the week surrounding October 15th is half of that. Or in other words, a week of cover crop growth in September can equal two in October. And when considering warmer season crops whose growth slows below 50 °F, a week of growth in September can equal three in October. Conversely, GDU accumulation can double from April to May, which allows some cover crops to double their biomass in just a few weeks.

Figure 2. Average daily growing degree days for Gibson City, Illinois. Data sourced from National Weather Service (n.d.) NOWData – NOAA Online Weather Data [data set], accessed 5 December 2022. https://www.weather.gov/wrh/Climate?wfo=lot/.

In addition to planting for maximum GDU accumulation, warm-season cover crops should reach flowering ahead of the average first frost date when possible. Most warm-season plants will need between 40 to 80 days of good growth to flower, with buckwheat, sudex and pearl millet taking the fewest days and sunn hemp taking longer.

For cool-season covers, the winter low temperature will determine their ability to overwinter. This can be easily determined by USDA plant hardiness zones, which are determined by the average winter minimum temperature, divided into 10 °F zones. However, be aware that warmer temperatures can exist in dense canopies in early winter and the insulating properties of snow can result in temperatures being warmer at the soil surface than in the air, which means that cover crops that should winterkill at a given air temperature may survive. Finally, knowing the average spring frost-free date will give an indication of when winter cover crops will begin to take off or when it is safe to plant warm season covers where early season cover is needed.

Selection Considerations

In addition to climate-related restrictions on cover crops, management expectations should also be considered. These include:

Seeding methods: Most cover crops can be seeded with a drill, although smaller seeds such as the brassicas may require a small seed box for accurate metering. Additionally, some planters are easily adapted to many cover crop species with a few modifications, and the use of 15-inch planters is somewhat common. However, the use disks for metering specific seed sizes limits the ability to plant a cover crop mix.

Cover crops can also be broadcast in various ways. Smaller seeded cover crops will broadcast better than larger seeds, as they are better adapted to shallower planting depths. Clovers, grasses, brassicas and small grains broadcast well, while peas do not. Seed weight and density can affect the ability to be spread via spinner spreaders, with small grains performing well. Smaller seeds may necessitate using a narrower spread pattern and may perform better when applied with an aerial boom. Air seeders can be widely adapted to existing tools, such as a vertical-tillage units, or attached to a combine header (Figure 3). However, broadcasting should be limited to scenarios where the seed has access to adequate moisture; and if conditions are dry, planting or drilling is preferred.

Termination and regrowth: Termination is the second major event in cover crop management and timing is typically dictated by the weather and the growth state of the crop. Herbicides are the most common way of cover crop termination, and applications for harder to terminate cover crops, such as annual ryegrass, should be made during warmer (>50 °F), sunny days, limiting options for early termination in some locations. Other methods, such as rolling or mowing are most effective at certain crop stages, often during flowering. Develop a plan for regrowth, especially for crops that were intended to be winterkilled, survived mowing or rolling, or are harder to terminate with herbicides.

Figure 3. There are a variety of ways to help reduce labor needs. This unit uses an air seeder to drop seed behind the header, placing seeds underneath the chaff spread by the combine. (Source: Zachary Larson. Used with permission.)

Time and labor: Balancing the labor requirements of harvesting cash crops and planting cover crops can be challenging; however, many farmers have found ways to do both in a timely fashion each year. Some farmers have success in seeding in the morning before crops dry out or use a part-time tractor operator. Others have turned to interseeding cover crops, which allows for early establishment and shifts labor requirements to an off time. In spring, timely termination is most important, especially when dealing with fast-growing cover crops such as cereal rye.

Residue management: Poorly managed cover crop residues have the potential to hinder germination of the cash crop, cause excessive variability in soil temperatures and wrap planter parts. The amount of residue produced by cover crops depends on the species, seeding rate, seeding date, and termination date, with cereal grains and sudex typically producing the most residue. Most farmers have found success in managing residue at cash crop planting by adding row cleaners, replacing factory closing wheels and adding guards to reduce wrapping. When planning for later termination, lower seeding rates can help reduce the overall amount of residue and allow for better light penetration to the soil surface, which can help to limit issues at planting.

Fertility effects: Cover crops that have a high carbon content, like mature small grains, can lead to the immobilization of soil nitrogen when they begin to decompose. From a management standpoint, cover crop selection, termination, and management of cash crop fertility are ways to minimize potential issues.

Ability to handle contingencies: Issues can arise when growing cover crops and having a plan can help mitigate potential problems. The most common issue, especially with fast-growing cover crops, is not terminating the cover in a timely fashion and dealing with excessive growth. Another common issue occurs when thick cover crop stands are terminated ahead of a large rain, and the residue greatly reduces drying of the soil surface. Thinking ahead and having the flexibility to modify plans and equipment should be considered when selecting cover crops.

Putting it All Together

Many resources are available for cover crop selection. Regional decision tools such as the Midwest Cover Crops Council and Northeast Cover Crops Council are available and others are in development. The book, Managing Cover Crops Profitably, is available as a PDF and covers many of the common cover crops, and many seed suppliers often offer similar resources as well.

Cover crop resources may rate species on a 1 to 5 scale for various agronomic and ecological functions. When selecting, start with the top two or three priorities and a realistic expectation of planting date. Exclude species that do not meet the requirements needed for planting and termination timing, as they aren’t likely grow well and will most likely be a poor return on investment. Generally, it’s much better plant a less-than-ideal cover crop that will grow well and produce a meaningful amount of biomass than plant an ideal cover crop late or in the wrong environment where it won’t perform. With realistic options identified, pick the species that rate the highest for your priorities and avoid those that rate low, as they may provide the lowest return on your seed dollar and occupy space where a better-suited crop could be.

As with many agricultural inputs, economics should drive the selection of the cover crop species, especially when there are multiple options that meet the goals and work within your cropping system. Since seeding rates vary widely, comparisons should be made on a cost per acre basis. However, don’t skip on desired functions in the name of cost savings, especially when a small increase in price results in a gain of functions or more desirable growth characteristics.

Finally, unless there is a specific function desires that the “tried and true” cover crops can’t offer, select the species that have been proven to work in your area. Many of the niche species can be more expensive and harder to find and there is generally less information on managing these species. Cover crop suppliers and experienced farmers can be excellent resources for knowing what works, and more importantly what doesn’t, and can help ensure a successful cover on your farm.

Cover crops can provide a wide variety of agronomic benefits. However, the is likely no single cover crop that can perform every intended function well in all environments. By considering the limitations of your cropping system and local climate and the unique functions and properties of different cover crop species available on the market, you can more confidently find what best fits your operation and offers the most value from the array of products on the market.

Sources:

Bullard, V. (2017). Winterkill cover crop demonstration, CAPMC Fact Sheet. U.S. Department of Agriculture, Natural Resources Conservation Service Lockeford Plant Materials Center. https://efotg.sc.egov.usda.gov/references/public/Ca/CoverCrop_CaseStudy_Winterkill_7-2017.pdf Clark, A. (Ed.) (2007). Managing cover crops profitably (3rd ed.). Sustainable Agriculture Network.
Northeast Cover Crops Council. (2022, July 5). Cover Crop Decision Support Tools. http://covercrop.tools/

Legal Statements: ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower’s fields.

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