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 winter 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 forage radish 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 grasses helps compensate for the decreases in nitrogen availability for the following crop when grasses are allowed to mature. In the mid-Atlantic region, the winter 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 winter 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. This may allow mowing instead of herbicide use, in no-till production systems.
If you want to accumulate a lot of organic matter, it’s best to grow a cover crop for the whole growing season (see figure 10.5a), which means no income-generating crop will be grown that year. This may be useful with very infertile or eroded soils. It also may help vegetable production systems when there is no manure available and where a market for hay crops justifies a longer rotation.
COVER CROP SELECTION AND PLANT PARASITIC NEMATODES
If nematodes become a problem in your crops (common in many vegetables such as lettuce, carrots, onions, and potatoes, as well as 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 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 nematode—it will actually stimulate nematode numbers. However, sudan grass, sorghum-sudan crosses, and ryegrass, as well as pearl millet (a grain crop from Africa, grown in the U.S. mainly as a warm-season forage crop) have been reported to decrease nematode numbers dramatically. 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 millet; sudan grass; 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 can secrete compounds from their roots that are toxic to nematodes.
Planting after economic crop harvest.
Most farmers sow cover crops after the economic crop has been harvested (figure 10.5b). In this case, as with the system shown in figure 10.5a, there is no competition between the cover crop and the main crop. The seeds can be no-till drilled instead of broadcast, resulting in better cover crop stands. We recommend against the use of tillage prior to cover crop seeding, as it negates most of the benefits of the cover crop. 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 10.5b) is severely limited in northern climates by the short growing season and severe cold. Winter 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 10.6a). With some early-maturing vegetable crops, especially in warmer regions, it is also possible to establish cover crops in late spring or early summer (figure 10.6b). Cover crops also fit into an early vegetable–winter grain rotation sequence (figure 10.6c).
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 frost-seeded in early spring. Seeding cover crops during the growth of economic crops (figure 10.5c) 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. Winter rye is able to establish well without seed covering, as long as sufficient moisture is present (figure 10.7). Farmers using this system usually 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. When growing is on a smaller scale, seed is broadcast with the use of a hand-crank spin seeder.
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, an intercrop, polyculture (if more than one crop will be harvested), and an orchard-floor cover. Intercropping has many benefits. Compared with bare soil, a ground cover provides erosion control, better conditions for using equipment during harvesting, 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. One way to decrease competition is to delay seeding the intercrop until the main crop is well established. This is sometimes done in commercial fruit orchards. Soil-improving intercrops established by delayed planting into annual main crops are usually referred to as 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 coverfree strip (figure 10.8). This provides more distance between the main crop and the intercrop rows.
Cover Crop Termination
No matter when you establish cover crops, they are usually killed before or during soil preparation for the next economic crop. This is usually done by mowing (most annuals are killed that way) once they’ve flowered, plowing into the soil, using herbicides, rolling and crimping in the same operation, or naturally by winter injury. In many 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. Good suppression of vetch in a no-till system has been obtained with the use of a modified rolling stalk chopper. Farmers are also experiencing good cover crop suppression using a crimper-roller that goes ahead of the tractor, allowing the possibility of notill planting a main crop at the same time as suppressing the cover crop (see figure 16.7). Although not recommended for most direct-seeded vegetable crops, this works well for many agronomic crops.
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.
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 more residue and better water conservation for the main crop. Cover crop mulch may more than compensate for the extra water removed from the soil during the later period of green manure growth. 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.
Some cover crops can 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. Cover crops that may become a weed problem include buckwheat, ryegrass, crown vetch, and hairy vetch. On the other hand, natural reseeding of subclover, crimson clover, or velvet bean might be beneficial in some situations.
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, and caution is recommended when manual fieldwork is performed.