Waterhemp survives after postemerge herbicide
click image to zoomWaterhemp that survived glyphosate applied at 0.75 lb ae/acre. The photograph was taken 21 days after glyphosate was applied to 4-inch-tall waterhemp. In the past 10 days, Aaron Hager with the University of Illinois reports he and his fellow researchers have received more calls and inquiries about waterhemp plants that have survived applications of postemergence herbicides. The most common scenario is having a "noticeable" percentage of plants survive after applying glyphosate in soybean (at rates from 0.75 to 1.5 lb ae/acre). Many producers have said that within about 7 to 10 days after glyphosate was applied, it became obvious the waterhemp plants would survive. In some instances it appears that survival could be attributable to an application rate too low for the size of plants, to rainfall soon after the application, or to poor coverage of the target vegetation. In other instances the best explanation appears to be the evolution of a glyphosate-resistant population. Remediation of resistance situations will likely be challenging, and it is altogether possible that herbicidal control of surviving plants will not be achievable.
click image to zoomFigure 1. Distribution of PPO- and glyphosate-resistant waterhemp populations in Illinois. PPO-resistant populations have been determined over several years, while the distribution of glyphosate-resistant populations is based on samples submitted in 2010 only. Options to control surviving plants, whatever their herbicide sensitivity/resistance profile, include interrow cultivation and hand roguing. Some may scoff at the suggestion, but in many areas of the mid-south and southeastern United States, these tactics represent the few remaining viable options to manage emerged populations of herbicide-resistant weeds.
Viable herbicide options for control of surviving waterhemp plants depend on their resistance profile. Keep in mind that more than one type of herbicide resistance may be present in any given field. For example, surveys in 2010 (Figure 1) indicated that about a third of glyphosate-resistant waterhemp populations also demonstrated resistance to PPO inhibitors, and virtually all were also resistant to ALS inhibitors. If plants survived for a reason unrelated to herbicide resistance, retreating them with glyphosate could provide effective control. Be sure to select an application rate appropriate for the size of the target plants (up to 1.5 lb glyphosate ae/acre/application), to include NIS (if recommended on the glyphosate product label) and/or AMS, and to apply at a spray volume sufficient for good coverage of the target vegetation.
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