In the last two decades, glyphosate has been used extensively throughout the Midwest for broadleaf and grass weed control in glyphosate-resistant corn and soybean cropping systems.
Its popularity stemmed from its broad-spectrum effectiveness after the introduction of glyphosate-resistant crops in the late 1990s. Its low cost and flexibility in application timing encouraged widespread adoption and use.
Reliance on glyphosate as the primary method of weed control — rather than part of an integrated strategy using several weed control options — has allowed for glyphosate-resistant weeds. Common ragweed, common waterhemp, giant ragweed, marestail, kochia and Palmer amaranth have been confirmed glyphosate-resistant in Nebraska.
With glyphosate no longer an option for the control of these glyphosate-resistant weed 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 by University of Nebraska Extension in 2015 and 2016 near Adams, where glyphosate-resistant common ragweed was first confirmed to evaluate herbicide programs for the control of glyphosate-resistant common ragweed in LibertyLink (glufosinate-resistant) soybean. Glyphosate-resistant common ragweed can be controlled effectively by using a diversified preplant and post-herbicide program in LibertyLink soybean.
COST BREAKDOWN: Cost of herbicide programs for controlling glyphosate-resistant common ragweed in dryland LibertyLink soybean, income from soybean yield, and gross profit margin in a field experiment conducted in Gage County in 2015 and 2016.
Common ragweed has an early-spring emergence in Nebraska, with 90% total emergence reached before mid-May. This necessitates using a preplant burndown herbicide to control emerged common ragweed before planting:
• 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.
• Liberty applied postemergence alone or in a tank mixture with Prefix, Pursuit or Warrant controlled 84% to 98% glyphosate-resistant common ragweed.
• For best control and to reduce the evolution of herbicide resistance, no single mode of action should be used preplant or postemergence.
• 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.
The cost of preplant followed by postemergence 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 (see table).
• Single application herbicide programs were cheapest but resulted in much less profit due to low soybean yield.
The preplant, preemergence, postemergence program provided equal control of glyphosate-resistant common ragweed as the preplant, early-post, late-postemergence program.
This report comes from UNL CropWatch.