10.4: Cover Crop Management
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)There are numerous management issues to consider when using cover crops. Once you decide what your major goals are for using cover crops, select one or more to try out. Consider using combinations of species. You also need to decide where cover crops best fit in your system: planted following the main crop, intercropped during part or all of the growing of the main crop, or grown for an entire growing season in order to build up the soil. The goal, while not always possible to attain, should be to have something growing in your fields (even if dormant during the winter) all the time. Other management issues include when and how to kill or suppress the cover crop, and how to reduce the possibility of interference with your main crops either by using too much water in dry climates or by becoming a weed in subsequent crops.
Mixtures of Cover Crops
Although most farmers use single species of cover crops in their fields, mixtures of different cover crops offer combined benefits. The most common mixture is a grass and legume, such as cereal rye and hairy vetch, oats and red clover, or field peas and a small grain. Other mixtures might include a legume or small grain with oilseed radishes, or even just different small grains mixed together. Mixed stands usually do a better job of suppressing weeds than a single species. Growing legumes with small grains helps compensate for the decreases in nitrogen availability for the following crop when small grains are allowed to mature. In the mid-Atlantic region, the cereal rye-hairy vetch mixture has been shown to provide another advantage for managing nitrogen: When a lot of nitrate is left in the soil at the end of the season, the rye is stimulated (reducing leaching losses). When little nitrogen is available, the vetch competes better with the rye, fixing more nitrogen for the next crop. A crop that grows erect, such as cereal rye, may provide support for hairy vetch and enable it to grow better. Mowing close to the ground kills vetch supported by rye easier than vetch alone. In no-till production systems, this may allow for mowing instead of herbicide use.
Florida farmer Ed James has found significant benefits to the health and productivity of his orange groves by using mixes of cover crops.
It helps to have a blend because if you have one species that doesn’t take, you aren’t left without any germination,” he says. “As the buckwheat begins to play out, the hairy indigo and sunn hemp start to come on. As that begins to play out, the brassicas are coming. We already have a monoculture with the trees, so the mix of cover crops makes the soil feel like it is getting a crop rotation.”
—GILES (2020)
Cover Crops and Nitrogen
Managing nitrogen supply is one of the critical challenges farmers face during a crop rotation; the aim is to have sufficient available N for the crops being grown while not having a lot of mineral N left in the soil after crop maturity, especially during seasons when it might leach out or be denitrified. Cover cropping can play an important role in N management, whether the need is to supply N for grains or vegetables, or to lower available N at the end of the season to reduce losses.
Estimating N available from cover crops. Legume cover crops can supply significant amounts of available N for the following crop. If a legume is productive and allowed to grow to the bud stage to gain sufficient size (biomass), quite a bit of N will be made available to the next crop, from 70 to well over 100 pounds per acre. But the amount of N supplied depends on the cover crop species (or mix of cover crops) and how long it’s allowed to grow. Hairy vetch and crimson clover are two of the many choices that farmers frequently turn to in order to produce a lot of N, but other legumes may prove useful as sources of N.
The amount of N that will be made available to the following crop depends on the stage of growth, the amount of growth (biomass), and the N content of the cover crop or cover crop mix. Small cover crops whose leaves are deep green, for example, in early spring, will contain a high percent of N, over 3 percent. But because there is so little mass of material, the plants contain low total amounts of N. The N percent of a cover crop such as cereal rye tends to decrease (from over 3 percent) as the plant grows more leaves and then when the stem elongates and flowering and maturity occurs, ending up well below 1 percent N with a C:N ratio of 80 or more. If the crop has a low percent N (around 1.5%–2% N), as is common with small grains when stems elongate and flowering begins, little to no N can be counted on to help the following crop because soil organisms use all the N present as they decompose the residue. (See Figure 9.3 and Table 9.4 for an explanation of the C:N ratio and its relation to percent N in residue.)
