Herbicide Resistance Update in Kansas and Colorado

A weed biotype that survives a herbicide treatment that previously was effective is considered to be herbicide resistant. There are numerous herbicide resistant weed species that are problematic in Kansas (Table 1) and Colorado.

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Figure 1. Herbicide Resistant Weed Species in Kansas2
Herbicide Site of ActionWeed Species
ALS InhibitorsKochia, Common waterhemp, Palmer amaranth, Russian thistle, Common cocklebur, Common sunflower, Shattercane, Cheat, Bushy wallflower, Flixweed, Japanese brome
Photosystem II (triazines)Kochia, Common waterhemp, Palmer amaranth, Downy brome, Redroot pigweed
EPSPS Inhibitor (glyphosate)Kochia, Common waterhemp, Palmer amaranth, Marestail, Giant ragweed, Common ragweed
PPO InhibitorsCommon waterhemp
Three way multiple resistance (ALS Inhibitors, Photosystem II (triazines), PPO inhibitors)Common waterhemp
Source: Modified from Herbicide resistant weeds. Department of Agronomy. Kansas State University. http://www.agronomy.k-state.edu/extension/weed-management/herbicide-resistance/
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According to Kansas State University, glyphosate-resistant pigweed species such as Palmer amaranth and common waterhemp are fairly common throughout Kansas.1 Many of the uncontrolled pigweeds from last season produced seed and will be a continuing problem in future years within those fields.

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The Roundup Ready® Xtend Crop System is an advanced weed management system, pending regulatory approvals, that is designed to help control herbicide resistant and tough-to-control broadleaf weeds in soybeans. Two of the components of the system are Roundup Ready 2 Xtend® soybeans and XtendiMax™ herbicide with VaporGrip™ Technology. Roundup Ready 2 Xtend® soybeans are built on the Genuity® Roundup Ready 2 Yield® technology. They are the only soybean products that provide tolerance to both dicamba and glyphosate.

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Figure 2

Figure 2. Relative volatility of various dicamba formulations.

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Dicamba and glyphosate herbicides are utilized to manage weeds pre-planting and as an in-crop option (up to and including beginning bloom (R1) stage) on Roundup Ready 2 Xtend® soybeans. XtendiMax™ herbicide with VaporGrip™ Technology is a new dicamba formulation which can provide a significant reduction in volatility compared to other commercially available dicamba formulations (Figure 2). XtendiMax™ herbicide with VaporGrip™ Technology, is now approved by the EPA for in-crop use and has state approval for in-crop applications in Colorado and Kansas.

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XendiMax herbicide with VaporGrip Technology also offers potential soil activity for up to 14 days on certain small-seeded broadleaf weeds depending on rainfall and soil type.

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The wide application window allows for control of small weeds (less than 4 inches) when they are more vulnerable, such as marestail at early burndown. The ability to use this product through the planting window will allow for Roundup Ready 2 Xtend soybeans to get a head start towards their high yield potential with minimal competition from weeds. With some other products, application restrictions limit use to 28 days or more before planting.

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A weed management program should include traditional residual herbicides in pre-emergence and post-emergence applications that have different sites of action. Targeting weeds that have not yet emerged or are small (less than 4 inches) will contribute to a more successful weed management program.

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Use of XtendiMax™ herbicide with VaporGrip™ Technology has various application requirements including nozzle selection, set-back distances from sensitive areas, spray volume, etc. Always read and follow the most current pesticide label requirements.

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Additional up-to-date information about the Roundup Ready® Xtend Crop System and the use of XtendiMax™ herbicide with VaporGrip™ Technology is available at the following websites.

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A key component to potentially reducing the frequency of unwanted herbicide resistant weeds is to use herbicides or tank mixes with different sites of action.

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A resource is available at http://takeactiononweeds.com/ which lists numerous herbicides by their site of action. The resource, which is supported by the United Soybean Board and industry, contains a Site of Action Lookup Tool and a downloadable Site of Action Chart. An additional source of herbicide recommendations focusing on reducing herbicide resistance issues is available from Roundup Ready PLUS® Crop Management Solutions, at: https://www.roundupreadyPLUS.com/.