INTRODUCTION
Cover crops are plant species that are introduced into crop rotations to provide beneficial services to the agro-ecosystem. Some of the most important environmental services provided by cover crops include soil protection from erosion, capture and prevention of soil nutrient losses, fixation of nitrogen by legumes, increase in soil carbon and associated improvements in soil physical and chemical characteristics, decrease in soil temperature, increase in biological diversity including beneficial organisms, and suppression of weeds and pests (Sustainable Agriculture Network, 1998). This chapter will focus on weed suppression by cover crops, but the need to manage cover crops to optimize the totality of impacts on the ecosystem will be emphasized at the conclusion.
Cover crops can be grouped into two categories: 1) annuals that are grown during an off-season or period of the year that is not favourable for crop production and that are killed before planting a cash crop; and 2) living mulches that grow at the same time as the cash crop for all or a portion of the growing season. Cover crops that are killed before planting a cash crop influence weed control primarily through the influence of their residue on weed germination and establishment. Examples of this kind of cover crop are Vicia villosa Roth, a winter annual legume, and Secale cereale L. a winter annual cereal, which are adapted to grow during the cold season in temperate climates and are killed before planting a cash crop when temperatures become warmer. Examples of cover crops that are adapted to hot summer fallow periods in tropical and sub-tropical climates include annual legumes such as Mucuna spp. and Crotalaria juncea L. or warm-season annual grasses such as Sorghum spp.
WEED CONTROL BY COVER CROP RESIDUE
Annual cover crops are usually killed before planting a cash crop. This can be performed either by incorporation of cover crop residue into the soil or by killing the cover crop chemically or mechanically and leaving the residue as a mulch on the surface of the soil.
Incorporated residue
Tillage has been shown to stimulate weed germination and emergence of many weed seeds through brief exposure to light (Ballar et al. 1992). When tillage is used to incorporate residue, many weed seeds will be stimulated to germinate by this operation. Therefore, when incorporating residue by tillage, weed management tactics must be available to control the potential increased load of weed seedlings.
Incorporated plant residues can become toxic to weeds by the release of allelopathic chemicals. There are numerous reports of allelopathy and of the isolation of allelopathic compounds from plants (Inderjit and Keating, 1999). However, this phenomenon can be inconsistent under natural conditions because the allelopathic potential of plants is affected by many factors such as the age of the plant, soil properties, and environmental conditions. Interactions of multiple stresses in the environment on the target plants also will affect the degree of allelopathic activity (Einhellig, 1996). Examples of successful weed control leading to an increase in crop yield following incorporated cover crop residue include incorporation of Sorghum bicolor L. stalks before Triticum aestivum L. (Cheema and Khaliq, 2000), incorporation of Brassica napus L. before Solanum tuberosum L. (Boydston and Hang, 1995), and incorporation of Trifolium incarnatum L. before Zea mays L. (Dyck et al. 1995).
As with the use of herbicides for the control of weeds, there must be sufficient selectivity between the activity of cover crop toxins on weeds and on crops. In order to be useful as a weed control practice, the crop must be relatively insensitive to allelochemicals in the environment. Small-seeded crop plants may be more sensitive to allelochemicals than large-seeded plants. Crop cultivar selection and appropriate residue management may be important approaches for maximizing allelopathic activity on weeds and minimizing deleterious effects on crops, including autotoxicity. The relative timing and placement of residue relative to crop seeds can be manipulated to reduce the level of toxicity that emerging crop seedlings are exposed to.
Surface residue
When cover crops are killed and residue is left on the soil surface in a no-tillage cropping system, many factors will contribute to weed suppression (Teasdale 1998, Liebman and Mohler, 2001). Absence of tillage itself lowers weed emergence because seeds that require a brief exposure to light during tillage operations are not induced to germinate. In addition, residue on the surface of soil can suppress weed emergence directly. The degree of weed control provided by cover crop residue on the surface of soil can vary according to cover crop species, residue biomass, and weed species. Weed suppression by cover crop residue increases according to a negative exponential relationship with increasing residue biomass. Residue levels that are naturally produced by cover crops can reduce weed emergence up to 90 percent. Annual species that are small-seeded and have a light requirement for germination are most sensitive to surface residue, whereas large-seeded annuals and perennial weeds are relatively insensitive. Weed suppression will decline during the course of the season as the residue decomposes.
