By: Mike Burton, Assistant Professor, Crop Science, NCSU; Randy Weisz, Crop Science Extension Specialist, NCSU; Alan York, Crop Science Extension Specialist, NCSU; Molly Hamilton, Crop Science Extension Assistant, NCSU
Weed pest management must be an ongoing consideration for organic producers in order to produce acceptable yields and crop quality. Using a system of weed management that includes multiple tactics can help to reduce losses that weeds cause in the short and long term. Here we have divided various tactics into two major categories, cultural and mechanical. Typically, cultural tactics are associated with enhancing crop growth or cover and mechanical tactics are some form of physical manipulation that is directed to kill, injure or bury weeds.
Weed Identification Guides
Crop Rotation: It is beneficial to have a rotation system that includes crops with different life cycles, growth patterns and management techniques in order to reduce the chance that weeds can proliferate over successive years. For example, a rotation could include a summer crop, winter crop, legume, grass, a cultivated crop (corn) and a non-cultivated crop (wheat or hay). Because some weeds are triggered to germinate by tillage, rotations of tilled and no-tillage systems (e.g. a forage or hay crop) may also be of benefit.
Cultivar and Cover Crop Selection: Competitive differences exist among crop cultivars. Tall cultivars and cultivars with rapid establishment, quick canopy closure, high leaf area index and yield stability are reportedly more competitive with weeds than short or dwarf cultivars, or cultivars (or seedlots) that are of low seed vigor, are slow growing, or have low leaf area index. Some weed species are suppressed by the exudation of crop-produced allelochemicals (naturally produced compounds that can affect the growth of neighboring organisms) in standing crops or in residues of allelopathic crops (e.g. rye cover crop). Results of studies conducted on wheat (Wu et al. 2000, 2001) and rye (Reberg-Horton 2002) have demonstrated that the production of allelochemicals varies widely with cultivar and can change during crop development. Efforts to enhance allelopathic characteristics of cultivars have begun in the small grains breeding program at NC State University and elsewhere.
Seed Quality: Seed cleanliness, percent germination and vigor are characteristics that can influence the competitive ability of the seedling crop. Planting un-clean (especially farmer-saved) seed which is often of lower quality than certified seed and may contain unknown quantities of weed seed or disease, may result in the introduction of pests not previously observed on the farm. There is also a risk of increasing weed density and moving weeds to parts of fields that were not previously infested. Percent germination and vigor are important to weed management because they collectively affect stand quality and time to canopy closure.
Planting – Sowing Date and Seeding Rate: Sowing date and seeding rate affect the crop population, which must be optimal in order to compete with weeds. Carefully maintained and adjusted planting equipment will insure that the crop seed is planted at the correct depth and is planted uniformly for uniform emergence.
Cover Crops: Cover crops can provide benefits of reduced soil erosion, increased soil nitrogen, and weed suppression through allelopathy, light interception, and the physical barrier of plant residues. Cover crops such as rye, triticale, soybean, cowpea, or clover can be tilled in as a green manure, allowed to winter kill, or be killed or suppressed by undercutting with cultivator sweeps, mowing, or rolling. Warm-season cover crops help to suppress weeds by establishing quickly and out-competing weeds for resources. It is important to carefully manage cover crops so that they do not set seed in the field and become weed problems themselves.
Fertility – Compost and Manures: Uncomposted or poorly composted materials and manures can be a major avenue for the introduction of weed seed content. However, soil fertility that promotes crop growth helps to reduce the chance that weeds will establish a foothold in an area of poor productivity, i.e. poor (noncompetitive) crop performance leaves the door open to pests (disease, insects and weeds).
Sanitation and Field Selection: Weeds are often spread from field to field on tillage, cultivation or mowing equipment. Cleaning equipment before moving from one field to another, or even after going through a particularly weedy section (or problematic weed patch), can prevent weeds from spreading between fields or within fields. When transitioning to organic, it may be a good idea to start with fields that are known to have low weed infestations. Fields with problem weeds, such as Italian ryegrass, wild garlic, johnsongrass or bermudagrass, should be avoided if possible, when first transitioning to organic management as these fields will be more difficult to manage when first practicing organic weed control.
Critical Period of Weed Interference: A critical period of weed interference is the time in a crop’s life when weed competition can adversely affect yield. Critical periods of weed interference will vary by crop, weed species and growing conditions. This concept does not apply to organic small grains because there are very few weed removal options once the crop is established. However, for soybeans a conservative critical period is roughly 2.5 weeks after planting to about 5 weeks after planting, and for corn between 3 weeks after planting and 6 weeks after planting. This is the period when weed competition will adversely affect the crop and weeds, therefore, need to be controlled. Some early competition (before 2.5 or 3 weeks after planting) can be tolerated, assuming you can remove the weeds later. This can be difficult with organic management as the weeds will be too large to effectively cultivate out, therefore the goal should be consistent weed control up to 4 or 5 weeks for soybeans and 6 weeks for corn after planting. After the crop reaches the end of this critical weed interference period, later-emerging weeds may look ugly in the field but they do not affect yield.