If you estimate (or measure) the mass of a cover crop at the time of termination and its percent N, you can then estimate the amount of N that may be available to the following crop by using Table 10.1.
| Cover crop total N | Estimated available N (pounds N per acre) | |
|---|---|---|
| % N in dry matter | pounds N per ton | |
| 1 | 20 | 0 |
| 1.5 | 30 | 10 |
| 2 | 40 | 14 |
| 2.5 | 50 | 20 |
| 3 | 60 | 28 |
| 3.5 | 70 | 37 |
| 1Modified from “Estimating plant-available nitrogen release from cover crops.” PNW 636. A Pacific Northwest Extension Publication (Oregon State University, Washington State University and University of Idaho). | ||
Minimizing residual N in fall. Another way to increase N availability to the following crop is through cover crops capturing end-of-season residual N and protecting it for use by the next commercial crop. At the end of the season in some cropping systems there may be significant amounts of residual N that then can be lost through leaching below the root zone or by denitrification over the winter and early spring. This is both an economic issue for the farm and an environmental issue. Corn-soybean crop alternation and corn-corn are especially prone to high N levels in the fall and to overwinter and early spring loss. Grass cover crops such as cereal rye can help by taking up mineral N in the fall. (As mentioned above, there are good reasons to use a grass-legume mix such as cereal rye-hairy vetch in a situation where you aren’t sure whether there is or isn’t a lot of N left at the end of the season.) When there may be a lot of mineral N throughout the root zone (not just near the surface), if planted early enough, a deep-rooted cover crop such as forage radish together with cereal rye can help retain N. The forage radish can bring up nitrate from deeper in the profile in the fall, and when frost kills the radish and the nitrate leaks out, it can be taken up by cereal rye.
Planting Cover Crops
There are three ways to time the planting of a cover crop in relation to your cash crops: 1) plant a cover crop for an entire growing season; 2) plant a cover crop after the harvest of a cash crop and before planting the next cash crop; and 3) interseeding, or planting a cover crop into a growing cash crop. The approach you take will depend on your reason for planting a cover crop, your cash crops, the length of the growing season and the climate.
Planting for an entire growing season. If you want to accumulate a lot of organic matter, it’s best to grow a high-biomass mix of cover crops for the whole growing season (see Figure 10.7a), which means no income-generating crop will be grown that year. This may be especially useful with very infertile or eroded soils and when transitioning to organic farming. This is sometimes done on vegetable farms when no manure is available and in fallow systems in the western United States, but grain/oilseed farmers will not normally give up a year of production in a field.
Planting after cash crop harvest. Most farmers sow cover crops after the cash crop has been harvested (Figure 10.7b). In this case, as with the system shown in Figure 10.7a, there is no competition between the cover crop and the main crop. The seeds can be no-till planted with a grain drill or a row crop planter (no need for a high clearance interseeder) instead of broadcast, resulting in better cover crop stands. If possible, tillage should be avoided prior to cover crop seeding to maximize the soil health benefits that cover crops provide. In milder climates, you can usually plant cover crops after harvesting the main crop. In colder areas, there may not be enough time to establish a cover crop between harvest and winter. Even if you are able to get it established, there will be little growth in the fall to provide soil protection or nutrient uptake. The choice of a cover crop to fit between main summer crops (Figure 10.7b) is severely limited in northern climates by the short growing season and severe cold. Cereal rye is probably the most reliable cover crop for those conditions. In most situations, there are a range of establishment options.
Cover crops are also established following grain harvest in late spring (Figure 10.8a). With some early maturing vegetable crops, especially in warmer regions, it is also possible to establish cover crops in early summer (Figure 10.8b). Cover crops also fit into an early vegetable-winter grain rotation sequence (Figure 10.8c).
If nematodes become a problem in your crops (common in many vegetables such as lettuce, carrots, onions and potatoes, as well as in some agronomic crops), carefully select cover crops to help limit the damage.
For example, the root-knot nematode (M. hapla) is a pest of many vegetable crops, as well as of alfalfa, soybeans and clover, but all the grain crops—corn, as well as small grains—are nonhosts. Growing grains as cover crops helps reduce nematode numbers. If the infestation is very bad, consider two full seasons with grain crops before returning to susceptible crops.
The root-lesion nematode (P. penetrans) is more of a challenge because most crops, including almost all grains, can be hosts for this organism. Whatever you do, don’t plant a legume cover crop such as hairy vetch if you have an infestation of root-lesion nematodes; it will actually stimulate nematode numbers. However, sudangrass, sorghum-sudan crosses and ryegrass, as well as pearl millet (a grain crop from Africa, grown in the United States mainly as a warm-season forage crop) have been reported to dramatically decrease nematode numbers. Some varieties appear better for this purpose than others. The suppressive activity of such cover crops is due to their poor host status to the lesion nematode, general stimulation of microbial antagonists and the release of toxic products during decomposition.
