Waterhemp in Clarksdale, MO.

It’s no shock to growers that the number of herbicide-resistant weeds in corn and soybean fields is increasing across the Midwest. As these weeds prove to be a difficult problem, field experts continue to work to find solutions for managing them.

University of Missouri researchers recently confirmed a waterhemp strain that resists 6 different herbicide sites of action (SOAs) after growers in north-central MO found a population of waterhemp that resisted several herbicides. They identified that the waterhemp population was resistant to the following:

• Chlorimuron (Group 2)
• 2,4-D (Group 4)
• Atrazine (Group 5)
• Glyphosate (Group 13)
• Fomesafen (Group 14)
• Mesotrione (Group 27)

The researchers noted that resistance to the SOAs above likely developed due to dependence on solo active ingredients. Unfortunately, the introduction of multiple SOAs occurred after resistance had already become a significant issue. By that point, adding another active ingredient did not provide multiple effective SOAs, it only worked to shift the selection pressure to the next active ingredient in the program.

A key detail to note about the study is that although some plants had stacked resistance, researchers said they did not find individual plants resistant to the 6 SOAs. In other words, overall management will still be improved from the use of multiple SOAs.

The best strategy for using herbicides is still to proactively include multiple effective SOAs in weed management programs and to not depend on any single SOA for control in either a pre- or post-emergence application.

In soybeans, the use of Syngenta premixes like Boundary®, BroadAxe® XC and Prefix® as soil-applied herbicides are key to managing multiple resistant pigweed species. In some areas, only glufosinate and dicamba are viable post-emergence options, but these products require a soil residual tank-mix partner in order to be effective. In such cases, Boundary, BroadAxe XC and Prefix are an excellent fit. In PPO-susceptible populations, Flexstar® and Flexstar GT 3.5 can be paired with strong residual pre-emergence products for effective management.

To combat weeds in corn, Syngenta recommends a 2-pass system of Acuron® or Acuron Flexi pre-emergence, followed by Halex® GT post-emergence for an additional SOA. Each corn herbicide premix contains multiple SOAs for built-in resistance management and provides long-lasting residual control, especially when layered in a 2-pass system.

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