Growing Small Farms: Cover Crops Research Review

Below is a review of recent cover crop research conducted in vegetable crops. Only those studies deemed most relevant and practical for farmers were included. A brief summary of the findings is given along with a reference. Please refer to the references page for the complete citation. Browse this page and then proceed to the section on Selecting Cover Crops.

Bell Pepper
A California study examined the rate of nitrogen release from a lana vetch cover crop in a pepper production system. Some plots were additionally treated with an organic fertilizer (feather meal). Yields were similar in all of the fertilizer treatments, but this may have been due to a disease outbreak late in the seaosn. The study showed that although the vetch provided significant amounts of nitrogen to the pepper, the long-season demands of a crop such as pepper may require supplemental applications of fertilizer later in the season for optimal yield.

Broccoli
A fall experiment performed in Maryland and Virginia (Abdul-Baki et al., 1997) investigated the effects of two cover crops in a no-till system on weed supression and broccoli yield. The cover cops included forage soybean (Glycine max), foxtail millet (Setaria italica), and a combination of the two. All treatments except the millet alone produced yields similar to the control plots that used a conventional till system. Weed populations were not high enough to reduce yields in any of the treatments.

A California study (Wyland et al., 1996) looked at the impacts of winter cover crops on nitrate leaching, soil water, pest populations, crop yield, and management costs in a broccoli production system. Merced rye (Secale cereale cv. Merced) and phacelia (Phacelia tanacetifolia cv. Phaci) were planted in November and incorporated the following March using reduced tillage techniques. The cover crops caused a 65-70% reduction in nitrate leaching. The phacelia cover crop caused increased broccoli yields and the economic analysis showed that winter cover crops could provide a favorable advantage.

Broccoli yield and weed supression under different tillage and weed control systems were examined in Virginia (Infante and Morse, 1996). Conventional and no-till systems were compared. Weed control treatments were compared under both tillage systems and included a control, a preemergent herbicide, and red clover (Trifolium pratense), white clover (Trifolium repens), and hairy vetch (Vicia villosa) cover crops. Broccoli yield and weed supression were equal to or higher under the no-till systems than the conventional systems. Broccoli yield was not affected by the cover crop treatments. The cover crops provided similar weed control as the conventional plots in three out of four sites.

Cabbage
An Illinois study (Mwaja et al., 1996) investigated the effects of rye (Secale cereale) and hairy vetch (Vicia villosa) cover crops on cabbage production. Two cover crop killing methods were used: an application of glyphosate + mowing, or mowing + disking. The conventional control plot used a pre-plant incorporated herbicide (Trifluralin) and conventional tillage. Cabbage yields were higher in the conventional system. Insect infestations were higher in the conventional system than in the glyphosate cover crop system. Three years of the cover crop + glyphosate system increased soil organic matter from 3.07-3.48%.

Celery
A California study addressed the use of cover crops to promote beneficial insect populations and control leafminers in celery. The cover crop used was sweet alyssum. The cover crop alone did not provide adequate pest control. However, the use of cover crops as part of an integrated system of pest management should be explored further.

Cowpea
A Florida study (McSorley, 1994) addressed the issue of using cover crops for nematode management. Cover crops included wheat (Triticum aestivum), rye (Secale cereale), lupine (Lupinus augustifolius and L. angustifolius), crimson clover (Trifolium incarnatum), and hairy vetch (Vicia villosa). Root-knot nematode population densities were low following rye and wheat cover crops, but high after the legumes. Ring nematode population densities were highest after wheat and lowest after lupine. Root-knot nemetode is a more serious agricultural pest than ring nematode. Maximum cowpea yields in sites infested with root-knot nematodes were achieved following rye cover crops combined with the use of nematode-resistant cowpea cultivars.

Eggplant
A Florida study (McSorley and Dickson, 1995) examined the effects of tropical rotation crops on plant-parasitic nematodes in eggplant. Tropical rotation crops included castor (Ricinus communis), American jointvetch (Aeschynomene americana), velvetbean (Mucuna deeringina), cowpea (Vigna unguiculata), cotton (Gossypium hirsutum), sorghum-sudangrass (Sorghum bicolor x S. sudanese), okra (Hibiscus esculentus), and soybean (Glycine max). All the rotation crops but the okra and soybean were effective in maintaining low levels of the root-knot nematode Meloidogyne incognita . The nematode populations remained low following a winter cover crop of rye (Secale cereale) but increased as the eggplant matured.

