Organic Field Crop Production and Marketing

in North Carolina

North Carolina Organic Grain Production Guide

Chapter 7: Weed Management

Mike Burton, Assistant Professor, Crop Science, NC State University

Randy Weisz, Crop Science Extension Specialist, NC State University

Alan York, Crop Science Extension Specialist, NC State University

Molly Hamilton, Crop Science Extension Assistant, NC State University

 

Weed pest management must be an ongoing consideration for organic farmers to achieve acceptable yields and crop quality. A system of weed management that includes multiple tactics will help reduce losses in both the short and long term. Various weed management tactics fall into two major categories: cultural and mechanical. Cultural tactics are associated with enhancing crop growth or cover, while mechanical tactics are used to kill, injure, or bury weeds. During a cropping season, successful organic weed management will rely on the cultural tactics described below to achieve competitive crop plants and will use the mechanical tactics to reduce the weed population that emerges in the crop. When a cash crop is not in the field, plant a cover crop or use an occasional shallow tillage to kill germinating and emerging weeds.

Cultural Tactics

Crop rotation

It is beneficial to use a rotation system that includes crops with different life cycles, growth patterns, and management techniques. This will 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 noncultivated crop (wheat or hay). Because some weeds are triggered to germinate by tillage, rotations of tilled and no-till crops (such as 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 and quick canopy closure are reportedly more competitive with weeds than short or dwarf cultivars or cultivars (or seedlots) that have low seed vigor, are slow growing, or are less bushy. Some weed species are suppressed by the exudation of crop-produced allelochemicals (naturally produced compounds that can affect the growth of other plants) in standing crops or in residues of allelopathic crops (for example, a rye cover crop). Results of studies conducted with wheat and rye have demonstrated that the production of allelochemicals varies widely with cultivar and can change in concentration during crop development. Allelopathic characteristics of cultivars are being investigated in the small grains breeding programs at several universities.

Seed quality

Seed cleanliness, percent germination, and vigor are characteristics that can influence the competitive ability of the seedlings. Seed that has not been carefully screened (especially farmer-saved seed) is often of lower quality than certified seed and may contain unknown quantities of weed seed or disease. Planting this seed may result in the introduction of pests not previously observed on the farm. There is also a risk that weed density will increase and that weeds will be introduced to previously uninfested parts of the field. Germination rate and vigor are equally important to weed management because they collectively affect stand quality and time to canopy closure.

Planting – sowing date, seeding rate, row spacing, and population.

Sowing date and seeding rate affect the final crop population, which must be optimum to compete with weeds. Carefully maintained and adjusted planting equipment will ensure that the crop seed is uniformly planted at the correct depth for optimum emergence. Narrower rows and a slightly increased plant population (up to 10 percent higher than usual) will also help the crop compete with weeds.

Cover crops

Cover crops can provide the 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 manage cover crops carefully 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 seeds. However, soil fertility that promotes early and sustained crop growth helps to reduce the chance that weeds will establish a foothold. Areas of poor productivity leave the door open to diseases, insect pests, 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 can prevent weeds from spreading between fields or within fields. A short investment of time to clean equipment can pay large dividends if it prevents the spread of problem weeds. When transitioning to organic systems, it is highly advisable 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, as these weed species will be difficult to manage.

Mechanical Tactics

A healthy, vigorous crop is one of the best means of suppressing weeds. However, some physical tactics are almost always needed to provide additional weed control. The methods described below can be used together 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 consider for mechanical weed control are weed species present and their size, soil condition, available equipment, crop species and size, and weather. Since it might not be necessary to use a tactic on the entire field, knowledge of weed distribution and severity can be valuable. Tillage, blind cultivation (shallow tillage of the entire field after planting), 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, which is 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, weed seeds often germinate in a flush after tillage. A moldboard plow will bury the weed seeds on or near the surface (those that come out of dormancy as the soil warms) 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 near 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. If there is enough time before planting, the stale seedbed technique can be used as an alternate approach. In this technique, soil is tilled early (a seedbed is prepared), which encourages weed flushes, and then shallow tillage, flaming, or an organically approved herbicide is used to kill the emerged or emerging weed seedlings. While this technique should not be used in erosion-prone 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 provides the best opportunity to destroy weeds that would otherwise be growing within the rows and that are not likely to be removed by subsequent mechanical tactics. Blind cultivation stirs soil above the level of seed placement (further emphasizing the need for accurate placement of the crop seed), causing the desiccation and death of tiny germinating weed seedlings. Crop seeds germinating below the level of cultivation should not be injured. The first blind cultivation pass is usually performed immediately before the crop emerges, and a second pass is performed about a week later. 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.

Between-row cultivation

Between-row cultivation should not be the primary mechanical weed control tactic, 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 1-inch tall and the crop is large enough not to be covered by soil thrown up 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. Complete all cultivation passes 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. Cultivation works best when the ground is fairly dry and the soil is in good physical condition.

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. 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.

Other methods of mechanical weed control may be effective and efficient depending on the available equipment, budget, and goals of the farm.

Flame weeding

Flame weeding provides fairly effective weed control on many newly emerged broadleaf species and can be used in tilled or no-till fields. Grasses may not be well controlled by flaming because their 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 hand weeding or topping (cutting off the weed 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 long-term problems caused by weeds that escaped management. Topping of flowering weeds can reduce seed set and the weed seed bank in the field.

Herbicides

Several herbicides have been approved for certified organic farming. These include acetic acid (distilled vinegar), clove oil, nondetergent soap-based pesticides, some corn gluten meal products, and boiling water. While these products have potential for controlling weeds in organic farming systems, no research has been conducted with them in grain crops in North Carolina. Therefore, we cannot give recommendations for their use in this state. The cost of herbicides approved for organic farming may also be prohibitively expensive for field crops. The Organic Materials Review Institute (OMRI) publishes a list of commercially available products that can be used in certified organic operations for weed control (www.OMRI.org). Conditions for use of an approved herbicide must be documented in the organic system plan as specified in the 2000 National Organic Plan.

No-till Organic Weed Control

Recent research on no-till organic agriculture shows some potential for 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 with high residue, mow or roll that cover crop, and no-till plant into the residue. This system, however, takes a lot of planning to work well. For more information on organic no-till farming, contact the Rodale Institute by mail at 611 Siegfriedale Road, Kutztown, PA 19530-9320, by telephone at 610-683-1400, or on the Web: http://www.rodaleinstitute.org.