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.
It is beneficial to have 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).
Cultivar and cover crop selection
Competitive differences exist among crop cultivars. Tall cultivars and cultivars with rapid establishment and quick canopy closure are 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 crop-produced allelo-chemicals (naturally produced compounds that can inhibit 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. Paul Murphy, NC State wheat breeder, is selecting for weed suppressive wheat lines, and his progress can be followed online.
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 increased plant populations will help the crop compete with weeds. When blind cultivation such as rotary hoeing will be used, seeding rates should be increased by 10 to 20 percent. If a particularly aggressive blind cultivation tactic such as springtooth harrowing will be utilized, seeding rates should be increased to at least 20 percent.
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 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. Soil fertility that promotes early and sustained crop growth, however, 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 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.
A healthy, vigorous crop is one of the best means of suppressing weeds. Some physical tactics, however, 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. Because 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.
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 echnique, 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 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. Blind cultivation will only kill weeds in the “white thread” stage. If the weed can be seen from the tractor, then it is too large to be destroyed by blind cultivation. Thus, early and timely blind cultivation is critical. However, more than two blind cultivation passes may cause some damage to soybeans. This damage results in decreased yield potential, but blind cultivation is still necessary to avoid much higher yield losses from weed competition. The first blind cultivation pass is usually performed immediately before the crop emerges, and subsequent passes occur approximately every five days afterwards. Blind cultivation cannot be performed when soybeans are in the crookneck stage during emergence. In North Carolina, farmers often need to perform three to five blind cultivation passes, especially in a less competitive crop like soybeans. 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 (Figure 7-2), tine weeders (Figure 7-1), spike tooth harrows, springtooth harrows, and chain link harrows.
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 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.
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.
The different types of between-row cultivators vary in how soil is moved, the size of the weeds that can be killed, and the precision of operation. Rolling cultivators (Figure 7-5) can be fine-tuned to throw precise amounts of soil towards the crop, peel soil away from the crop, or even do both during a single pass.
This flexibility makes them a mainstay on most organic farms. S-tine cultivators are also frequently used in combination with rolling cultivators to remove larger weeds. In general, smaller weeds are easier to control than large weeds with one exception. Wide sweep cultivators (Figure 7-3 and Figure 7-4) that are mounted on parallel linkages can run parallel to the soil surface.
They are able to sever the roots of large weeds and kill them, but small weeds are able to reroot. Most wide sweep cultivators can be angled to control smaller weeds by changing the angle of the blade and can also throw soil when needed.
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 be performed only 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.
*Illustrations by John Gist were reprinted with permission from Steel in the Field, published by the Sustainable Agriculture Research and Education (SARE) outreach office, USDA. Citation of SARE materials does not constitute SARE’s or USDA’s endorsement of any product, organization, view or opinion. Visit the SARE website for more information about SARE and sustainable agriculture.
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. The cost of herbicides approved for organic farming may be prohibitively expensive for field crops. The OMRI publishes a list of commercially available products that can be used in certified organic operations for weed control. Conditions for use of an approved herbicide must be documented in the organic system plan as specified in the 2000 NOP.
Daren Hubers farms nearly 1,800 acres of conventional corn–beans production and 240 acres of organic grain production. His farm is located in the Blacklands (denoting soils high in organic matter) of eastern North Carolina. The farm consists of two main soil types: Ponzer muck and Newholland mucky loamy sand.
Overview: Corn (organic and conventional) is generally planted between March 25 and April 15 and is harvested starting the third or fourth week of August. Group III and IV soybeans are planted on conventional land, and Group V and VI beans are planted on organic land as a double crop after wheat. Daren plants organic corn and beans on 20-inch rows, with a target plant population of 34,000 corn plants per acre and 150,000 soybean plants per acre. Conventional beans are planted at 100,000 population.
Weed Management: Weed management is the biggest challenge to organic production on this farm. Daren feels that starting with a clean, wellprepared field is an important practice to keep ahead of the weeds. Weed control is difficult in their light, chaffy soils because tillage and cultivation often make the soil too “fluffy” and loose, causing sandblasting by soil that damages plants. Managing weeds in soybeans is especially difficult because soybeans are not as competitive with weeds as corn and require more tillage passes to achieve adequate weed control. In corn and soybeans on this farm, weed management starts with a 35-foot flex-tine weeder made by Kovar Manufacturing. Daren expanded the width of the harrow by adding booms and teeth. Initially, the teeth had a 45-degree angle to them and were too aggressive in the soil (pulling plants out), so he cut off the ends (angled portion) of all the teeth and it performed well. Daren tries to go over fields every other day from the time of planting until crop emergence.
After the crop emerges, Daren uses a rotary hoe once or twice, and then cultivates between rows alternating between a rolling cultivator and S-tine cultivator. Daren has also experimented with organic herbicides, specifically clove oil. He used a directed sprayer to target the bottom of soybean plants in the row. By employing this method, expensive herbicides were not broadcast and the most difficult weeds to eliminate were targeted by the spray. Daren was hoping to eliminate some cultivation passes by using an herbicide, and though partially effective, the herbicides were not economically efficient.
Cultivating large fields of 20-inch row organic corn and soybeans is tedious and slow. It is commonly said that the worst pest in a field of beans is the cultivator operator—when he drifts over rows. Daren helped eliminate this problem by investing in real time kinematic (RTK) satellite navigation. The RTK navigation system allows him to use a satellite GPS to locate his rows while he is planting. When he sets up his tractor and cultivator to the location of the planted rows, he can let the tractor drive itself, making good time, and not worrying about drift. The accuracy on the system is 1 to 2 cm. He also uses the system when blind cultivating with the harrow before the crop has emerged to ensure that tractor tires stay between the rows.
Publication date: Feb. 10, 2014
Other Publications in North Carolina Organic Grain Production Guide
- Chapter 1: Introduction
- Chapter 2: Organic Crop Production Systems
- Chapter 3: Crop Production Management - Corn
- Chapter 4: Crop Production Management - Organic Wheat and Small Grains
- Chapter 5: Crop Production Management - Organic Soybeans
- Chapter 6: Soil Management
- Chapter 7: Weed Management
- Chapter 8: Rolled Cover Crop Mulches for Organic Corn and Soybean Production
- Organic Certification for Field Crops: A Guide
- Chapter 10: Marketing Organic Grain Crops and Budgets
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