COVER CROPS AND GREEN MANURES
by Vern Grubinger
Vegetable and Berry Specialist
University of Vermont Extension

Cover crops and green manures are grown primarily for reasons other than short term economic gain. In other words, they are not produced for sale, but rather for the benefits they provide to the production of subsequent cash crops. Cover crops are so-called because they protect otherwise bare soil against erosion; green manures improve soil fertility. Because a cover crop is inevitably added to the soil, it becomes a green manure, so the terms are reasonably interchangeable.

 Potential benefits of cover crops are numerous:
1) Reducing the impact of wind and water passing over the soil surface can reduce erosion.
2) Adding organic matter to soil improves its physical condition, or structure.
3) Competing for light, water and nutrients may suppress weeds.
4) Legume cover crops add "free" symbiotically-fixed nitrogen to the farming system.
5) Crops growing late in the season can capture and "recycle" soluble nutrients otherwise lost.
6) Providing cropping system diversity may create habitats for beneficial insects.

Since a single cover crop planting cannot effectively provide all of the potential benefits listed above, the grower must prioritize the desired benefits before deciding what to plant. For example, only legume cover crops will provide free nitrogen, but they are not very competitive with weeds. For the field in question, what are the nitrogen needs for the subsequent crop, and how heavy is the weed pressure? The identification of a cover crop priority, combined with a cover cropping strategy that fits a rotational plan with cash crops will greatly narrow down the choices of possible cover crop species. In addition, other factors to consider include: seed cost, winter hardiness (if applicable), and suitability to soil conditions, tillage equipment and the crop to follow.

Cover crop strategies refer to how the cover crop fits into a crop rotation plan. There are 4 general strategies for the use of cover crops:

1) Fallow crops that require taking land out of cash crop production for all or part of a season.
2) Winter cover crops that are sown in late summer or autumn and remain in place until spring.
3) Smother crops that are grown during a spring, summer or fall window between cash crops.
4) Interseeded cover crops that may remain in place for varying amounts of time.

A fundamental goal of cover cropping is to avoid bare soil between cash crop plantings. This not only protects soil, but captures sunlight and produces biomass that enhances soil quality. Numerous side benefits accrue from this approach as well, such as improved trafficabilty of fields and reduced compaction, enhance aesthetics, and potential for animal feed production.

Fallow cover crops provide the best rotation benefit in terms of resting the land from cultivation, avoiding crop families likely to host vegetable diseases, and adding biomass to the soil to help maintain its structure and the supply of organic carbon and nitrogen. However, fallow crops can be the most costly in terms of lost production because they are grown in lieu of a cash crop. The cost of land, the pressure to crop your land, and the need for resting land must all be considered in deciding whether to fallow cover crop.

Certainly, an intentional fallow cover is far better than letting weeds take over and/or leaving some soil bare and exposed to erosion. Fallows can be accomplished with a year-long cycle of cover crop planting and incorporation, such as field pea-buckwheat-winter rye, or a single sowing, such as red clover plus an annual grass like oats or a perennial grass like orchard grass, that might be left in place for a year or two.

Winter cover crops are usually sown after the cash crop is harvested and residues are plowed or disked in. The most popular choice for a winter cover crop after vegetable crops is winter rye because it can put on vegetative growth as long as the temperature is above 38 degrees F. Oats or other small grains can also be sown late, although they can't match rye's ability to put on growth in cold weather. These grasses are good at mopping-up available nitrogen that may be left over from fertilizer, manure applications, or mineralization of soil organic matter. Besides ability to grow at low temperatures, ability to overwinter is a major consideration. Rye will reliably grow the following spring, while oats will reliably winter-kill in most of New England. There are advantages and drawbacks to either scenario, the main issue being the amount of residue that will have t be dealt with come spring.

Interseeding, or undersowing a cover crop into a standing cash crop, is a way get a jump on the traditional winter cover crop season. This can lead to an increase in cover crop biomass production, and presumably, better erosion control and soil organic matter enhancement because the cover crop is in place longer. Earlier establishment also increases your choices of cover crops compared to waiting to sow a cover crop until after a full-season vegetable crop.

In the case of a very late vegetable harvest, or just not getting around to the job, it may even be too late for winter rye to put on enough growth to protect the soil over winter. That's where interseeding can be an advantage by assuring that some winter protection gets established and the soil is not left bare.

