by Sharad C. Phatak and Juan Carlos Diaz-Perez
Cover crops are poised to play increasingly important roles on North American farms. In addition to slowing erosion, improving soil structure and providing fertility, we are learning how cover crops help farmers to manage pests (390). With limited tillage and careful attention to cultivar choice, placement and timing, cover crops can reduce infestations by insects, diseases, nematodes and weeds. Pest-fighting cover crop systems help minimize reliance on pesticides, and as a result cut costs, reduce your chemical exposure, protect the environment, and increase consumer confidence in the food you produce.
Farmers and researchers are using cover crops to design new strategies that preserve a farm’s natural resources while remaining profitable. Key to this approach is to see a farm as an “agro-ecosystem”— a dynamic relationship of the mineral, biological, weather and human resources involved in producing crops or livestock. Our goal is to learn agricultural practices that are environmentally sound, economically feasible and socially acceptable.
Environmentally sustainable pest management starts with building healthy soils. Research in south Georgia (see Georgia Cotton, Peanut Farmers) shows that crops grown on biologically active soils resist pest pressures better than those grown on soils of low fertility, extreme pH, low biological activity and poor soil structure. There are many ways to increase biological activity in soil. Adding more organic material by growing cover crops or by applying manure or compost helps. Reducing or eliminating pesticides favors diverse, healthy populations of beneficial soil flora and fauna. So does reducing or eliminating tillage that causes losses of soil structure, biological life or organic matter. These losses make crops more vulnerable to pest damage.
Farming on newly cleared land shows the process well. Land that has been in a “cover crop” of trees or pastures for at least 10 years remains productive for row crops and vegetables for the first two to three years. High yields of agronomic and horticultural crops are profitable, with comparatively few pesticide and fertilizer inputs. After that period—under conventional systems with customary clean tillage—annual crops require higher inputs. The first several years of excessive tillage destroys the food sources and micro-niches on which the soil organisms that help suppress pests depend. When protective natural biological systems are disrupted, pests have new openings and crops are much more at risk.
Cover crop farming is different from clean-field monocropping, where perfection is rows of corn or cotton with no thought given to encouraging biological diversity. Cover crops bring more forms of life into the picture and into your management plan. By working with a more diverse range of crops, some growing at the same time in the same field, you’ve got a lot more options. Here’s a quick overview of how these systems work.
Georgia Cotton, Peanut Farmers Use Cover Crops to Control Pests
TIFTON, Ga .—Here in southwestern Georgia, I’m working with farmers who have had dramatic success creating biologically active soil in fields that have been conventionally tilled for generations. We still grow the traditional cash crops of cotton and peanuts, but with a difference. We’ve added cover crops, virtually eliminated tillage, and added new cash crops that substitute for cotton and peanuts some years to break disease cycles and allow for more biodiversity.
Our strategies include no-till planting (using modified conventional planters), permanent planting beds, controlled implement traffic, crop rotation and annual high-residue winter cover crops. We incorporate fertilizer and lime prior to the first planting of rye in the conversion year. This is usually the last tillage we plan to do on these fields for many years. Together, these practices give us significant pest management benefits within three years.
Growers are experimenting with a basic winter cover crop>summer cash crop rotation. Our cover crops are ones we know grow well here. Rye provides control of disease, weed and nematode threats. Legume crops are crimson clover, subterranean clover or cahaba vetch. They are planted with the rye or along field borders, around ponds, near irrigation lines and in other non-cropped areas as close as possible to fields to provide the food needed to support beneficials at higher populations.
When I work with area cotton and peanut growers who want to diversify their farms, we set up a program that looks like this:
- Year 1. Fall—Adjust fertility and pH according to soil test. Deep till if necessary to relieve subsurface soil compaction. Plant a cover crop of rye, crimson clover, cahaba vetch or subterranean clover.
- Spring—Strip-till rows 18 to 24 inches wide, leaving the cover crop growing between the strips. Three weeks later, plant cotton.
- Year 2. Fall—Replant cereal rye or cahaba vetch, allow crimson or subclover hard seed to germinate. Spring Strip-till cotton.
