CORN
Crop Observation and Recommendation Network

February 17-March 3, 2003
C.O.R.N. 2003-4

In This Issue:

A) Another ALS-Resistant Weed in Ohio!
B) Glyphosate, Glyphosate, and More Glyphosate
C) Herbicide Programs for Non-GMO Soybeans

A) Another ALS-Resistant Weed in Ohio! - Stachler and Loux CORN Questions

Greenhouse studies this winter have confirmed that an Ohio population of common lambsquarters is resistant to Harmony GT, an ALS-inhibiting herbicide. The population also has some tolerance to Raptor. This population originates from the Leipsic, Ohio area (Northeast Putnam County). Limited information is available on the crop and herbicide history, but soybeans were grown in 2000 and 2001 and Synchrony STS and Raptor were used those years. Michigan State has also confirmed ALS-resistant common lambsquarters in a few Michigan populations. Their populations are cross-resistant, meaning they will not be controlled by Harmony GT or Raptor.

B) Glyphosate, Glyphosate, and More Glyphosate - Loux CORN Questions

A number of glyphosate products are currently available, including Roundup WeatherMax, Roundup Original, Glyphomax Plus, Touchdown, Cornerstone, and others. Most are labeled for preplant burndown in any crop and postemergence use in Roundup Ready corn and soybeans, and directions for use are generally similar. In OSU research, we have not observed differences in weed control or crop injury when comparing the various glyphosate products. However, differences among formulations may result in differences in toxicity to the user, recommended rate, rainfastness, and adjuvant recommendations, and users should consult product labels for specific directions. For example, the rainfast interval indicated on the Roundup WeatherMax label is 30 minutes, while some other glyphosate product labels specify rainfast intervals of one hour or more. The true indicator of the activity of a glyphosate formulation is not the total pounds of glyphosate shown on the label, but rather the pounds of glyphosate acid (or acid equivalent). This is the figure that should be considered when comparing the rates and costs of glyphosate formulations. The following table shows the amount of product required to provide the equivalent of 0.75 lbs glyphosate acid.

Some premix products that contain glyphosate include:

Backdraft SL (BASF), a premix of imazaquin (Scepter) plus glyphosate for control of emerged weeds and residual control of broadleaf weeds. This product is targeted primarily for preplant or preemergence application to any type of soybean, but can be applied postemergence on Roundup Ready soybeans. The use rate of Backdraft SL (5 pints/A) provides the equivalent of 2.1 oz/A of Scepter 70DG plus 0.56 lbs acid equivalent of glyphosate.

Extreme (BASF), a premix of imazethapyr (Pursuit) plus glyphosate for postemergence plus residual weed control in Roundup Ready soybeans. This product may improve velvetleaf and morningglory control, compared to glyphosate alone. The residual control from the imazethapyr may also help control annual grasses, black nightshade, and a few other weeds that can emerge after postemergence applications. The use rate provides the equivalent of a labeled rate of Pursuit plus 0.56 lbs acid equivalent of glyphosate.

Fieldmaster (Monsanto), a premix of acetochlor, atrazine, and glyphosate, is labeled for postemergence use in Roundup Ready corn. However, the rate of residual herbicide in this premix is higher than needed when combined with glyphosate for an early postemergence herbicide treatment in Roundup Ready in corn. The 4-quart rate provides the equivalent of 1.1 qts Harness, 1.5 qts Atrazine 4L, and 20 oz Roundup UltraMa x. A more economical approach for postemergence applications in Roundup Ready corn would be tank-mixing approximately half-rates of Degree Xtra, Harness Xtra, Bicep II Magnum, etc with glyphosate.

Ready Master ATZ (Monsanto), a premix of glyphosate (Roundup Ultra Max) plus atrazine. This product can be applied preemergence on any corn hybrid, and postemergence on Roundup Ready hybrids only. The 2-quart rate provides the equivalent of 1 lb/A of atrazine plus 22 ounces of Roundup WeatherMax.

C) Herbicide Programs for Non-GMO Soybeans - Loux and Stachler CORN Questions

We have received many questions about the most effective herbicide programs for non-GMO soybeans. Some of the issues for weed control in non-GMO soybeans are:

• 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 non-GMO soybeans over the past several years.
  
• Unlike glyphosate, which can control large weeds at high rates if necessary, other postemergence (POST) herbicides are most effective when weeds are less than 6 inches tall. Timing of POST application is therefore critical for effective weed control in non-GMO soybeans.
  
• Most POST herbicides will cause some degree of injury to soybeans, and a second POST application (if needed) can further slow the growth of previously injured soybeans. So, try to avoid a program consisting of multiple POST applications.
  
