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April Weed of the Month: Why Scout for Herbicide Resistant Weeds?

A growing problem that is costing farmers more money each year is herbicide resistant weeds. The primary driver for resistant weeds comes from the repeated use of the same herbicide with the same mode of action year after year.

What can be done? Scouting for herbicide resistant weeds throughout the growing season and even when harvest time comes around can be very beneficial. The earlier the detection, the more time there is allotted to eliminate the plant or patch by mowing, hand removal, and/or herbicide application before the plant(s) produces seed. Scouting for these weeds and defining where and who the "escapes" are can help determine what formulation of herbicide to use or what could be used next season. If seed from an herbicide resistant weed is allowed to be produced, the potential for resistance is greater the next generation. Roger Becker, a weed scientist at the University of Minnesota informs landowners that, "it is a much easier task to control resistant weeds early on in the invasion process before the seed bank has a chance to buildup, creating the potential for a high population expressing in subsequent crops, and ensure several years of effort to manage the problem due to seed dormancy." He also mentions that, "managers should rotate different modes of action for the herbicides that they use. Additionally, use of soil applied herbicides that have residual activity coupled with post emergence herbicide options with different modes of action will help delay the development of herbicide resistance."

How does a person identify an herbicide resistant weed in the field? There is no definitive way to positively identify a resistant plant in the field but there can be a greater suspicion, especially if targeted chemical controls are not effectively managing a plant population. If there is a suspicion, plants samples can be taken and brought to a lab to be tested. Jeff Gunsolus, another University of Minnesota weed scientist, says that, "there are lab-based diagnostic tests available for one mechanism of glyphosate resistance and one of two known mutations for resistance to PPO herbicides (e.g. Cobra and Flexstar). These are useful tests but unfortunately, a negative test for resistance does not guarantee that resistance due to another resistance mechanism isn't possible. The much more extensive and labor intensive weed grow outs and multiple rate applications are still the best way to determine resistance. Hence most farmers and consultants rely on past precedent and in-field symptoms."

One thing is for sure, no plant can be identified or differentiated for herbicide resistance with the naked eye in the field. Drones are being used in some parts of the country to look for these herbicide resistant plants, but they still have to be ground-truthed and laboratory tested to positively show herbicide resistance.

The University of Minnesota Extension has a webpage where they talk about weed management. Click here for the link.

 

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