ISSUE 8   July 3, 2008


Glyphosate has been the herbicide of choice for weed control for a decade and for good reason. It controls weeds well without injuring soybeans and its price kept dropping, which made the choice easier. However, the trend of low priced glyphosate changed in 2008. Glyphosate prices have spiked because of reduced glyphosate supplies and increased demand.

Does this change how glyphosate is used in soybeans? Glyphosate will still be the backbone for most corn and soybean weed control programs, but the higher price may have a couple consequences. First, some people may be tempted to reduce glyphosate rates, which may lead to poor control if the weeds are too large when sprayed. Some people also suggest that reduced rates may contribute to a shift towards more tolerant weeds. Regardless of price, apply the glyphosate rate that is appropriate for the weed species and size in fields. Avoid the temptation of reducing rates below those needed for effective control just to save costs.

The second consequence of higher glyphosate prices is that it may open the door for some other herbicide options. Consider the preemergence (PRE) herbicides. The cost of several PRE herbicides that are recommended as foundation treatments in Roundup Ready soybeans may be similar to the current cost of glyphosate. "Foundation treatments" often refer to PRE herbicides that are applied at lower rates to provide early season weed suppression. They can help to control weeds that might be tougher to control with glyphosate alone or provide early season weed suppression so that glyphosate can be applied closer to soybean canopy closure. As a result, more consistent and higher levels of weed control are typical (based on university test results). Adding a PRE herbicide also brings another herbicide mode of action into the weed management program, which may reduce the selection pressure for glyphosate-resistant weeds.

Conditions where 2 glyphosate applications may be required:

  • 30-inch row soybeans
  • Weeds with extended emergence (ie waterhemp, common ragweed)
  • Tough to control weeds (See page 107 in the ND Weed Control Guide)
  • Standard 2 pass glyphosate* = $12 + $12 = $24/A
    PRE herbicide* and POST glyphosate = $12 + $12 = $24/A

    *Actual costs will vary depending on herbicide and rate, but can be very similar.

    Most of the foundation herbicide programs have been promoted to improve broadleaf weed control and many of the common options are listed below. Some other herbicides provide effective control of a mix of broadleaf and grass weeds while others are primarily effective on grass weeds. Glyphosate is only one of many items with a rapidly changing price. The prices of crops, fertilizers, seed, fuel, and land are all in flux and it may be tempting to cut costs. However, remember that the goal of a weed management program is to protect yield. The old saying about being penny wise and pound foolish certainly applies to weed management. Herbicides are a wise investment. This investment may now include more than just glyphosate. The price increase of glyphosate should make us consider if other herbicide options have a fit in soybean systems and provide benefits to improve overall weed management.



    I have received a few calls about the need for residual activity with postemergence herbicide applications. Iím going to focus on grass weeds, which were the basis for the calls. Of course, every situation differs. In some areas, weeds had emerged before it rained to fully activate preemergence herbicides. These fields may still get some benefit from the preemergence herbicides, but the escapes still need to be controlled. Other fields that are being treated entirely postemergence may have a greater need for residual activity as there was no residual herbicide in the system yet.

    Letís consider a crop planted on May 1 for an example. We would typically like to have 8 weeks of residual weed control provided by a preemergence herbicide. By that time (July 1), the corn should be canopied and shading out most weeds emerging during the rest of the season. Plus, weed germination is also greatly diminished by that time of the summer. If we are making a postemergence application on June 1, we normally donít need 8 weeks of residual activity anymore. Rather, about 4 weeks of residual activity should get us to the same July 1st date. With the cool weather this may be pushed back. We can get that residual activity two different ways. One way is to get the residual activity from the postemergence herbicide. The postemergence grass herbicides differ in the amount of residual activity that they provide. Under conditions where a rain is received a couple days after application, which would activate their residual, I would rank the herbicides in the following general order: glyphosate (no residual), Liberty (< 1 week), Option (about 1 week), Accent and Stout (1-2 weeks), Steadfast and Resolve (2-3 weeks). Of course, these estimates will vary on the weed species, their density, rainfall, etc. and should be used as a relative guide. So, some of these herbicides can provide some residual and may be sufficient especially if cleaning up escapes from a poorly activated preemergence herbicide. Others would benefit from additional residual activity.

    A second way to get residual activity is to add a preemergence herbicide to the tank mix. A full rate is not needed anymore since 4 weeks have passed since planting in this example. A half rate should be adequate in many cases at this point in the season. Using a half rate also makes the addition of a residual herbicide more palatable when considering the cost. Herbicides to consider include those in the acetanilide family (i.e. Dual, Harness, Outlook, etc.) and Prowl. These herbicides do not have postemergence activity on grasses, but can provide the desired residual activity. Their benefit would be most obvious with herbicides like glyphosate and Liberty.

    A wide range of field situations and needs certainly exist in corn fields across the area. However, if the time point in the season (the time between planting and canopy) and some basic principles are considered, appropriate decisions on the need and benefits of residual grass herbicides can be made.

    Rich Zollinger
    Extension Weed Specialist

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