Lettuce
A California study (Jackson et al., 1993) examined the use of winter cover crops to minimize nitrate losses in lettuce production. Cover crops included rye (Secale cereale cv. Merced), annual ryegrass (Lolium multiflorum), white mustard (Brassica alba), oilseed radish (Raphanus sativus cv. Renova), white senf mustard (Brassica hirta cv. Martigena), and phacelia (Phacelia tanacetifolia cv. Phaci). The cover crops were incorporated directly on the lettuce beds before the lettuce was direct seeded. The study showed that soil nitrate concentrations were reduced by the cover crops during a short winter fallow period. Lettuce yield was unaffected by the cover crops.

Another California study addressed the use of cover crops to promote beneficial insect populations and control aphids in lettuce. The cover crop used was sweet alyssum. The cover crop alone did not provide adequate pest control. However, the use of cover crops as part of an integrated system of pest management should be explored further.

Muskmelon
A Kansas study (Singogo et al., 1996) found that fall-planted cover crops produced good high-quality muskmelon yields. The cover crops included alfafa (Medicago sativa), hairy vetch (Vicia villosa), Austrian winter pea (Pisum sativum subsp. arvense), and winter wheat (Triticum aestivum). Muskmelons were grown on plastic mulch and drip irrigation with either a cover crop, a cover crop + cow manure, or synthetic nitrogen fertilizer (at 70 or 100 kg/ha). The melon yields produced from the legume cover crop plots were similar to the plots from both synthetic fertilizer rates. The additional manure did not have an effect on melon yield.

Snap Bean
An Illinois study (Mwaja et al., 1996) investigated the effects of rye (Secale cereale) and hairy vetch (Vicia villosa) cover crops on snap bean production. Two cover crop killing methods were used: an application of glyphosate + mowing, or mowing + disking. The conventional control plot used a pre-plant incorporated herbicide (Trifluralin) and conventional tillage. Snap bean yields were higher in the conventional system. Three years of the cover crop + glyphosate system increased soil organic matter from 3.07-3.48%.

Sweet Corn
An experiment performed in New Mexico (Guldan et al., 1996) examined the effect of timing of cover crop interseeding on sweet corn yield and cover crop biomass and nitrogen content. Five leguminous cover crops were included: hairy vetch (Vicia villosa), barrel medic (Medicago truncatula), alfafa (Medicago sativa), black lentil (Lens culinaris), and red clover (Trifolium pratense). The legumes were interseeded at last cultivation at the V9 (early) or blister (late) stage. Timing of seeding had no effect on corn yield. Legume biomass and nitrogen content were higher in the early seeded treatment. Barrel medic produced the highest biomass and nitrogen content of the five crops.

Weed control in a no-till sweet corn system with cover crops was examined in an Arkansas study (Burgos and Talbert, 1996). Cover crops were used alone or in combination with herbicides. Cover crops included hairy vetch (Vicia villosa), rye (Secale cereale), wheat (Triticum aestivum), and hairy vetch (Vicia villosa) + rye (Secale cereale). Certain herbicides performed better in the no cover crop plots. None of the cover crops suppressed yellow nutsedge. All the cover cops except hairy vetch reduced sweet corn yield. Another study (Ilnicki and Enache, 1992) found that a subterranean clover (Trifolium subterraneum) living mulch provided excellent weed control and did not reduce corn yields.

A Maine study (Dyck and Liebman, 1994) investigated the effects of crimson clover (Trifolium incarnatum) mulch, synthetic nitrogen fertilizer, and their interaction on weed control in sweet corn. The clover residue suppressed lambsquarters (Chenopodim album) emergence by 27%, while the nitrogen fertilizer increased lambsquarters emergence by 75%. Sweet corn biomass was not affected by the clover residues.