- Year 3. Fall—Plant rye. Spring Desiccate rye with herbicides. No-till plant peanuts.
- Year 4. The cycle starts again at year 1.
Vegetable farmers frequently use fall-planted cereal rye plowed down before vegetables, or crimson clover strip-tilled before planting vegetables. The crimson clover matures, drops hard seed, then dies. Most of the seed germinates in the fall. Cereal/legume mixes have not been more successful than single-crop cover crop plantings in our area.
Some vegetable farmers strip-till rows into rye in April. The strips are planted in early May to Southern peas, lima beans or snap beans. Rye in row middles will be dead or nearly dead. Rye or crimson clover can continue the rotation.
Vegetable farmers also broadcast crimson clover in early March. They desiccate the cover, strip-till rows, then plant squash in April. The clover in the row middles will set seed then die back through summer. The crimson strips will begin to regrow in the fall from the dropped seed, and fall vegetables may be planted in the tilled areas after the July squash harvest.
Insecticide and herbicide reduction begins the first year, with no applications needed by the third or fourth year in many cases.
The farmers get weed control by flail mowing herbicide-killed, fall-planted rye, leaving about 6 inches of stubble. Alternatively, they could use a roller to kill the rye crop. One or two post-emerge herbicide applications should suffice in the first few years. I don’t recommend cultivation for weed control because it increases risks of soil erosion and damages the protective outer leaf surface layer (cuticle) that helps prevent plant diseases.
We see changes on farms where the rotations stay in place for three or more years:
Insects. Insecticide costs under conservation tillage are $50 to $100/A less than conventional crop management in the area for all kinds of crops. The farmers using the alternative system often substitute with insect control materials such as Bacillus thuringiensis (Bt), pyrethroids, and insect growth regulators that have less severe environmental impact than chemical pesticides. These products are less persistent in the field environment, more targeted to specific pests and do less harm to beneficials. By planting cover crops on field edges and in other non-crop areas, these farmers are increasing the numbers of beneficials in the field environments.
Pests that are no longer a problem on the cover-cropped farms include thrips, bollworm, budworm, aphids, fall armyworm, beet armyworm and white flies. On my no-till research plots with cover crops and long rotations, I’ve not used insecticides for six years on peanuts, for eight years on cotton and for 12 years on vegetables. I’m working with growers who use cover crops and crop rotations to economically produce cucumbers, squash, peppers, eggplant, cabbage, peanuts, soybeans and cotton with only one or two applications of insecticide—sometimes with none.
- Weeds. Strip-tilling into over-wintered cover crops provides acceptable weed control for relay-cropped cucumbers (325). Conventional management of rye in our area is usually to disk or kill it with broad-spectrum herbicides such as paraquat or glyphosate. Rye can also be killed with a roller, providing an acceptable level of weed control for the subsequent cash crop.
- Diseases. I’ve been strip-tilling crimson clover since 1985 to raise tomatoes, peppers, eggplant, cucumbers, cantaloupes, lima beans, snap beans, Southern peas and cabbages. I’m using no fungicides. Our research staff has raised peanuts no-tilled into cereal rye for the past six years, also without fungicides.
- Nematodes. If we start on land where pest nematodes are not a major problem, this system keeps them from becoming a limiting factor. Even though the conventional wisdom says you can’t build organic matter in our climate and soils, we have top-inch readings of 4 percent organic matter in a field that tested 0.5 percent four years ago.
We are still learning, but know that we can rotate crops, use cover crops and cut tillage to greatly improve our sustainability. In our experience, we’ve reduced total costs by as much as $200 per acre for purchased inputs and tillage. Parts of our system will work in many places. Experiment on a small scale to look more closely at what’s really going in your soil and on your crops. As you compare insights and share information with other growers and researchers in your area, you’ll find cover crops that help you control pests, too.