Residues on the surface of soil can vary greatly in dimension, structure, distribution pattern, and spatial heterogeneity. Several physical properties of mulches have been explored that may contribute to weed suppression by the physical impedance of weed emergence (Teasdale and Mohler, 2000). The ‘mulch area index’ is a pivotal property for defining many important mulch properties. It is defined as the projected area of mulch material per unit soil area and can be determined by multiplying the residue mass per unit area by the area-to-mass ratio as measured from a sub-sample of residue material. The ‘solid volume fraction’ is another important mulch property that is defined as the fraction of mulch volume composed of solid material. Together, these two indices can predict weed suppression by a wide variety of mulch types ranging from Z. mays stalks with a low area-to-mass ratio to Quercus leaves with a high area-to-mass ratio. This suggests that residue with a large number of layers and a small amount of empty internal space will be most suppressive.
Residue also influences the microclimate of the soil by intercepting incoming radiation (Teasdale and Mohler, 1993). Interception and reflection of short-wave radiation by residue 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 seed germination requirements to determine the pattern of weed seedling emergence observed in any given season.
Light extinction by cover crop mulch follows a similar negative exponential decline in relation to mulch area, as light extinction by a crop canopy declines as a function of leaf area. Since mulch mass is linearly related to mulch area, a similar exponential relationship holds between light extinction and mulch mass. 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.
Cover crop residue on the soil surface can reduce maximum soil temperature by 2-5o C and raise minimum soil temperature by 1o C in temperate climates although this will vary according to radiation intensity, soil moisture, and soil type. Greater differences will probably be observed in tropical or drier areas of the world. Most weed seed will germinate over a wide range of temperatures and, therefore, a reduction in maximum soil temperature by residue will usually have little influence on germination. Because of the decrease in maximum and increase in minimum soil temperature, the daily soil temperature amplitude also 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 that have 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.
Residue in most fields will have a relatively heterogeneous spatial distribution. This can be caused by relatively uneven stands of cover crop plants within a field resulting in areas with locally heavy and thin residue after cover crop desiccation. Even when there are relatively uniform stands of cover crops, uneven residue at a microsite level can be detected. For example, greater than 50 percent of sites measured under seemingly uniform Vicia villosa mulch permitted greater than 10 percent light transmittance to the soil level (Teasdale and Mohler, 1993). This can be explained by the exponential relationship between soil cover and mulch area index (Teasdale and Mohler, 2000). Assuming a random distribution of mulch material, it will require increasingly more mulch to achieve each successive unit increase in soil coverage by mulch. For example, it takes an increase in mulch area index from 1.4-1.9 (= 0.5) to raise soil cover from 75 to 85 percent but it takes an increase from 1.9-3.0 (= 1.1) to raise soil cover from 85-95 percent. Even a relatively high mulch area index of 4 will leave 2 percent of soil uncovered. Thus, cover crop residue will rarely provide complete ground cover and cannot be expected to provide either complete or full-season weed control. Cover crops can contribute to weed control but herbicides or other weed control tactics are required to optimize weed control and crop yield.
Every weed control tactic, including cover crops, exert a selective pressure on weed populations and will select for those species that are best adapted to that system. Perennial and selected large-seeded annual weeds that have minimal requirements to break seed dormancy and sufficient energy reserves to penetrate mulch layers will be most likely to establish and reproduce in a cover crop mulch. Also, species that have a similar phenology to the cover crop but that can survive the cover crop management system will become problematic. For example, we have observed that Lolium multiflorum Lam. can become established with a V. villosa cover crop or Digitaria sanguinalis (L.) Scop. can establish with a spring planted Glycine max (L.) Merr. cover crop and both species can regrow and reproduce after the cover crop is mowed in preparation for planting a cash crop. Thus, cover crops must be used in rotations that prevent the buildup of species adapted to that cover crop system.
Practical applications
Cover crops that produce high amounts of biomass will enhance weed suppression by leaving high amounts of suppressive residue
Vigorous species that are well adapted and planted at optimum planting dates will be most useful. For example, Vigna unguiculata (L.) Walp. is adapted to hot, dry conditions and produced 8.2-9.6 Mg/ha of residue as a cover crop that effectively suppressed weeds in a desert climate (Hutchinson and McGiffen, 2000). Mixtures of cover crops that have complementary resource requirements is another approach to increasing cover crop biomass. Often, a combination of grasses and legumes make effective cover crop mixtures for the same reasons they make effective intercropping partners. A polyculture of V. villosa plus T. incarnatum plus S. cereale produced higher biomass and suppressed weeds more than each species in monoculture (Teasdale and Abdul-Baki, 1998).