Forage-type pearl millet, sudangrass and brassicas such as mustard, rapeseed, oilseed radish and flax, all provide some biofumigation effect because when they decompose after incorporation, they produce compounds that are toxic to nematodes. Marigolds, grown sometimes as companion plants in gardens, can secrete compounds from their roots that are toxic to nematodes.
Interseeding. The third management strategy is to interseed cover crops during the growth of the main crop. Cover crops are commonly interseeded at planting in winter grain cropping systems or are frost-seeded in early spring. Seeding cover crops during the growth of cash crops (Figure 10.7c) is especially helpful for the establishment of cover crops in areas with a short growing season. Delaying the cover crop seeding until the main crop is off to a good start means that the commercial crop will be able to grow well despite the competition. Good establishment of cover crops requires moisture and, for small-seeded crops, some covering of the seed by soil or crop residues. High clearance grain drills can be used to obtain good seed-to-soil contact when interseeding a cover crop (Figure 10.9). Cereal rye is able to establish well without seed covering, as long as sufficient moisture is present. Farmers using this system will broadcast seed during or just after the last cultivation of a row crop. Aerial seeding, “highboy” tractors, or detasseling machines are used to broadcast green manure seed after a main crop is already fairly tall, like with corn. When growing is on a smaller scale, seed is broadcast with the use of a hand-crank spin seeder. This works best for some of the grasses, and its success depends on the soil surface being moist for germination and establishment to occur.
Intercrops and living mulches. Growing a cover crop between the rows of a main crop has been practiced for a long time. It has been called a living mulch or an orchard-floor cover, with the cover crop established before the main crop. Intercropping, with the cover crop established at or soon after planting, has many benefits. Compared with bare soil, a ground cover provides erosion control, better conditions for using equipment during harvest, higher water-infiltration capacity, and an increase in soil organic matter. In addition, if the cover crop is a legume, a significant buildup of nitrogen may be available to crops in future years. Another benefit is the attraction of beneficial insects, such as predatory mites, to flowering plants. Less insect damage has been noted under polyculture than under monoculture.
Growing other plants near the main crop also poses potential dangers. The intercrop may harbor insect pests, such as the tarnished plant bug. Most of the management decisions for using intercrops are connected with minimizing competition with the main crop. Intercrops, if they grow too tall, can compete with the main crop for light, or may physically interfere with the main crop’s growth or harvest. Intercrops may compete for water and nutrients. Using intercrops is not recommended if rainfall is barely adequate for the main crop and supplemental irrigation isn’t available. Soil-improving intercrops established by delayed planting into annual main crops are usually referred to as interseeded cover crops. Herbicides, mowing and partial rototilling are used to suppress the cover crop and give an advantage to the main crop. Another way to lessen competition from the cover is to plant the main crop in a relatively wide cover-free strip (Figure 10.10). This provides more distance between the main crop and the intercrop rows. When establishing orchards and vineyards, one way to reduce competition is to plant the living mulch after the main perennial crops are well established.
In the past, the recommendation was to leave a week or two between the time the cover crop was killed and when the cash crop was planted. That is still the best approach in certain situations, such as in a dry spring. In fact, in a dry spring, terminating a few weeks ahead of the cash crop may be needed.
However, more and more farmers are now "planting green," where the cash crop is directly seeded into a still living cover crop. (In a 2019–2020 national survey, 54% of farmers reported that they plant green. See the box “Farmers Say Cover Crops Help the Bottom Line.”) Most often, the cover crop is sprayed with an herbicide shortly after the cash crop is planted.
Mechanical control of the cover crop is another option. For example, good suppression of hairy vetch in a no-till system has been obtained with the use of a modified rolling stalk chopper at early bloom. Farmers are also experiencing good cover crop suppression using cereal rye and a roller-crimper that goes ahead of the tractor, allowing the possibility of no-till planting a main crop at the same time as suppressing the cover crop (see Figure 16.10). Although not recommended for most direct-seeded vegetable crops, this has been successfully used for soybeans, corn and cotton.
