NDSU Crop and Pest Report

ISSUE 15   August 19, 2004


Concern for herbicide residues after applications of Tordon and Curtail have been sprayed and CRP is broken out into cropland have been asked.

The half-life of Tordon is about 13 months in North Dakota. Beans are sensitive to approximately 4 ppb. One quart of Tordon per acre would be about 250 ppb in one acre at a 3 inch depth. Thus, 1 quart of Tordon could take up to 7 years to dissipate to meet soybean tolerance. The breakdown would be faster if the field were plowed to mix the Tordon soil residue with untreated (deeper soil) and if the CRP crop were left to add organic matter. However, I doubt anyone could spray Tordon this year and plant soybeans for at least 2 to 3 years. A simple soil bioassay of soybean seeded in a pot with soil from the CRP field will let you know when there is crop safety.

Curtail has a much shorter residue. Page 114 of the ND Weed Guide shows for Curtail a rotational restriction to soybeans of 10.5 months. However, that is with crop rates, which are usually about 50% or less than rangeland rates. Doubling the rate will not double the waiting time, it is not linear. However, it will increase it by at least another 12 months.

The bottom line is if they have used Tordon or Curtail, they will have to plant back to wheat, flax, or corn. Even then a bioassay is mandatory with the selected crop first.

Dr. Rod Lym
NDSU Perennial/Noxious Weed Scientist



EPA has released its schedule for completing pesticide reregistration eligibility decisions (REDs) Interim REDs (IREDs), and tolerance reassessments decisions (TREDs) during the next several years. By following this schedule, the agency plans to meet it statutory deadlines to complete tolerance assessment and all food use REDs and IREDs by August 3, 2006, and REDs for other nonfood use persticides subject to reregistration by October 3, 2008. The schedule is available at:




It is widely known that the large-scale adoption of Roundup Ready soybeans significantly lowered the value of of herbicide inputs into this market. More recently, glyphosate has come off-patent which has driven the price of glyphosate products down. Part of these inputs dollars have been recaptured in the price of Roundup Ready seed. However, the monopoly of the Roundup Ready trait in providing glyphosate resistance may also see competition in the future. The Roundup Ready trait is provided by a gene, which produces an insensitive target site (i.g. glyphosate cannot bind to the enzyme, which is blocked in conventional crops).

Recently, an article in the journal Science reported that scientists had developed a gene that produces an enzyme, which efficiently detoxifies glyphosate. When this gene was inserted into corn, it provided a 6-fold level of glyphosate resistance at the 0.75 lb ae/A rate (1 qt of 3 lb/gallon formulation). The authors for this article were from Verdia Inc., Maxygen Inc., and Pioneer Hi-Bred International. The article did not state if there were plans for commercializing this technology. However, if it were developed, a new round of competition could enter the market.



Monsanto has been informed by the United States Food and Drug Administration (FDA) that the Agency has completed its consultation process for Roundup Ready wheat for food and feed safety. Completing the US FDA review and consultation confirms that Roundup Ready wheat is as safe as conventional wheat for all food and feed uses. Previously, Monsanto announced its intent to defer all ongoing efforts to commercialize Roundup Ready wheat in order to accelerate and focus efforts to introduce biotechnology traits in other crops. All other regulatory submissions necessary to introduce Roundup Ready wheat have been withdrawn and Monsanto is not moving to commercialize the product at this time.



Weeds poke heads through soybean canopy this time of the year tempting growers to load up the sprayer for one last trip across the field. The impact of the weeds on crop yields have already occurred but weed seed production and harvest problems can be reduced.

The adoption of Roundup Ready technology has made late_season treatments more attractive because of minimal potential for crop injury (except for wheel tracks) and the ability of glyphosate to control large weeds. However, the potential value of these late season treatments should be evaluated prior to committing to spraying. Realize that impact on yields has already occurred, thus little yield benefit is likely to be achieved. If the weeds have not initiated seed set at the time of application it should be possible to reduce seed production. However, if the fruiting structure is visible, it is unlikely that killing the weeds at this late date will influence seed production or viability of the seed. Many people think that late_season treatments will reduce the viability of seeds that are produced, but research has consistently shown that seeds that have been initiated at the time of application are unlikely to be greatly influenced. The other possible benefit of the late season treatments is harvesting efficiency, and there may be situations where this may make the treatment worthwhile.



"Dicamba Injury to Soybeans" is a new publication from University of Wisconsin that discusses the three most common sources of dicamba exposure to soybeans, symptoms of dicamba exposure on soybeans and the potential for yield loss. This publication contains a photo gallery of dicamba injury symptoms and dicamba injury mimics on soybean. It is available from the Nutrient and Pest Management Program at (608) 265-2660. Or you can check it out on the web at http://ipcm.wisc.edu/

Richard Zollinger
NDSU Extension Weed Specialist

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