To successfully interseed cover crops, the timing of the two crops must be right. Sowing must be delayed enough to minimize competition with the vegetable crop, but early enough so the cover crop can survive competition with the vegetable and then withstand the harvest traffic.
The best timing depends on the vegetable - cover crop combination, and of course your location. On your own farm, it's important to start small as you experiment with a new cropping system.

Small, slow-growing vegetable crops are poor candidates for interseeding. Carrots, onions and the like will suffer from the competition. Vigorous vegetables, like winter squash and sweet corn, are much more suitable. Interseeding requires good soil-seed contact, sufficient irrigation or rainfall, and decent weed control early in the season so that the cover crop has a chance to establish well. Right after last cultivation is a good time to interseed a cover crop - there's a nice seed bed for the cover crop, weeds have just been beaten back, and the vegetable canopy is not yet fully closed, so there's still some light available to help get the cover crop going.

With a crop like winter squash, interseeded cover crops should be sown just before the vines were starting to run, after cultivation has left a clean swath of soil between the crop rows.  Since this would be early in the summer, the timing isn't right for winter rye, oats, or other crops that will get too tall by harvest. Low-growing red clover, perennial ryegrass or annual ryegrass would be better choices. High seeding rates will help assure establishment.

With crops like peppers, tomatoes, cabbage or kale, last cultivation usually occurs in August, depending on spacing and cultural practices (like staking). By mid- to late-August, the option is open to sow winter cover crops like hairy vetch and rye, rye alone, or oats. These can be broadcast into the crop mechanically or hand-scattered in the rows. Some growers have fabricated custom-seeders for dropping seed between rows and cultipacking the soil.

A concern with interseeding is the possibility of disease enhancement, since crop residues are not plowed under at the end of the growing season. Hence, interseedings are not advisable if there are serious disease problems in the crop. It would then be better to disk the field and try an get winter rye on. Another concern is yield reductions due to competition. In dry years, this could really be a problem. Interseedings are pretty risky unless irrigation is available.

Some growers have tried interseeding with crops like cabbage and found that the broadcast seeds get caught in the leaves, sprout, and make a mess. Hank Bissell of Starksboro, Vermont has found an innovative way to overcome this. He set up his crew with 5 gallon buckets of cover crop seed, and they race down the rows hand-broadcasting seed under the leaves of the crucifers. In smaller fields, it doesn't take that long. Innovative interseeders, unite!

Smother crops are grown to suppress weeds. Because they must put on a lot of growth rapidly to do the job, they also tend to be good producers of organic residues that can help maintain soil organic matter levels. Smother crop species must be timed to the season in which they make most rapid growth - and as such can be divided into cool season or warm season crops. Weather conditions and cultural practices that encourage good stands and rapid early growth of smother crops are essential if they are to get a jump on weeds and effectively out-compete them.

Traditionally, smother crops are warm season summer crops that grow when weeds, especially broadleaf species, are also most plentiful. Summer is also the time when the stored food reserves of perennial weeds are lowest, making them most susceptible to suppression by shading. Summer smother species include buckwheat, Japanese millet, and sorghum Sudan-grass. These crops require soil to be fully warmed in order to establish well.

Cool season crops like oats, rye, vetch, or combinations can also smother weeds, besides protecting and improving soil. The key is quick and thick establishment, which is obtained with proper timing, seeding method that distribute seed evenly and cover it properly, and high seeding rates. Cool season smother crops may be die back as temperatures rise and thus be ineffective at smothering warmer-season broadleaf weeds, but if enough residue remains the "dead mulch" can sometimes provide good weed control.

Certified seed may increase the cost of cover cropping, but the investment is often worthwhile. Uncertified seed is more likely to be variable in performance and to contain noxious weed seeds that may be a problem for years to come.

Cover crop species fit into 4 main categories: grasses, other non-legumes, legumes, and mixtures.

Grasses are generally selected when nitrogen contribution to the soil is not a priority. They tend to grow rapidly and thus are better at short-term weed suppression than are legumes, which establish slowly.