Cover crop residue that decomposes slowly will extend the period of weed suppression
Slow decomposition is associated with residue material that has a high carbon-to-nitrogen ratio. For example, residue of S. cereale which has a higher carbon-to-nitrogen ratio than the legume V. villosa had a more extended period of weed suppression than V. villosa (Mohler and Teasdale, 1993). Also, equipment such as a mower that shreds residue would enhance decomposition compared to equipment such as a roller that keeps the residue intact.
Low amounts of residue can stimulate weed emergence
Occasionally, more weeds will emerge in low levels of cover crop residue (1- 2 mg/ha) than in uncovered control plots (Mohler and Teasdale, 1993; Teasdale and Mohler, 2000). Low levels of residue are not sufficient to inhibit weeds from emerging but can create an environment more favourable for germination and emergence. This residue can retard evaporation of soil moisture and provide more uniform moisture conditions for germination and emergence than exists at the surface of bare soil. Also, nitrogenous compounds released into the germination zone, particularly from legume cover crops, can stimulate germination of selected weed species.
Creating a mulch with multiple layers of densely packed material
Since mulch area index and solid volume fraction are important determinants of weed suppression, management practices that create the maximum mulch area and solid volume or, conversely, that minimize empty mulch volume, will maximize weed suppression. Mulch composed of broad leafy material held in a matrix of grass stems as might be obtained from a legume-grass cover crop mixture may be more effective than mulch composed primarily of stems or leaves alone. Also, use of implements such as rollers or stalk choppers that pack or compress the mulch as part of the desiccation process may maximize the suppressive potential of cover crop mulches. The use of cover crops that will provide uniform stands and minimize gaps is recommended. This will maximize the area with optimum amount of residue and minimize the area with ineffective or stimulatory levels of residue.
LIVING MULCHES
Living mulches are plants grown with a cash crop. They are usually not grown for harvest or direct profit but, instead, to provide ecological benefits including protecting soils from erosion, improving soil fertility, providing traffic lanes, suppressing weeds, and reducing pest populations (Hartwig and Ammon, 2002). Low-growing legumes and grasses are typically used for this purpose. Forage and turf species often are used as living mulches because their growth habit is lower than most crops and they are relatively easy to establish and manage. Legumes are often included in cropping systems where improvements in soil fertility and quality are a primary goal, whereas grasses are often included where durability and traffic ability are important. Living mulch such as V. unguiculata or Mucuna spp. also may produce edible parts that can supplement income produced by the primary crop with which it is intercropped.
Established living mulches can protect crop plants by forming a barrier to weed and other pest organisms originating in soils. Living mulches also create a more diverse community that can reduce insect pest levels by attracting natural enemies of pests or by creating an environment that is more difficult for pests to find and multiply on crop plants. The major constraint to using living mulches is competition for water and nutrients leading to reduced crop yield. Creative management approaches are required to alleviate the detrimental effect of living mulches on crops while enhancing the benefits to weed and pest management.
Weed suppression by living mulches and the problem of selectivity
Because weed and living mulch plants compete for the same resources, weeds can be suppressed by the introduction of living mulches into cropping systems. If a cover crop becomes established before the emergence of weeds, then the presence of green vegetation covering the soil creates a radiation environment that is unfavourable for weed germination, emergence, and growth. Several requirements for breaking dormancy and promoting germination of weed seeds in soils (light with a high red-to-far red ratio and high daily soil temperature amplitude) are reduced more by living mulches than by desiccated residue (Teasdale and Daughtry, 1993). Once established, living mulch also can use the light, water, and nutritional resources that would otherwise be available to weeds. Allelopathy is another mechanism by which living mulches may suppress weeds (Fujii, 1999). However, this is difficult to separate experimentally from mechanisms relating to competition for growth resources. Weeds can escape suppression by living mulches through gaps in the mulch canopy, by morphological and physiological capabilities to access resources despite the presence of competitive living mulch, or by emergence and growth patterns that avoid the most competitive period of living mulch growth.
Cover crops that grow during periods when crops are not present in a rotation can aid in maintaining ground cover and occupying a niche that would otherwise be occupied by weeds. For example, cover crops planted in the fall provided a ground cover that protected the soil from erosion and suppressed weeds during a summer fallow in the Canadian prairies (Moyer et al. 2000). In addition, cover crops planted in the fall can become living mulch for a crop that is relay-planted into the living cover crop in the following year. Enache and Ilnicki (1990) developed a system whereby Trifolium subterraneum L. was initially planted in the fall and produced a cover of dense, low vegetation that remained alive until natural senescence several weeks after corn was relay-planted in the spring. The subsequent mulch continued to suppress weeds throughout the remainder of the season until volunteer T. subterraneum emerged in the fall and established a naturally recurring cover crop. Weed biomass was reduced by 53-94 percent by this living mulch whereas weed biomass in desiccated S. cereale mulch ranged from an 11 percent decrease to a 76 percent increase compared to a no-mulch control. Likewise, weed biomass was reduced by 52-70 percent in live V. villosa treated in a manner similar to that described for T. incarnatum whereas weed biomass ranged from a 41 percent reduction to a 45 percent increase in desiccated V. villosa residue compared to treatment without a cover crop (Teasdale and Daughtry, 1993). Thus, a living cover crop is capable of greater weed suppression than desiccated cover crop residue.