—Sharad C. Phatak
In balanced ecosystems, insect pests are kept in check by their natural enemies (409). These natural pest control organisms —called beneficials in agricultural systems—include predator and parasitoid insects and diseases. Predators kill and eat other insects; parasitoids spend their larval stage inside another insect, which then dies as the invader’s larval stage ends. However, in conventional agricultural systems, synthetic chemical treatments that kill insect pests also typically kill the natural enemies of the insects. Conserving and encouraging beneficial organisms is a key to achieving sustainable pest management.
You should aim to combine strategies that make each farm field more hospitable to beneficials. Reduce pesticide use, and, when use is essential, select materials that are least harmful to beneficials. Avoid or minimize cultural practices such as tilling and burning that kill beneficials and destroy their habitat. Build up the sustenance and habitat that beneficials need. Properly managed cover crops supply moisture, physical niches and food in the form of insects, pollen, honeydew and nectar.
By including cover crops in your rotations and not spraying insecticides, beneficials often are already in place when you plant spring or summer crops. However, if you fully incorporate cover crops into the soil, you destroy or disperse most of the beneficials that were present. Conservation tillage is a better option because it leaves more of the cover crop residue on the surface. No-till planting only disturbs an area 2 to 4 inches wide, while strip-tilling disturbs an area up to about 24 inches wide between undisturbed row middles.
Cover crops left on the surface may be living, temporarily suppressed, dying or dead. In any event, their presence protects beneficials and their habitat. The farmer-helpful organisms are hungry, ready to eat the pests of cash crops that are planted into the cover-crop residue. The ultimate goal is to provide year-round food and habitat for beneficials to ensure their presence within or near primary crops.
We’re just beginning to understand the effects of cropping sequences and cover crops on beneficial and insect pest populations. Researchers have found that generalist predators, which feed on many species, may be an important biological control. During periods when pests are scarce or absent, several important generalist predators can subsist on nectar, pollen and alternative prey afforded by cover crops. This suggests you can enhance the biological control of pests by using cover crops as habitat or food for the beneficials in your area.
This strategy is important for farmers in the South, where pest pressure can be especially heavy. In south Georgia, research showed that populations of beneficial insects such as insidious flower bugs (Orius insidiosus), bigeyed bugs (Geocoris spp.) and various lady beetles (Coleoptera coccinellidae) can attain high densities in various vetches, clovers and certain cruciferous crops. These predators subsisted and reproduced on nectar, pollen, thrips and aphids, and were established before key pests arrived. Research throughout Georgia, Alabama and Mississippi showed that when summer vegetables were planted amid “dying mulches” of cool-season cover crops, some beneficial insects moved in to attack crop pests.
When crops are attacked by pests, they send chemical signals that attract beneficial insects. The beneficials move in to find their prey (420).
Maximizing natural predator-pest interaction is the primary goal of biologically based Integrated Pest Management (IPM), and cover crops can play a leading role. For example:
- Colorado potato beetles were observed at 9 a .m. attacking eggplant that had been strip-till planted into crimson clover. By noon, assassin bugs had clustered around the feeding beetles. The beneficial bugs destroyed all the beetles by evening.
- Cucumber beetles seen attacking cucumber plants were similarly destroyed by beneficials within a day.
- Lady beetles in cover crop systems help to control aphids attacking many crops.
Properly selected and managed, cover crops can enhance the soil and field environment to favor beneficials. Success depends on properly managing the cover crop species matched with the cash crops and anticipated pest threats. While we don’t yet have prescription plantings guaranteed to bring in all the needed beneficials—and only beneficials—for long lists of cash crops, we know some associations:
- We identified 13 known beneficial insects associated with cover crops during one growing season in south Georgia vegetable plantings (53, 55, 57).
- In cotton fields in south Georgia where residues are left on the surface and insecticides are not applied, more than 120 species of beneficial arthropods, spiders and ants have been observed.
- Fall-sown and spring-sown insectory mixes with 10 to 20 different cover crops work well under orchard systems. These covers provide habitat and alternative food sources for beneficial insects. This approach has been used successfully by California almond and walnut growers participating in the Biologically Intensive Orchard Systems (BIOS) project of the University of California (184).