Winter Rye is a common winter cover crop, sown after cash crops are harvested in the fall. It is very hardy, adapted to a wide range of conditions, and seed is inexpensive. The latest-sown cover crop, it can be planted from late summer to mid-October in most areas. Rye produces a lot of biomass in the spring. This helps maintain soil organic matter levels, but rank growth can be difficult to incorporate. Partial incorporation may leave rye as a weed. Sow 2 bu/acre (112 lb).

Oats as a winter cover crop can protect the soil without requiring intensive management in the spring, because they are frost-killed. Shallow incorporation of residues may still be necessary before crop planting.  Enough growth is needed before first frost to adequately protect the soil, so plant by late August, at a rate of about 100 lb/acre. Oat residues left on the soil surface may chemically suppress weed growth, in addition to acting as a physical barrier. Oats may also be used as a spring cover, usually in mixture with cool season legume(s) such as Canada field pea and/or hairy vetch.

Other small grains such winter wheat, barley, and triticale have been successfully used as winter and early spring cover crops. Some growers find them easier to incorporate than winter rye because they are less vigorous. All the small grains provide good rotation crops with vegetable since they tend not to be hosts of vegetable diseases. In addition, mature grain, can be combined off for a cash crop and the straw and stubble are an excellent soil conditioner.

Ryegrass is a cover crop that produces an extensive root system good at capturing left-over nitrogen late in the season after cash crops have been harvested.  It is well-suited to undersowing, after last cultivation of a vigorous vegetable like sweet, squash, or many crucifers, in order to establish a winter cover prior to harvest. Annual, or Italian ryegrass is less expensive than perennial ryegrass, and is more likely to winter-kill; however, it may overwinter in milder areas, and perennial ryegrass may winter-kill in harsher zones. These crops form a dense sod that reduces erosion. Sow from mid-summer to September at 30 to 40 lb/acre.

Sudangrass and sorghum-Sudangrass are fast-growing, warm season crops that require good fertility and moisture to perform well. Under such conditions, their tall, rank growth provides excellent weed suppression. Such heavy growth can be difficult to cut and incorporate. Mid-season mowing allows for regrowth before the crop winter-kills. Sow from late spring to late summer at 30-40 lb/acre.

Japanese millet is also a warm season crops that puts on rapid growth if planted after early June. Not as tall or coarse as sorghum-Sudangrass, it can be easier to incorporate come fall or the following spring, when a heavy disking of the dead stalks can sometimes be sufficient to prepare a seed bed. Seeding rates of 30-40 lb/acre are suggested.

Other non-legumes include buckwheat, and cruciferous cover crops.

Buckwheat is a fast-growing summer annual that can be used to protect the soil and suppress weeds in-between spring and fall cash crops. It has a reputation for growing well on acid and low phosphorus soils, but these traits are not common on vegetable farms. It decomposes rapidly, so is easy to incorporate, but does not contribute a lot of organic matter to the soil. Seed matures in 75 to 90 days, and the crop can be disked after re-seeding, mowed early and allowed to regrow, or incorporated at flowering, prior to setting seed. Sow in late spring to mid-summer at 60-70 lb/acre. To smother weedy fields, some growers plant a fallow cycle of two successive crops of buckwheat followed by winter rye.

Forage brassicas include the varieties of several species of  mustard-family crops that have been developed primarily for animal feed, including rape, turnip, and oilseed radish. These cool-season crops may help break-up compacted soils or pans with their tap roots, and their value as a forage may provide a dual use. However, given their susceptibility to diseases and pests common to cruciferous vegetables, they are not ideal candidates for cover cropping in terms of a whole-farm crop rotation plan.

Legumes are sown when "free" nitrogen is desired for a subsequent cash crop with a high nitrogen demand. Legumes generally require good drainage and fertility. Most grow slowly at first so they do not compete much with weeds until well-established. Seed should be drilled for best stands, inoculation with the proper Rhizobium strain helps assure nodule formation and good nitrogen fixation.

Sowing with a nurse crop such as oats, or in mixes with perennial grasses is common. Beware that legumes provide good habitat for tarnished plant bug, a major pest of many vegetable and berries, and after mowing or plowing, these may move into cash crops.

Red Clover is short-lived perennial that is somewhat tolerant of soil acidity or poor drainage. Mammoth red clover produces more biomass for plow-down than medium red clover, but does not regrow as well after mowing. Mammoth will often establish better than medium in dry or acid soils. Sow in early spring or late summer. Can be undersown in mid-summer into corn, winter squash before it vines, and other crops if soil moisture is plentiful. Sow at 10-15 lb/acre.

