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Herbicide-resistant weeds are spreading, and it’s important to know how to recognize them. “The problem is much more widely spread than what I think many of the growers recognize,” says Mike Owen, Iowa State University Extension weed specialist.

How do you recognize the problem?
It starts by understanding what resistance visually looks like, and knowing what herbicide resistance is by definition. Owens describes herbicide resistance as the inherited ability of a plant to survive a normally lethal dose of herbicide from the repeated use of the same control tactic.

“In most instances, growers think it’s the train-wrecked fields, it’s those that are total disasters, but the truth be told, resistance is scattered plants and pockets of plants out in fields they can observe, above the soybean canopy, in September and October as they are combining,” says Owen.

Three different factors affect weed adaptation to the environment, says Owen.

1. Genetic, such as:

  • Mutation rates
  • Mode of inheritance 
  • Mechanism of resistance

2.  Biological, such as:

  • Species life cycle of the plants
  • Germination biology
  • Population size and density of the plants

Understanding the biology of the weed is very important, says Jason Norsworthy, University of Arkansas weed scientist. Knowing key factors including when it emerges, growth rate, and when flowering occurs can help you to find its weakness. 

3. Operational, such as:

  • Herbicide efficacy
  • Frequency of use
  • Timing of use
  • Herbicide use patterns – including rotations or mixtures
  • Crop rotation
  • Herbicide rate
  • Cultivation practices

Here is where farmers can take matters into their own hands. Knowing the weed biology and using diverse management practices, including planting into weed-free fields, can help to delay the ability of the weed to become resistant. Other practices, such as managing field borders and preventing weed seed movement, can also help.

“We have to manage for the weed before it becomes a major issue,” says Norsworthy. Scouting fields and identifying potential problem weeds can help you do so.

Moving forward

“There is absolutely nothing new in the pipeline that is coming to solve
these resistance issues in terms of new herbicide mode of action,” says
Norsworthy. “There’s nothing coming in the next five to eight years.”

herbicides are only used to control the few weeds that come through a
residual herbicide,” says Norsworthy. “Our first line of defense is not a
postemergence herbicide.”

Owens points to diversity in management plans. These strategies include cultural, mechanical, and herbicidal strategies, he says. The key is making sure the chosen herbicide is effective. “The way to know whether or not they are effective is to look at the herbicide use history, and also to recognize the resistances that do exist in your particular area,” says Owen.

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