Weed suppression is one of the important contributions cover crops can provide for cropping systems. Two types of cover crops will be discussed: winter annual cover crops that typically provide a mulch of cover crop residue after being killed when the summer crop is planted and living mulches that grow during part or all of the crop growing season. Winter annual cover crops have been most successfully incorporated into crop rotations and are most widely used to date. Living mulches often compete excessively with the crop and have received attention mostly from researchers to date.

Weed Control by Cover Crop Residue

The degree of weed control provided by cover crops can vary according to cover crop species, residue quantity, and weed species.

Research has demonstrated that:

1. Weed suppression by cover crop residue increases with increasing residue quantity – natural levels of typical cover crop residues can be expected to reduce weed emergence by 75 to 90%
2. Weed suppression will decline during the course of the season according to the rate of residue decomposition
3. Residues with a large number of layers and small amount of empty internal space will be most suppressive.
4. Annual species that are small-seeded and have a light requirement for germination such as common lambsquarters and pigweeds are sensitive to surface residue, whereas, large-seeded annuals and perennial weeds are relatively insensitive.

Practical application of these results suggests that best weed control can be obtained by:

1. using cover crops that produce high amounts of biomass
2. using cover crops that do not decompose rapidly
3. using cover crop management implements that pack or compress the mulch
4. avoid fields with high populations of perennial or large-seeded annual weeds

Generally, cover crop residue can be expected to provide early-season weed suppression but not full-season weed control. As a result, cover crops can contribute to weed control in reduced-tillage systems but herbicides or other weed control tactics are required for achieving optimum weed control and crop yield. However, cover crops can permit a reduction of herbicide inputs and a shift toward total postemergence herbicide programs. Early weed suppression provided by cover crop residue should permit crops to become established before weeds. Postemergence herbicides can control later-emerging weeds until the crop has grown past the critical period for weed control. This approach could reduce herbicide losses to the environment by replacing preemergence herbicides that are frequently detected in ground and surface waters with postemergence herbicides that are used at lower rates and are less persistent.

Factors Influencing Weed Emergence through Cover Crop Residue

The formation of a physical barrier by cover crop residue is an important factor that can prevent emergence of weed seedlings. But residue also influences the microclimate of the soil by intercepting incoming radiation. Interception and reflection of short-wave radiation by mulch elements reduce the quantity of light available to the soil surface, the heat absorbed by soils during the day, and the amount of soil moisture evaporated from soils. These effects can interact with a multitude of seed germination requirements to determine the pattern of weed seedling emergence observed in any given season.

Light transmittance to the soil surface declines exponentially with increasing residue biomass. Many weed species require light to activate a phytochrome-mediated germination process prior to emergence. Emerging weeds also require light for initiation of photosynthesis before seed reserves are depleted. Extinction of light by residue can be an important factor inhibiting weed emergence through residue; in fact, weed suppression is highly correlated with light extinction.

Light transmittance through cover crop residue is highly heterogeneous despite the appearance of uniform soil coverage. Natural rates of hairy vetch or rye residue may have up to 60% of sites transmitting more than 10% of incoming radiation whereas twice the natural residue rate may still permit up to 25% of sites transmitting more than 10% of incoming radiation. These sites that transmit a high fraction of radiation may explain why complete weed control often is not obtained by a seemingly uniform layer of residue.

Natural residue levels on the soil surface can reduce maximum soil temperature by 2 to 5oC and raise minimum soil temperature by 1oC in temperate climates although this will vary according to radiation intensity, soil moisture, and soil type. Most weed seed will germinate over a wide range of temperatures and, therefore, the degree of reduction in maximum soil temperature by residue is not sufficient to prevent germination. However, reduced maximum soil temperatures may delay the emergence of many species. Because of the decrease in maximum and increase in minimum soil temperature, soil temperature amplitude is reduced by residue. High temperature amplitudes often are required to break the dormancy of selected weed species and, therefore, a reduction in soil temperature amplitude by cover crop residue can prevent germination of weed species with this requirement.

Residue on the soil surface increases soil moisture by increasing infiltration of rainfall and by decreasing evaporative moisture loss. Higher soil moisture under cover crop residue could either benefit or retard weed germination depending on species requirements. Under saturated soil conditions, residue could slow evaporation and reduce germination of species inhibited by excess soil moisture. Under droughty conditions, retention of soil moisture could enhance weed germination and seedling survival.