White Clover is a low-growing perennial, tolerant of shade and slightly acid soil. Ladino types are taller than the white, wild white, or Dutch types, which are all terms for the low-growing strains. The clovers do not compete well with weeds unless mowed, in which case their ability to grow laterally and low to the ground gives them an advantage. Thus, they are suitable for use in mowed walkways or alleys. Seed tends to be expensive, although stands can last for many years, especially if mowed or grazed, since the lateral-growing stolons continue to root. Sow in spring or fall at 10-12 lb/acre.

Sweet clover is a biennial crop, except for the annual types called Hubam. It is deep-rooted and adapted to a wide range of soils and thus is a good soil-improving crop, as a provider of free N and "biological subsoiling". Yellow Sweet clover is earlier maturing and somewhat less productive than white Sweet clover. Sow in early spring or late summer at 15-20 lb/acre. Heavy growth is produced in spring after overwintering. The tall, lush growth may be difficult to incorporate without proper equipment. This should be done in late spring, or by mid-summer at flowering since growth will cease after that. Deep rooted crops may deplete soil of moisture, which can be a problem for subsequent crops in dry years.

Hairy vetch is becoming increasingly popular as a hardy, winter annual cover crop. It is adapted to many soil types, and when properly established, has performed well throughout the northeast To ensure winter survival, it must be sown in late August to early September in most areas. Sow at 30 to 40 lb/acre, or mix 25 to 30 lb/acre with 1 bu/acre of winter rye or oats. The combination is recommended to assure good soil cover over the winter, as vetch puts on little growth in the autumn. Using rye allows the vetch to climb in the spring, which can reduce matting and facilitate cutting before incorporation if desired. Close mowing at flowering will also kill the vetch and leave a weed-smothering residue. Vetch can be spring-planted and used as a fallow, as it will provide good cover through late summer. Use "pea" type inoculant.

Alfalfa requires deep, well-drained soil with a pH near neutral for good growth. It is a long-lived perennial that is probably not worth the expense in a short-term rotation. Fixes large amounts of nitrogen if maintained for several years. Seed early spring or late summer at 15-25 lb/acre. Attracts tarnished plant bug, which can damage nearby crops when alfalfa is mowed.

Canada field pea is a cool-season, frost tolerant  crop used to provide spring or fall nitrogen fixation and ground cover, usually in combination with a grass nurse crop such as oats or triticale (a rye/wheat hybrid). Plant as early as possible in the spring, on well-limed, moist soils, at a rate of about 100-200 lb/acre. Soil-seed contact is important, so cover seeds well after broadcasting or drill several inches deep. Incorporate at flowering, 5-8 weeks after planting.

Cowpea is a warm-season, traditionally southern cash crop that can be grown as a summer cover crop that provides nitrogen fixation and some weed suppression during the warmest summer months in New England. Cowpeas are very frost sensitive. Forage cultivars may be better cover crops as they produce more biomass than horticultural varieties. Drill at 30 or 40 lb/acre, use rates twice as high if broadcasting. Good soil seed contact and well-drained soils are needed to establish strong stands. Nitrogen fixation rates can be substantial. Use cowpea/peanut type inoculant.

Soybeans are sensitive to frost and drought, but when grown as a cover crop, late-season effect of drought on pod fill is not of concern. Short-season varieties that set seed early are probably not best for extended cover cropping, since foliage begins to decline once pods are near maturity. Care should be taken to avoid damaging seed when handling, and planting must place seed deep enough and firmly enough into soil to assure that seed is in contact with enough moisture to result in good germination. Inoculate with soybean type Rhizobium.

References:

Managing Cover Crops Profitably, second edition. Sustainable Agriculture Network Publications, Hills  Building, UVM, Burlington, VT

Sarrantonio, M. 1994. Northeast Cover Crop Handbook, Rodale Institute, Emmaus, PA

Schmid, O. and R. Klay. 1984. Green Manuring, Principles and Practice (translated by Will  Brinton) Woods End Agricultural Institute, Mt. Vernon, ME

Schonbeck, M. 1988. Cover Cropping and Green Manuring on Small Farms in New England and  New York: An Informal Survey, New Alchemy Institute, E. Falmouth, MA

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