A healthy, vigorous crop is one of the best means of suppressing weeds. However, some physical tactics are discussed below that can be used in conjunction with good cultural practices to kill or suppress weeds – leaving the advantage to the crop. The goals of mechanical weed control are to eliminate the bulk of the weed population before it competes with the crop and to reduce the weed seed bank in the field. Important factors to think about for mechanical weed control are: weed species present and size, soil condition, available equipment, crop species and size, and the weather. Since it might not be necessary to use a tactic on the entire field, knowledge of weed distribution and severity can be valuable. Obviously, farmers will want to minimize tillage and cultivation to reduce environmental costs (e.g. erosion, destruction of soil structure, or loss of soil organic matter) and operating costs (e.g. fuel and equipment)—and these goals must be balanced with weed management. Tillage, blind cultivation, and between-row cultivation are important aspects of mechanical weed control.
Tillage: Proper field tillage is important to creating a good seedbed for uniform crop establishment—often a critical part of a crop’s ability to compete with weeds. Tillage should also kill weeds that have already emerged. In the spring, when the soil is warm, weeds often germinate in a flush after tillage. A moldboard plow will bury the weed seeds on the surface (those that were being cued to come out of dormancy by warming soil) and bring up dormant weed seeds from deeper in the soil. These weed seeds will normally be slower to come out of dormancy than weed seeds previously on the surface. Chisel plowing or disking does not invert the soil and can result in an early flush of weeds that will compete with the crop (Martens and Martens 2002). The stale seedbed technique can be used, if there is enough time before planting, as an alternate approach. In this technique, soil is tilled early (i.e. a seedbed is prepared), encouraging weed flushes, and then shallow tillage (or flaming) is used again to kill the emerged or emerging weed seedlings. While this technique should not be used in erodible soils, it can be used to eliminate the first flush or flushes of weeds that would compete with the crop.
Blind cultivation: Blind cultivation is the shallow tillage of the entire field after the crop has been seeded. Generally, it is used without regard for the row positions. It is the best opportunity to destroy weeds that would otherwise be growing within the rows and would not likely be removed by subsequent mechanical tactics (Martens and Martens 2002). Blind cultivation stirs above the level of seed placement (further emphasizing the need for accurate crop seed placement), causing the desiccation and death of tiny germinating weed seedlings. Crop seeds germinating below the level of cultivation should suffer little, if any, injury. The first blind cultivation pass is usually performed immediately before the crop emerges, and a second pass is performed about a week later (Martens and Martens 2002). This depends, of course, on weather, soil and crop conditions and weed pressure. Blind cultivation is most effective when the soil is fairly dry and the weather is warm and sunny to allow for effective weed desiccation. Blind cultivation equipment includes rotary hoes, tine weeders, spike tooth harrows, springtooth harrows and chain link harrows (Martens and Martens 2002).
Between-row cultivation: Between-row cultivation should not be the primary mechanical weed control, but should be used as a follow-up tactic to control weeds that escaped previous efforts. Between-row cultivation should be implemented when weeds are about one inch tall and the crop is large enough to not be covered up by dirt thrown during the cultivation pass. Usually more than one cultivation pass is needed. It may be useful to reverse the direction of the second (and alternate) cultivation pass in order to increase the possibility of removing weeds that were missed by the first cultivation. Planting corn in furrows can allow more soil to be moved on top of weeds and may be a useful practice on some farms. All cultivation passes should be done before the canopy closes or shades the area between the rows. After this time, the need for cultivation should decrease, as shading from the crop canopy will reduce weed seed germination, and equipment operations can severely damage crop plants. Cultivating works best when the ground is fairly dry and the soil is in good physical condition (Martens and Martens 2002). There are many types of cultivator teeth, shanks and points. Choose the cultivating equipment that works best in your soils. Points for cultivator teeth vary in type and width. Half sweeps (next to the row) and full sweeps (between rows) are probably the most versatile and common, but each type of point works best under certain conditions and on certain weed species (Martens and Martens 2002). Using fenders on cultivators at the first pass can keep the soil from covering up the crop. Cultivator adjustments are very important and should be made to fit the field conditions. Tractor speed should also be modified through the field to compensate for variability in soil type and moisture.
There are also other methods of mechanical weed control that may be effective and efficient, depending on the equipment, budget, and goals of the farm.