The level of ecological sustainability depends on the grower’s interests, management skills and situation. Some use no insecticides while others have substantially reduced insecticide applications on peanut, cotton and vegetable crops.
- In Georgia, Mississippi and South Carolina, minimally tilled crimson clover or cahaba vetch before cotton planting have been successful in reducing fertilizer N up to 50 percent and insecticide inputs by 30 to 100 percent.
- Many farmers are adopting a system of transplanting tomatoes, peppers and eggplant into a killed hairy vetch or vetch/rye cover crop. Benefits include weed, insect and disease suppression, improved fruit quality and overall lower production cost.
- Leaving “remnant strips” of a cover when most of the crop is mowed or incorporated provides a continuing refuge and food source for beneficials, which might otherwise leave the area or die. This method is used in orchards when continued growth of cover crops would cause moisture competition with trees.
- Insect movement is orchestrated in a system developed by Oklahoma State University for pecan growers. As legume mixtures senesce, beneficials migrate into trees to help suppress harmful insects. Not mowing the covers from August 1 until shuck split of the developing pecans lessens the unwanted movement of stink bugs, a pest which can damage green pecans (261). In California, lygus bugs on berseem clover or alfalfa are pests of cash crops. Be careful that cover crop maturity or killing a cover doesn’t force pests into a neighboring cash crop.
Growers traditionally have been advised to turn under plant debris by moldboard plowing to minimize crop losses due to diseases (321, 322, 403, 405, 406). Now we realize that burying cover crop residues and disrupting the entire soil profile eliminates beneficial insect habitats and the benefits of weed control by crop residues. The increased use of conservation tillage increases the need to manage crop disease without burying cover crops.
In the field, although plants are exposed to a wide diversity of microorganisms, plant infection by microorganisms is rare (314). A pathogen has to cross many favor beneficial plant barriers before it can cause disease to insects. roots, stem or leaves. You can use cover crops to reinforce two of these barriers.
Plant cuticle layer. This often waxy surface layer is the first physical barrier to plant penetration. Many pathogens and all bacteria enter the plant through breaks, such as wounds, or natural openings, such as stomata, in this cuticle layer. This protective layer can be physically damaged by cultivation, manipulation, spraying and sand-blasting from wind erosion, as well as by the impact and soil splashing from raindrops and overhead irrigation. Spray adjuvants may also damage the waxes of the cuticle resulting in more disease, as with Botritys cinerea rot in grapes (356, 367). In well-developed minimum- till or no-till crop systems with cover crops, you may not need cultivation for weed control (see below) and you can minimize spraying. Organic mulches form living, dying or killed covers that hold soil, stop soil splashing and protect crops from injury to the cuticle.
Select Covers that Balance Pests, Problems of Farm
Many crops can be managed as cover crops, but only a few have been studied specifically for their pest-related benefits on cash crops and field environments.
Learn all you can about the impacts of a cover crop species to help you manage it in your situation. Here are several widely used cover crops described by their effects under conservation tillage in relation to insects, diseases, nematodes and weeds.
• Cereal Rye (Secale cereale)—This winter annual grain is perhaps the most versatile cover crop used in the continental United States. Properly managed under conservation tillage, rye has the ability to reduce soil-borne diseases, nematodes and weeds. Rye is a non-host plant for root-knot nematodes and soil-borne diseases. It produces significant biomass that smothers weeds when it is left on the surface and also controls weeds allelopathically through natural weed-suppressing compounds.
As it grows, rye provides habitat, but not food, for beneficial insects. Thus, only a small number of beneficial insects are found on rye.
Fall-planted rye works well in reducing soil- borne diseases, root-knot nematodes and broadleaf weeds in all cash crops that follow, including cotton, soybean and most vegetables. Rye will not control weedy grasses. Because it can increase numbers of cut worms and wire worms in no-till planting conditions, rye is not the most suitable cover where those worms are a problem ahead of grass crops like corn, sweet corn, sorghum or pearl millet.