Cover Crop Termination
No matter when you establish cover crops, they are usually killed or drastically weakened before or during soil preparation for the next cash crop. This preparation is usually done by one of the following approaches: mowing once they’ve flowered (most annuals can be killed that way), using herbicides and no-till, plowing into the soil (with or without use of herbicides), or mowing, rolling and crimping and no-till planting in the same operation, or naturally by winter injury. In some cases it is a good idea to leave a week or two between the time a cover crop is tilled in or killed and the time a main crop is planted. Studies have found that a sudex cover crop is especially allelopathic and that tomatoes, broccoli and lettuce should not be planted until six to eight weeks to allow for thorough leaching of residue. This allows some decomposition to occur and may lessen problems of nitrogen immobilization and allelopathic effects, as well as avoiding increased seed decay and damping-off diseases (especially under wet conditions) and problems with cutworm and wireworm. It also may allow for the establishment of a better seedbed for small-seeded crops, such as some of the vegetables. Establishing a good seedbed for crops with small seeds may be difficult because of the lumpiness caused by the fresh residues.
Cover crops can also be terminated by partially or wholly harvesting the biomass. You might argue that cover crops should be grown for the purpose of improving the soil, not to be harvested or grazed. But sometimes farmers use a hybrid or adaptive system, especially if they have livestock. For example, if a cereal rye crop comes up quickly after the winter in a warm spring, a farmer might decide to harvest the extra biomass for hay or haylage. In other cases it might be worthwhile to allow animals to graze the cover crop, which still cycles much of the carbon and nutrients (see Chapter 12). Even though much of the aboveground biomass is harvested, the soil still benefits from the root biomass and, in the case of grazing, from the manure.
Management Cautions
Cover crops can cause serious problems if not managed carefully. They can deplete soil moisture; they can become weeds; and, when used as an intercrop, they can compete with the cash crop for water, light and nutrients. They also tend to be somewhat costly in terms of seed and establishment, so you want to ensure that the benefits pay off.
In drier areas and on droughty soils, such as sands, late killing of a winter cover crop may result in moisture deficiency for the main summer crop. In that situation, the cover crop should be killed before too much water is removed from the soil. However, in warm, humid climates where no-till methods are practiced, allowing the cover crop to grow longer means fewer problems with soil being very wet or saturated at planting, and more residue and water conservation for the main crop later in the season. Cover crop mulch may more than compensate for the extra water removed from the soil during the later period of green manure growth.
Greater formation of large (macro) pores with cover crops leads to more rainfall infiltration, while higher organic matter levels following their use leads to greater soil waterholding capacity. Surface residue also slows runoff of rainfall, which allows more to infiltrate into the soil. In addition, greater mycorrhizal fungi presence following cover crops may aid water uptake, and cover crops may lead to cash crops rooting deeper and reaching more water. Considering all their effects, cover crops normally greatly enhance the water status of soils for cash crops. In addition, in very humid regions or on wet soils, the ability of an actively growing cover crop to “pump” water out of the soil by transpiration may be an advantage (see Figure 15.8). Letting the cover crop grow as long as possible results in more rapid soil drying and allows for earlier planting of the main crop.
Using bin-run cover crop seed that hasn't been properly cleaned can result in introducing weed seeds into fields. And on rare occasions cover crops may become unwanted weeds in succeeding crops. Cover crops are sometimes allowed to flower to provide pollen to bees or other beneficial insects. However, if the plants actually set seed, the cover crop may reseed unintentionally. On organic farms the hard seed of vetch allows it to become a pest in small grains such as wheat. Cover crops that may become a weed problem include buckwheat, ryegrass and hairy vetch, but there is usually no concern with timely termination. On the other hand, natural reseeding of subclover, crimson clover or velvet beans might be beneficial in some situations.
Another issue to consider is that a cover crop might harbor a disease of crop plants and form a habitat bridge from one growing season to another. For example, oilseed radishes increase clubroot in broccoli. Finally, thick-mulched cover crops make good habitat for soil organisms and also for some undesirable species. Animals like rats, mice and snakes (in warm climates) may be found under the mulch, which might affect yields and crop quality, and caution is recommended when manual fieldwork is performed.