Flame weeding: Flame weeding provides fairly effective weed control on many emerged broadleaf species and can be used in tilled or no-tillage fields. Grasses are often not well controlled by flaming (growing points are often below the soil surface). Flame weeding should only be performed when field moisture levels are high and when the crop is small.
Hand weeding and Topping: Walking fields and performing hand weeding or topping weeds (cutting off the tops) can vastly increase familiarity with the condition of the crop and distribution of weeds or other pests. Farmers who are familiar with problem locations can remove patches of prolific weeds before they produce viable seeds and reduce problems caused by weeds that escaped management in the long run. Topping weeds can reduce seed set and, therefore, the weed seed bank in the field.
No-till organic weed control. There is growing interest in organic no-till production. Advantages such as reduced cultivation, reduced soil erosion and organic matter additions have made no-till organic an attractive idea. Recently, research has been done on no-till organic agriculture in many parts of the country, and there is potential for some organic systems to be much less reliant upon mechanical weed control. The basic premise for no-till organic weed control is to plant a cover crop that will produce a high biomass, then mow or roll that cover crop and no-till plant into the residue. This system, however, takes a lot of planning to work well. Weed control may be a challenge if the cover crop residue is not able to smother germinating weeds effectively. In North Carolina, tillage is an important technique for controlling insect pests such as Hessian fly, wireworms and cutworms in organic wheat and corn production. These insect pests may become problems in organic no-till systems. The following articles and sites provide more information about organic no-till, especially the New Farm site:
The New Farm®
The Rodale Institute
There are a number of pesticides approved for use in certified organic production systems—mainly non-synthetic compounds or biocontrols. Some may be used to treat soybean diseases. These include: neem (and its derivatives), hydrogen peroxide, potassium bicarbonate, pesticidal soaps and copper products. While these products do have potential for controlling pests in corn, no research has been done on them in field corn in North Carolina and, therefore, we can give no recommendations for their use in the state. The cost of organically approved pesticides may be prohibitively expensive for field crop production. Conditions for use of a pesticide must be documented in the organic system plan (NOP 2000). See the Pest Management Web Resources for more information, or check out the OMRI site.
Herbicides. There are a few organically approved herbicides that can be used in organic production. These include acetic acid (distilled vinegar) on its own or in combination with citric acid, products that contain clove oil, soap-based herbicides (non-detergent), some corn gluten meal products, and hot or boiling water. These herbicides are reported to work best on young weeds. Organically approved products can be found on the OMRI website. While these products do have potential for controlling some weed pests, no research has been done on them in North Carolina and, therefore, we can give no recommendations for their use in the state. The cost of these organically approved herbicides may be prohibitively expensive for field crop production.
Martens, M. and Martens, K. 2002. Organic weed control: cultural and mechanical methods. Acres. 32 (8). Available at: http://www.acresusa.com/toolbox/reprints/Organic%20weed%20control_aug02.pdf
Reberg-Horton, S.C., J. Burton, D. Danehower, G. Ma, D. Monks, P. Murphy, N.N. Ranells, J.Williamson, N.G. Creamer. 2005. Changes over time in the allelochemical content of ten cultivars of rye (Secale cereale L.) Journal of Chemical Ecology. 31:179-193.
Wu, H., T. Haig, J. Pratley, D. Lemerle, and M. An. 2001. Allelochemicals in wheat (Triticum aestivum L.): Cultivar difference in the exudation of phenolic acids. J. Chem. Ecol. 49: 3742-3745.
Wu, H., J. Pratley, D. Lemerle, and T. Haig. 2001. Allelopathy in wheat (Triticum aestivum).
Ann. Appl. Bot.139: 1-9.
Wu, H., J. Pratley, D. Lemerle, and T. Haig. 2000. Evaluation of seedling allelopathy in 453 wheat (Triticum aestivum) accessions against annual ryegrass (Lolium rigidum) by the equal-compartment-agar method. Aust. J. Agric. Res. 51:937-944.
Several weed identification guides are available for purchase through various publishers, and NCSU Ag Communications offers an excellent and inexpensive resource entitled Identifying Seedling and Mature Weeds that was developed for the Southeastern USA. However, it does not include some troublesome weed species in NC grain crop production. Another recommended guide is Weeds of the Northeast. Contact information for each publisher is listed at the end of this document. A few Internet guides are also available; their URLs at the time of printing were:
Weed Identification Guides
Identifying Seedling and Mature Weeds (AG-208), North Carolina Agricultural Research Service and Cooperative Extension Service, Publications Office, Box 7603, NCSU, Raleigh, NC 27695-7603; $10
Weeds of the Northeast
Uva, Neal and DiTomaso (1997)
Cornell University Press
P. O. Box 6525, Ithaca, NY 14851-6525