• Wheat (Triticum aestivum)— a winter annual grain, wheat is widely adapted and works much like rye in controlling diseases, nematodes and broadleaf weeds. Wheat is not as effective as rye in controlling weeds because it produces less biomass and has less allelopathic effect.
• Crimson Clover (Trifolium incarnatum) —Used as a self-reseeding winter annual legume throughout the Southeast, fall-planted crimson clover supports and increases soil-borne diseases, such as the pythium-rhizoctonia complex, and root-knot nematodes. It suppresses weeds effectively by forming a thick mulch. Crimson clover supports high densities of beneficial insects by providing food and habitat. Because some cultivars produce “hard seed” that resists immediate germination, crimson clover can be managed in late spring so that it reseeds in late summer and fall.
• Subterranean Clover (Trifolium subterraneum)— a self-reseeding annual legume, fall-planted subterranean clover carries the same risks as crimson clover with soil-borne diseases and nematodes. It suppresses weeds more effectively in the deep South, however, because of its thick and low growth habit. Subclover supports a high level of beneficial insects.
• Cahaba White Vetch (Vica sativa X V. cordata)—This cool-season annual legume is a hybrid vetch that increases soilborne diseases yet suppresses root-knot nematodes. It supports beneficial insects, yet attracts very high numbers of the tarnished plant bug, a serious pest.
• Buckwheat (Fagopyrum esculentum)— a summer annual non-legume, buckwheat is very effective in suppressing weeds when planted thickly. It also supports high densities of beneficial insects. It is suitable for sequential planting around non-crop areas to provide food and habitat for beneficial insects. It is very attractive to honeybees.
A well-planned crop rotation maximizes benefits and compensates for the risks of cover crops and cash crops. Planting rye in a no-till system substantially reduces root-knot nematodes, soil-borne diseases and broadleaf weeds. By using clovers and vetches in your fields and adding beneficial habitat in non-cultivated areas, you can increase populations of beneficial insects that help to keep insects pests under control. Mixed plantings of small grains and legumes combine benefits of both while reducing their shortcomings.
As pesticides of all types (fungicides, herbicides, nematicides and insecticides) are reduced, the field environment becomes increasingly resilient in keeping pest outbreaks in check. Plantings to further increase beneficial habitat in non-cultivated areas can help maintain pollinating insects and pest predators, but should be monitored to avoid build-ups of potential pests. Researchers are only beginning to understand how to manage these “insectary plantings.”
Editor’s Note: Each cover crop listed here, except for cahaba vetch, is included in the charts and is fully described in its respective section. Check the Table of Contents (p. 4) for location. —Sharad C. Phatak
Plant surface microflora. Many benign organisms are present on the leaf and stem surface. They compete with pathogens for a limited supply of nutrients. Some of these organisms produce natural antibiotics. Epiphytic bacteria adhere to plant surfaces forming multicellular structures known as biofilms (339). These biofilms play an important role in plant disease. Pesticides, soaps, surfactants, spreaders and sticking agents can kill or disrupt these beneficial microorganisms, weakening the plant’s defenses against pathogens (356, 367). Cover crops can help this natural protection process work by reducing the need for application of synthetic crop protection materials. Further, cover crop plant surfaces can support healthy populations of beneficial microorganisms, including types of yeasts that can migrate onto a cash crop after planting or transplanting.
Soilborne pathogenic fungi limit production of vegetables and cotton in the southern U.S. (404, 405, 406, 407). Rhizoctonia solani, Pythium myriotylum, Pythium phanidermatum and Pythium irregulare are the most virulent pathogenic fungi that cause damping-off on cucumbers, snap beans, and other vegetables. Sclerotium rolfsii causes rot in all vegetables and in peanuts and cotton. Infected plants that do not die may be stunted because of lesions caused by fungi on primary or secondary roots, hypocotyls and stems, and may have reduced yields of low quality. But after two or three years in cover cropped, no-till systems, damping-off is not a serious disease, as experience on south Georgia farms and research plots shows. Increased soil organic matter levels may help in reducing plant disease incidence and severity by enhancing natural disease suppression (252, 424).
