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C.O.R.N
Newsletter
2004-04
February 18, 2004 -
March 3, 2004
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Weed Management In Conventional (Non-GMO) Soybeans
by
Mark Loux
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Producers of non-GMO soybeans should strongly consider the use of a two-pass herbicide program – a preplant herbicide treatment that includes burndown plus residual herbicides followed by a broad-spectrum postemergence treatment. Total postemergence (or burndown without residual followed by postemergence) should be used only in fields with low weed populations and no history of ALS resistance. Use of a residual herbicide in the preplant treatment increases the cost of weed management, but will result in more consistent control of annual grasses, lambsquarters, black nightshade, giant ragweed, and ALS-resistant weeds.
Some of the issues for weed control in non-GMO soybeans are:
1. Planting non-GMO soybeans into a weed-free seedbed is essential, since postemergence herbicides will not adequately control typical spring no-till weeds. No-till normal soybeans therefore require an effective preplant burndown treatment, which should usually include glyphosate and 2,4-D.
2. Populations of ALS-resistant weeds, especially common and giant ragweed and marestail, may be present in non-GMO soybean fields. ALS resistance will be more likely if a field has been planted with normal soybeans over the past several years. We recommend planting non-GMO soybeans in rotation with corn and Roundup Ready soybeans (and wheat if possible), which will help minimize weed problems in the year that non-GMO soybeans are planted.
3. Unlike glyphosate, which can control large weeds at high rates if necessary, other postemergence herbicides are most effective when weeds are less than 6 inches tall. Timing of postemergence application is therefore critical for effective weed control in non-GMO soybeans.
4. Most postemergence herbicides will cause some degree of injury to soybeans, and a second postemergence application (if needed) can further slow the growth of previously injured soybeans. So, try to avoid a program consisting of multiple postemergence applications. Use of a two-pass herbicide program that includes residual herbicides will limit the need for “rescue” applications late in the season.
5. Giant ragweed can be extremely difficult to control with a total preemergence or total postemergence program in non-GMO soybeans. Problems here include: some of the giant ragweed may be ALS-resistant; application too early results in poor control of later-emerging giant ragweed; application too late results in poor control of large giant ragweed and possible yield loss due to weed interference.
6. Lambsquarters can be difficult to control with postemergence herbicides in non-GMO soybeans. HarmonyGT and Raptor are the only postemergence herbicides that control lambsquarters, and neither controls large lambsquarters well. Both herbicides are also among the more injurious to soybeans, which can limit tank-mixing possibilities.
7. Perennial weeds are not effectively controlled by most postemergence herbicides, and should be controlled in the previous crop if possible.
Recommendations for herbicide programs in non-GMO soybeans:
* We strongly suggest use of a preplant/preemergence (PRE) followed by postemergence (POST) approach in non-GMO soybeans. Total PRE programs should be used only in fields that have low annual grass populations and extremely low populations of giant ragweed, annual morningglory, and cocklebur. In addition, do not use total PRE programs in fields with ALS-resistant common or giant ragweed or perennial weeds. Total POST programs should be used only in fields with low weed populations.
* In a PRE followed by POST program, the PRE herbicide should provide residual activity on the more problematic weeds that are in the field, such as ragweeds, lambsquarters, nightshade, and marestail. Residual activity on grasses is less of a concern due to the effectiveness of POST grass herbicides (Select, Fusion, Poast), but PRE products that suppress grasses early in the season can be advantageous in heavy grass pressure.
* In no-till soybeans, the PRE should provide effective burndown of emerged weeds before soybean emergence. Where possible, apply at least one week before planting and include 2,4-D ester. Use of glyphosate may not be necessary in the burndown if applied early enough in spring, but should be used in all fields with dandelion, Canada thistle, and where weed size requires.
* Where the field received an herbicide application the previous fall, it is still advisable to apply a burndown treatment before soybeans emerge. In this situation, the weed population at the time of planting will consist primarily of small weeds that emerge in March or April. These can be controlled with 2,4-D alone, a low rate of glyphosate, or a combination of 2,4-D with residual herbicides.
* Increasing the 2,4-D rate to 1 lb/A can improve burndown of marestail, dandelion, and other tough winter weeds. This rate must be applied at least 30 days before planting with the exception of Weedone 650 and E-99, which can be applied at 1 lb/A (1 1/3 pts of a 6 lb/gal formulation) up to 15 days before soybean planting.
* In fields that have been primarily nonGMO soybeans or a rotation of nonGMO soybeans with corn over the past four or five years (or in any field that is known to contain ALS-resistant weeds), be extremely cautious about relying on ALS inhibitors (FirstRate, Classic, Synchrony, Raptor) for POST control of common or giant ragweed. Consider use of Flexstar or a combination of Flexstar with reduced rate FirstRate, Cloassic, or Synchrony in these fields.
* Use a PRE followed by POST approach for control of giant ragweed. PRE herbicides with activity on giant ragweed include Scepter, Backdraft, Canopy XL, FirstRate, and Gangster. All of these herbicides contain ALS inhibitors, so the POST component should not rely solely on FirstRate, Classic, Synchrony or Raptor, which are also ALS inhibitors. We suggest either Flexstar or Cobra be used in the POST treatment to control giant ragweed that escape the PRE treatment (Flexstar has been more effective and less injurious to soybeans than Cobra in OSU research). A reduced rate of FirstRate, Synchrony, or Classic can be mixed with the Flexstar to improve control of large plants, but we suggest using close to the full rate of Flexstar in order to adequately control ALS-resistant plants. Keep in mind that Flexstar should be applied when ragweed plants are no more than 4 to 8 inches tall for most effective control.
* Consider a PRE followed by POST approach for lambquarters, which can be difficult to control with either of the two POST herbicides that can be effective in normal soybeans - Harmony GT and Raptor. Most PRE herbicides will effectively control lambsquarters through the growing season. The only PRE soybean herbicides that do not control lambsquarters are Dual II Magnum, Outlook, alachlor, Axiom, and low rates of Domain and Boundary. In triazine-resistant areas Sencor, Domain, Boundary, or Axiom will not control lambsquarters.
* In fields with marestail, make sure the burndown effectively controls emerged plants, and include herbicides with residual activity on marestail if applied before the 2nd week of May. Do not rely on FirstRate, Classic, or Synchrony to control plants that emerge after planting or escape burndown treatments, since ALS resistance is present in many marestail populations. The most effective residual herbicides for control of marestail, including ALS-resistant, are Sencor, Valor, Gangster, Authority, and higher rates of Canopy XL. Where the marestail are not ALS-resistant, FirstRate or Python can be used for residual control. |
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C.O.R.N. is a summary of crop observations, related
information, and appropriate recommendations for Ohio
Crop Producers and Industry. C.O.R.N. is produced by
the Ohio State University Extension Agronomy Team, State
Specialists at The Ohio State University and Ohio Agricultural
Research and Development Center. C.O.R.N. Questions
are directed to State Specialists, Extension Associates,
and Agents associated with Ohio State University Extension
and the Ohio Agricultural Research and Development Center
at The Ohio State University.
Information presented above and where
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by Ohio State University Extension is implied. Although
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of consulting the pesticide label and adhering to those
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on a nondiscriminatory basis without regard to race,
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origin, gender, age, disability or Vietnam-era veteran
status.
Issued in furtherance of Cooperative
Extension work, Acts of May 8 and June 30, 1914, in
cooperation with the U.S. Department of Agriculture,
Keith L. Smith, Director, Ohio State University Extension.
TDD # 1 (800)
589-8292 (Ohio only) or (614) 292-1868
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