Tomato
A study conducted in Illinois, Indiana, and Kentucky (Masiunas et al., 1995) determined the impact of rye cover crops on weed populations in tomato. The experiment compared method of tillage (conventional vs. reduced-till), rye seeding density (56, 110, and 170 kg/ha) and method of kill (glyphosate + mowing vs. mowing alone). The control plots used herbicides and conventional till for weed control. The rye residue was left on the soil as a mulch before the tomatoes were transplanted. Rye seeding density had no effect on rye biomass, weed control, or tomato yield. Although supplemental weed management was necessary in all treatments, in most cases tomato yields in the reduced till + glyphosate plots were equal to or better than the yields in the conventional till plots.

An Indiana study (Smeda and Weller, 1996) also investigated the effects of rye cover crops in transplant tomato production. Rye was killed either by mowing or glyphosate at various times prior to planting. Time of kill had an effect on weed suppression. In one year, rye killed 4 weeks before tomato planting did not effectively control weeds 5 weeks after planting, while rye killed 2 weeks before planting provided up to 97% weed control 5 weeks after tomato planting. Clippings from mowed rye placed on tomato beds provided up to 60% weed control 8 weeks after planting. Tomato yields were not reduced by rye residues.

A Maryland study (Abdul-Baki et al., 1996) looked at the impact of winter annual cover crop mulches on fresh-market tomato production. Tomato yields under hairy vetch (Vicia villosa), crimson clover (Trifolium incarnatum), and a mixture of rye (Secale cereale) plus hairy vetch mulches were compared to tomatoes grown with black plastic mulch. In two of three years, the cover crop mulches produced higher tomato yields and heavier fruits than the plastic mulch. A similar, earlier study (Teasdale and Abdul-Baki, 1995) compared soil temperature and tomato growth and yield under plastic vs. hairy vetch (Vicia villosa) mulch. Maximum soil temperatures were higher in the plastic mulch treatments and these produced greater early tomato root and shoot growth. However, tomato yields in the cover crop plots were equal to or higher than the plastic mulch plots.

Yet another study led by Abdul-Baki (Abdul-Baki and Teasdale, 1993) examined the effects of using winter annual cover crop mulches in a no-till tomato production system in Maryland. Five mulch treatments were used: hairy vetch (Vicia villosa), subterranean clover (Trifolium subterraneum), paper mulch, black plastic, and no mulch. The cover crops were sown into the beds in the fall and then flail-mowed in the spring immediately before transplanting the tomatoes in May. The tomatoes were planted with minimal disturbance of the soil or mulch cover. The vetch mulch produced the highest tomato yield of all the treatments. The subterranean clover mulch produced lower tomato yields than the conventional system or vetch system but outproduced the paper and no-mulch systems.

Four tomato production systems differing in cover crop and chemical inputs were compared at two sites in an Ohio study (Creamer et al., 1996): a conventional system, an integrated system utilizing cover crop mixtures and reduced chemical inputs, an organic system with cover crops and no chemicals, and a no-input system with cover crops and no additional management or inputs. The cover crop mixture included hairy vetch (Vicia villosa), crimson clover (Trifolium incarnatum), rye (Secale cereale), and barley (Hordeum vulgare). The cover crop treatments provided the same level of weed control as the conventional treatments. Fruit yield was similar for all the systems at one site but higher in the conventional system at the other site.

An Illinois study (Mwaja et al., 1996) investigated the effects of rye (Secale cereale) and hairy vetch (Vicia villosa) cover crops on tomato production. Two cover crop killing methods were used: an application of glyphosate + mowing, or mowing + disking. The conventional control plot used a pre-plant incorporated herbicide (Trifluralin) and conventional tillage. Tomato yields in the cover crop systems were similar to the conventional system in two out of three years. Three years of the cover crop + glyphosate system increased soil organic matter from 3.07-3.48%.

A California study evaluated the effectiveness of a summer sudangrass (Sorghum vulgare var. sudanese) cover crop in controlling Verticillium wilt in tomatoes. Other treatments included a control and plots treated with metham. In one of two trials, tomato yield increase 20% in the cover crop plots. The cover crop did not reduce verticillium incidence.