MINNEAPOLIS — Tall waterhemp (Amaranthus tuberculatus) is expanding its reach across Minnesota, and herbicide-resistant populations are becoming more commonplace. Most waterhemp populations have been resistant to ALS (Group-2) herbicides, such as Pursuit, for a while. In addition, glyphosate-resistant (Group-9) populations were first reported in 2007, and PPO-resistant (Group-14) populations were confirmed in southern Minnesota the past two growing seasons. Herbicides in Group-14 include Cobra, Flexstar and Spartan. To add to management challenges, some waterhemp populations have developed resistance to two or all three herbicide groups. In this situation, what herbicide control options are left?

The case for layering residual herbicides

Once postemergence applications of ALS-, glyphosate- and PPO-herbicides have lost effectiveness against waterhemp, a farmer planting Roundup Ready soybeans would have no viable postemergence herbicide options left to control waterhemp. Switching to a LibertyLink or dicamba-tolerant soybean would provide postemergence control options, but late application or overuse of these systems is not a long-term solution either. As history has shown, over-reliance on any one herbicide over time can be a great way to select for resistance to that herbicide.

The extended emergence pattern of waterhemp poses further challenges. Even when a residual herbicide is applied at planting, late-emerging waterhemp may not be controlled as the amount of herbicide remaining in the soil may be below the amount needed for effective control. The length of effective control is influenced by the herbicide applied and the application rate.

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Source: Morning Ag Clips
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