• Giant ragweed control is extremely difficult to control with a total PRE or total POST program in non-GMO soybeans (or in a glyphosate/2,4-D burndown followed by POST program). 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.
  
• Planting non-GMO soybeans into a weed-free seedbed is essential, since POST herbicides will not adequately control typical spring no-till weeds. No-till non-GMO soybeans therefore require an effective preplant burndown treatment.

Recommendations for herbicide programs in non-GMO soybeans:

• We strongly suggest use of a preplant/preemergence (PRE) followed by 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 (almost none) of giant ragweed, annual morningglory, and cocklebur. In addition, do not use total PRE programs in fields with ALS-resistant common ragweed or perennial weeds.
  
• 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, since mixtures of Sencor plus 2,4-D, Canopy plus 2,4-D, etc can control small weeds. Where it is still available, Canopy SP is more effective than Canopy XL in burndown treatments in early spring.
  
• 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 as discussed previously.
  
• Increasing the 2,4-D rate to 1 lb/A (1 quart of 4 lb/gal ester formulations) 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, 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 non-GMO soybeans or a rotation of non-GMO soybeans with corn over the past four or five years, be extremely cautious about relying on ALS inhibitors (FirstRate, Classic, Synchrony, Raptor) for POST control of common ragweed. Consider use of Flexstar or a combination of Flexstar with reduced rate of an ALS inhibitor 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, Squadron, Canopy SP and XL, FirstRate, Gauntlet, 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 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 ragweeds 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 non-GMO soybeans - Harmony GT and Raptor. Most PRE herbicides will effectively control lambsquarters through the growing season. In fact, 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 fields with marestail, make sure the burndown effectively controls emerged plants, and include herbicides with residual activity on marestail. 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. Sencor, Valor, Authority, and high rates of Canopy XL (4.5 oz/A) appear to provide residual marestail control, although we have limited data on this.

A brief summary of the spectrum of residual broadleaf control for various PRE herbicides follows. This summary assumes application at full rates - application at reduced rates will reduce control of tough broadleaf weeds or generally shorten the duration of residual control.

• Authority - residual control of lambsquarters, pigweeds, waterhemp, black nightshade, Pennsylvania smartweed; suppression of velvetleaf, annual morningglory.
  
• Backdraft/Scepter - residual control of lambsquarters, black nightshade, pigweeds, Pennsylvania smartweed, velvetleaf, common ragweed; partial control of giant ragweed, morningglory, cocklebur; no residual control of ALS-resistant weeds.
  
• Boundary - residual control of annual grasses, lambsquarters (except triazine-resistant), pigweeds, waterhemp; partial control of Pennsylvania smartweed.
  
• Canopy SP - residual control of lambsquarters, pigweeds, Pennsylvania smartweed, velvetleaf, common ragweed, marestail, waterhemp; partial control of giant ragweed, cocklebur, morningglory, ALS-resistant common ragweed; poor control of ALS-resistant giant ragweed.
  
• Canopy XL - residual control of lambsquarters, pigweeds, Pennsylvania smartweed, velvetleaf, common ragweed, waterhemp, black nightshade; partial control of giant ragweed, cocklebur, morningglory; no residual control of ALS-resistant common or giant ragweed.
  
• Domain - residual control of annual grasses, lambsquarters (except triazine-resistant), pigweeds, waterhemp; partial control of common ragweed, Pennsylvania smartweed, black nightshade.
  
• FirstRate - residual control of lambsquarters, pigweeds, Pennsylvania smartweed, velvetleaf, common ragweed; partial control of giant ragweed, morningglory, cocklebur; no residual control of ALS-resistant weeds.
  
• Gauntlet - residual control of lambsquarters, pigweeds, Pennsylvania smartweed, velvetleaf, common ragweed, waterhemp, black nightshade; partial control of giant ragweed, cocklebur, morningglory; no residual control of ALS-resistant common or giant ragweed.
  
• Gangster - residual control of lambsquarters, pigweeds, Pennsylvania smartweed, velvetleaf, common ragweed, marestail, waterhemp, black nightshade; partial control of giant ragweed, cocklebur, morningglory, ALS-resistant common ragweed; poor control of ALS-resistant giant ragweed.
  
• Prowl - residual control of annual grasses, pigweeds, lambsquarters; partial control of waterhemp.
  
• Python - residual control of lambsquarters, pigweeds, Pennsylvania smartweed, velvetleaf; partial control of common ragweed; no control of ALS-resistant weeds.
  
• Sencor - residual control of lambsquarters, pigweeds, waterhemp, Pennsylvania smartweed; partial control of common ragweed, velvetleaf.
  
• Valor - residual control of lambsquarters, pigweeds, waterhemp, Pennsylvania smartweed, velvetleaf, marestail, black nightshade; partial control of common ragweed.

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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.

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