Nebraska: Gaining The Upper Hand Against Resistant Common Ragweed

Over-reliance on glyphosate as the primary method of weed control has clearly pushed the selection of glyphosate-resistant weeds. In Nebraska, we have confirmed glyphosate-resistant populations of common ragweed, common waterhemp, giant ragweed, marestail, kochia, and Palmer amaranth.

With glyphosate no longer an option for the control of these glyphosate-resistant populations, growers are looking for herbicides with different modes of action. A diversified herbicide program for weed management needs to be used to avoid evolving resistance to multiple modes of action.

A diversified herbicide program should include at least two effective modes of action in each application and should rotate modes of action between years to keep selection pressure low.

A field study was conducted in 2015 and 2016 near Adams to evaluate herbicide programs for the control of glyphosate-resistant common ragweed in LibertyLink (glufosinate-resistant) soybean. Adams was where glyphosate-resistant common ragweed was first confirmed in the state.

Glyphosate-resistant common ragweed can be controlled effectively by using a diversified preplant and POST herbicide program in LibertyLink soybean.

Remember This: Common Ragweed Gets An Early Start

Common ragweed has an early spring emergence period in Nebraska with 90% total emergence reached before mid-May. This necessitates using a preplant burndown herbicide to control emerged common ragweed before planting (Figure 1).

• Herbicide options such as 2,4-D, Authority First, Optill plus Outlook, paraquat, Sharpen plus 2,4-D, or Valor XLT effectively controlled common ragweed before planting and up to three weeks after application (Figure 2).
• Liberty applied post-emergence alone or in a tank-mixture with Prefix, Pursuit, or Warrant controlled 84%-98% glyphosate-resistant common ragweed (Figure 3).
• For best control and to reduce the evolution of herbicide resistance no single mode of action should be used preplant or post-emergence.
• Single applications (only preplant or only POST) resulted in less weed control, greater weed biomass, and less soybean yield.
• Three applications (preplant, pre-emergence, late post-emergence or preplant, early post-emergence, late post-emergence) did not result in greater common ragweed control, weed biomass reduction, or soybean yield (Figure 4 and 5).

The cost of preplant followed by post-emergence herbicide programs ranged from $59.17 to $93.09 per acre with application and provided the highest gross profit margins compared to no weed control and single- or triple-application herbicide programs (Table 1).

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• Single application herbicide programs were cheapest but resulted in much less profit due to low soybean yield.

The pre-plant/pre-emergence/post-emergence program provided equal control of glyphosate-resistant common ragweed as the pre-plant/early-post/late-post-emergence program.

Source URL: https://cropwatch.unl.edu/2018/herbicide-options-managing-glyphosate-resistant-common-ragweed-libertylink-soybean