In soils with high levels of disease inoculum, however, it takes time to reduce population levels of soil pathogens using only cover crops. After tests in Maine with oats, broccoli, white lupine (Lupinus albus) and field peas (Pisum sativum) researchers cautioned it may take three to five years to effectively reduce stem lesion losses on potatoes caused by R. solani (240). Yet there are single-season improvements, too. For example, in an Idaho study, Verticillium wilt of potato was reduced by 24 to 29 percent following sudangrass green manure. Yield of U.S. No.1 potatoes increased by 24 to 38 percent compared with potatoes following barley or fallow (394).
Nematodes are minute roundworms that interact directly and indirectly with plants. Some species feed on roots and weaker plants, and also introduce disease through feeding wounds. Most nematodes are not plant parasites, but feed on and interact with many soil-borne microorganisms, including fungi, bacteria and protozoa. Damage to the crop from plant-parasitic nematodes results in a breakdown of plant tissue, such as lesions or yellow foliage; retarded growth of cells, seen as stunted growth or shoots; or excessive growth such as root galls, swollen root tips or unnatural root branching.
If the community of nematodes contains diverse species, no single species will dominate.
This coexistence would be the case in the undisturbed field or woodland described above.
In conventional crop systems, pest nematodes have abundant food and the soil environment is conducive to their growth. This can lead to rapid expansion of plant parasitic species, plant disease and yield loss. Cropping systems that increase biological diversity over time usually prevent the onset of nematode problems. Reasons may include a dynamic soil ecological balance and improved, healthier soil structure with higher organic matter (5, 245, 424). In Michigan, to limit nematodes between potato crops, some potato growers report that two years of radish improves potato production and lowers pest control costs (270, 271).
Once a nematode species is established in a field, it is usually impossible to eliminate it. Some covers can enhance a resident parasitic nematode population if they are grown before or after another crop that hosts a plant-damaging nematode species.
If a nematode pest species is absent from the soil, planting a susceptible cover crop will not give rise to a problem, assuming the species is not introduced on seed, transplants or machinery (357). Iowa farmer Dick Thompson reports that researchers analyzing his fields found no evidence that hairy vetch, a host for soybean cyst nematode, caused any problem with the pest in his soybeans. This may be due to his use of compost in strip-cropped fields with an oats/hairy vetch>corn>soybean rotation.
You can gradually reduce a field’s nematode pest population or limit nematode impact on crops by using specific cover crops. Nematode control tactics involving covers include:
- Manipulating soil structure or soil humus
- Rotating with non-host crops
- Using crops with nematicidal effects, such as brassicas
Cover crops may also improve overall plant vitality to lessen the nematode impact on yield. But if you suspect nematode trouble, send a soil sample for laboratory analysis to positively identify the nematode species. Then be sure any cover crops you try aren’t alternate hosts for that pest species. Area IPM specialists can help you.
Using brassicas and many grasses as cover crops can help you manage nematodes. Cover crops with documented nematicidal properties against at least one nematode species include sorghum-sudangrass hybrids (Sorghum bicolor X S. bicolor var. sudanese), marigold (Tagetes patula), hairy indigo (Indigofera hirsuta), showy crotalaria (Crotolaria spectabilis), sunn hemp (Crotalaria juncea), velvetbean (Mucuna deeringiana), rapeseed (Brassica rapa), mustards and radish (Raphanus satiuus).
You must match specific cover crop species with the particular nematode pest species, then manage it correctly. For example, cereal rye residue left on the surface or incorporated to a depth of several inches suppressed Columbia lance nematodes in North Carolina cotton fields better than if the cover was buried more deeply by moldboard plowing. Associated greenhouse tests in the study showed that incorporated rye was effective against root-knot, reniform and stubby root nematodes, as well (20).
Malt barley, corn, radishes and mustard sometimes worked as well as the standard nematicide to control sugar beet nematode in Wyoming sugar beets, a 1994 study showed. Increased production more than offset the cover crop cost, and lamb grazing of the brassicas increased profit without diminishing nematode suppression. The success is conditional upon a limited nematode density. The cover crop treatment was effective only if there were fewer than 10 eggs or juveniles per cubic centimeter of soil. A moderate sugar beet nematode level was reduced 54 to 75 percent in about 11 weeks, increasing yield by nearly 4 tons per acre (231).