Living mulches also can be intercropped with a primary cash crop by planting shortly before, at the same time as, or shortly after planting the primary cash crop. These secondary intercropped species are often referred to as smother crops (Liebman and Staver, 2001). Smother crops should be species that establish more rapidly than weeds and whose peak period of growth coincides with that of early weed emergence but does not coincide with that of the crop. Ideally the smother crop should suppress weed establishment during the critical period for weed establishment, i.e. the period when emerging weeds will cause a loss in crop yield (Buhler et al. 2001). The smother crop will then senesce following this critical period for weed competition, thereby minimizing subsequent competition between the smother crop and the primary crop during the remainder of the season. One approach is the use of low-growing, fast-establishing, fast-maturing annuals planted with longer-season grain crops. For example, using annual Brassica and Medicago spp., Buhler et al. (2001), observed various levels of weed control depending on seasonal, species, and timing variables. However, good control of weeds was usually associated with crop yield loss.
In tropical systems, Chikoye et al. (2001), planted several smother crops with various growth habits in a Z. mays-Manihot esculenta Crantz intercrop system and found that Mucuna cochinchinensis (Lour.) A. Chev., Lablab purpureus L. and Pueraria phaseoloides (Roxb.) Benth. were effective for reclaiming fields heavily infested with the difficult-to-control perennial weed, Imperata cylindrica (L.) Beauv. After three years, rhizome biomass of I. cylindrica was reduced by 94 percent by annually weeding five times, 89 percent by M. cochinchinensis, 77 percent by L. purpureus, 74 percent by V. unguiculata, and 55 percent by P. phaseoloides. Akobundu et al. (2000) observed that Mucuna spp. suppressed I. cylindrica until the subsequent cropping season when Z. mays yield was higher and hand weeding was reduced by 50 percent compared to plots without cover crop. Mucuna deeringiana (Bort) Merr. and Canavalia ensiformis (L.) DC. living mulches reduced weed biomass and improved Z. mays yields in a traditional slash-and-burn system in Mexico (Caamal-Maldonado, 2001). Liebman and Dyck, (1993) reviewed literature where one or more primary crops were intercropped with a smother crop and found that weed biomass was lower with than without the smother crop in 47 cases, variable in three cases, and higher in four cases. Thus smother crops can be effective tools for managing weeds as well as improving soil fertility and providing additional food production if edible reproductive parts are produced by the cover crop.
The major hurdle to the adoption and use of living mulches is lack of selectivity. Typically, a living mulch that is competitive enough to suppress weeds will also suppress crop growth and yield. Much of the research with living mulches has focused on documenting and alleviating this problem (Liebman and Staver, 2001, Teasdale, 1998). Several approaches have been used to reduce competition between the living mulch and cash crop species without eliminating the desirable attributes and benefits of the living mulch (see practical applications below). These attempts to achieve selectivity have met with varying success but often lack consistency.
Practical Applications
Ideal living mulch for weed suppression should have the following characteristics:
ability to provide a complete ground cover of dense vegetation;
rapid establishment and growth that develops a canopy faster than weeds;
selectivity between suppression of weeds and the associated crop.
Means for achieving selectivity between weeds and the associated crop include:
1. Using low-growing living mulch that competes primarily for light. In this case, as long as the living mulch becomes established before the weeds, it would maintain weed suppression by excluding light but would not impact taller growing crops and would not compete with the crop excessively for soil resources such as water and nutrients.
2. Planting the living mulch so that the time of peak growth of the living mulch does not coincide with the critical period during which competition would have the greatest impact on crop yield.
3. Reducing crop row spacing and/or increase crop population to enhance the competitiveness of the crop relative to the living mulch.
4. Providing supplemental water and nitrogen to compensate for resources used by living mulch plants.
5. Suppressing the living mulch so as to reduce its competitiveness with the crop.
Means for suppressing living mulch include:
a) A broadcast application of an herbicide at a rate that is suppressive but not lethal.
b) A banded application of a herbicide to kill the living mulch in the crop row so as to reduce competition within the row area but permit weed suppression by the living mulch between rows.
c) Strip tillage to provide suitable planting conditions without competition within the crop row but to permit weed suppression by the living mulch between rows.
d) Mowing to reduce the height and vigour of the living mulch.