Chemical compounds released from cover crop residue have potential to stimulate or inhibit weed germination and growth. Nitrates released by legume residue can stimulate germination of selected weed species. On the other hand, research has demonstrated the presence of toxins known as allelochemicals that inhibit germination and growth of many weed species. In natural environments it is difficult to separate allelopathic effects from the physical effects described above. It also can be difficult to determine whether growth inhibition by residues with high carbon/nitrogen ratios is due to allelopathy or immobilization of nitrogen.

Living Mulches

There are many approaches to incorporating living mulches into cropping systems. A few examples of living mulches include perennial sod-like mulches that are maintained for many years once established, a cover crop into which the cash crop is relay planted, and a cover crop overseeded into a cash crop. Generally, living mulches can suppress weeds if they are well-established before emergence of weeds and maintain uniform coverage of the soil; that is, if they become occupants of the niche normally occupied by weeds. In almost every case where living mulches are competitive enough to successfully displace weeds, they also are competitive enough to reduce crop growth and yield as well.

Several approaches have been taken to minimize the impact of living mulches on crop productivity:

1. broadcast suppression of the living mulch with a sublethal herbicide dose
2. band-killing the living mulch within the crop row with a lethal herbicide dose
3. strip-tillage within the crop row
4. increasing the competitiveness of the crop through manipulation of the population and plant
   spacing
5. providing sufficient nutrients and water to compensate for resources used by the living mulch

Successful Approaches to Using Cover Crops for Vegetable Production

Hairy vetch has been demonstrated to be a valuable cover crop for both agronomic and vegetable crops. A hairy vetch cover crop that is planted in fall will consistently produce high biomass with a high nitrogen content in most areas of the country except the extreme north. This cover crop can be easily killed by herbicide, mowing, or rolling in the spring to leave a uniform mulch on the soil surface which will reduce erosion, suppress weeds, and release nitrogen. Growing fresh-market tomatoes in hairy vetch residue can reduce herbicide and nitrogen inputs and eliminate the cost of installing and disposing of plastic mulches. Because of sizable yield increases and cost reductions, this system has provided substantial increases in economic returns. This system is most applicable to vegetable crops with postemergence herbicides registered for control of both broadleaved and grass weeds.

Although hairy vetch has many benefits, it has weaknesses as well. Hairy vetch captures very little excess nutrient in soils during fall and winter months. It also suppresses weeds for only a limited period of time because of rapid decomposition. Rye has many characteristics that are the opposite of hairy vetch. Rye is superior at capturing nutrients in fall and winter and provides a more persistent weed suppressive mulch in summer. However, a rye cover crop can remove excess soil moisture and can immobilize nitrogen if left to grow too long in spring; yield losses in corn and tomatoes often are observed in this situation.

A mixture of hairy vetch and rye can provide a broader spectrum of benefits than either cover crop alone. We have observed higher mulch biomass and improved weed control by this mixture than by either species alone. The carbon:nitrogen ratio of the mixture remains low enough to prevent nitrogen immobilization as long as the rye component is seeded below monoculture seeding rates (a seeding rate of 40 lb/A of vetch plus 40 lb/A of rye has been effective in our research). Cover crop mixtures have proven to be an effective means of increasing the weed suppressive capability of cover crops as well as maintaining many of the other benefits of both species.

References:

• Abdul-Baki, A.A. and J.R. Teasdale. 1997. Sustainable Production of Fresh-Market Tomatoes and Other Summer Vegetables with Organic Mulches. USDA-ARS Farmers Bull. 2279, Revised.
• Kelly, T.C., Y.C. Lu, A.A. Abdul-Baki, and J.R. Teasdale. 1995. Economics of a hairy vetch mulch system for producing fresh-market tomatoes in the mid-Atlantic region. J. Amer. Soc. Hort. Sci. 120:854-860.
• Teasdale, J.R. 1998. Cover crops, smother plants, and weed management. pp. 247-270 In J.L.
• Hatfield, D.D. Buhler, and B.A. Stewart, eds. Integrated Weed and Soil Management. Ann Arbor Press, Chelsea, MI.
• Teasdale, J.R. and A.A. Abdul-Baki. 1998. Comparison of mixtures vs. monocultures of cover crops for fresh-market tomato production with and without herbicide. HortScience 33:1163-1166.

Information on our site was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.

By: John R. Teasdale, USDA-ARS Weed Science Laboratory, Bldg. 001 Room 323, Beltsville, MD 20705, 301-504-5504, teasdale@asrr.arsusda.gov

Published: Proceedings. 1999. New England Vegetable and Berry Growers Conference and Trade Show, Sturbridge, MA. p. 347-350.

Reviewed: T. Jude Boucher, UConn IPM, 2012

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