Cover crops are widely used as smother crops to shade and out-compete weeds (412). Cereal grains establish quickly as they use up the moisture, fertility and light that weeds need to survive. Sorghum-sudangrass hybrids and buckwheat are warm-season crops that suppress weeds through these physical means and by plant-produced natural herbicides (allelopathy).
Cereal rye is an overwintering crop that suppresses weeds both physically and chemically. If rye residue is left on the soil surface, it releases allelo chemicals that inhibit seedling growth of many annual small-seeded broadleaf weeds, such as pigweed and lambsquarters. The response of grassy weeds to rye is more variable. Rye is a major component in the killed organic mulches used in no-till vegetable transplanting systems.
Killed cover crop mulches last longer if the stalks are left intact, providing weed control well into the season for summer vegetables. Two implements have been modified specifically to enhance weed suppression by cover crops. The undercutter uses a wide blade to slice just under the surface of raised beds, severing cover crop plants from their root mass. An attached rolling harrow increases effectiveness (95, 96, 97). A Buffalo rolling stalk chopper does no direct tillage, but aggressively bends and cuts crops at the surface (303). Both tools work well on most legumes when they are in mid-bloom stage or beyond.
Killed mulch of a cover crop mix of rye, hairy vetch, crimson clover and barley kept processing tomatoes nearly weed-free for six weeks in an Ohio test. This length of time is significant, because other research has shown that tomato fields kept weed-free for 36 days yield as much as fields kept weed-free all season (97, 150). The roller is another method used to terminate the cover crop (13). The roller flattens and crimps the cover crop, forming a flat mat of cover crop residue that effectively control weeds.
Cover crops can also serve as a “living mulch” to manage weeds in vegetable production. Cover crops are left to grow between rows of the cash crop to suppress weeds by blocking light and outcompeting weeds for nutrients and water. They may also provide organic matter, nitrogen (if legumes) and other nutrients mined from underneath the soil surface, beneficial insect habitat, erosion prevention, wind protection and a tough sod to support field traffic.
To avoid competition with the cash crop, living mulches can be chemically or mechanically suppressed. In the Southeast, some cool-season cover crops such as crimson clover die out naturally during summer crop growth and do not compete for water or nutrients. However, cover crops that regrow during spring and summer—such as subterranean clover, white clover and red clover— can compete strongly for water with spring- planted crops unless the covers are adequately suppressed.
In New York, growing cover crops overseeded within three weeks of potato planting provided good weed suppression, using 70 percent less herbicide. Yield was the same as, or moderately reduced from, the standard herbicide control plots in the two-year study. Hairy vetch, woollypod vetch, oats, barley, red clover and an oats/ hairy vetch mix were suppressed as needed with fluazifop and metribuzin (341).
Cover crops often suppress weeds early, then prevent erosion or supply fertility later in the season. For example, shade-tolerant legumes such as red clover or sweetclover that are planted with spring grains grow rapidly after grain harvest to prevent weeds from dominating fields in late summer. Overseeding annual ryegrass or oats at soybean leaf yellowing provides a weed-suppressing cover crop before frost and a light mulch to suppress winter annuals, as well.
Healthy soils grow healthy weeds as well as healthy crops, making it difficult to manage weeds in conservation tillage without herbicides. Long term strategies for weed management should include:
- Reducing the weed seed bank
- Preventing weeds from going to seed
- Cleaning equipment before moving to different fields and farms
- Planting cover crops to help manage weeds in conservation tillage
Cover crops can play a pest-suppressing role on virtually any farm. As we find out more about the pest management benefits of cover crop systems, they will become even more attractive from both an economic and an environmental perspective. Traditional research will identify some new pieces of these biologically based systems. However, growers who understand how all the elements of their farm fit together will be the people who will really bring cover crops into the prominence they deserve in sustainable farming.