COVER CROPS AS PART OF AN INTEGRATED WEED MANAGEMENT SYSTEM
Holistic management principles and a shift to a systems approach for crop protection are vital to combating agricultural weeds as well as other pests. Ecologically-based weed management focuses on preventive practices and natural processes of population regulation with herbicides or cultivation used as interventions only when needed. Emphasis is placed on maximizing the beneficial ecological processes within farming systems that can maintain weed populations at low, manageable levels. Although agricultural systems are simplified compared to natural ecosystems, there are abundant opportunities to redesign and manage agricultural systems to reduce weed populations.
The live and dead plant materials associated with the use of cover crops in agricultural systems are particularly well suited to developing ecologically-based weed management systems. Generally, a more diverse biological and physical environment at the surface of soils such as that associated with cover crops offers opportunities for regulating and minimizing weed populations. Liebman and Gallandt (1997) propose that successful integrated weed management systems can be developed by combining several strategies or ‘little hammers’ that would cumulatively reduce the relative fitness of weeds versus crops. An integrated system, including cover crops in combination with other strategies, could improve weed control compared to reliance on each strategy alone. Not all weed management strategies are equally compatible with cover crops, however. For example, soil-active herbicides can be adsorbed by cover crop residue and are less effective with than without cover crops. Mechanical cultivation is often not as efficient in reduced tillage systems where living and/or dead cover crop vegetation can interfere with cultivating equipment and where untilled soil is less susceptible to fragmenting and desiccating weed seedlings as is a clean, well-tilled soil. Cover crops should be more compatible with control measures such as post-emergence herbicides or biocontrol agents that act on the foliage of weeds after emergence than practices that act through the soil medium. Most important, long-term strategies need to be developed to maintain weed populations at low levels through suppressive crop rotations, crop population/row spacing, and fertility management.
Ultimately, weed management is one of the many potential benefits of using cover crops. Cover crop management therefore must be designed to optimize all of the potential benefits that can be derived from cover crops and to minimize the negative impacts of cover crops. For example, high levels of cover crop biomass may be desirable for erosion control and weed suppression but may interfere with planting operations, maintain soil at temperatures that are too cold in spring, or compete with the crop for limited soil moisture. Management practices that encourage rapid degradation of cover crops such as mowing may reduce effectiveness for weed suppression but may enhance release of nitrogen that can stimulate early crop growth. Soil moisture depletion by cover crops will become the primary management consideration in those areas of the world where soil moisture is the limiting factor in crop production. Cover crop management requires an understanding of all the potential impacts on cropping systems, the definition of the most important goals to be achieved by using cover crops, and a balanced approach for achieving those goals.
Practical applications
1. Integrate cover crops into a long-term preventive approach to managing weeds that includes a rotational plan to minimize populations as well as appropriate interventions for controlling weeds that do emerge.
2. Rotate cover crops within crop rotations. Continuous use of the same cover crop species or cover crops with the same pattern of planting and growth will select for weed species that are adapted to these species and patterns. Also, cover crops can serve as hosts for nematodes and pathogens and may increase populations of these pests. Cover crops should be rotated in the same way that crops are rotated to reduce buildup of populations of detrimental weeds and pests.
3. Cover crops can permit a reduction of herbicide inputs. Weed suppression provided by cover crop residue usually permits crops to become established before weeds. Many soil-applied pre-plant or pre-emergence herbicides will be adsorbed to the cover crop residue and become ineffective; use of these products with high levels of cover crop residue may not be economical. However, post-emergence herbicides that are applied to foliage of emerged weeds can be used more effectively with cover crop systems. They may be used only as needed and can be selected for the specific weed species that need to be controlled. This approach could reduce herbicide losses to the environment by replacing pre-emergence herbicides that may be persistent and are often detected in ground and surface waters with post-emergence herbicides that are used at lower rates and are less persistent.
4. Balance management of cover crops for weed suppression with other management requirements. The primary goals of cover crop management may derive from other important benefits of cover crops such as nitrogen contribution to a cash crop or alleviating high soil temperatures. Alternately, the need to minimize negative influences of cover crops such as depletion of soil moisture reserves or interference with planting operations can become important considerations. Successful management of cover crops requires a balanced plan to maximize the benefits and minimize their negatives in order to achieve a productive and sustainable agro-ecosystem.
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