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How To Effectively Use The Herbicide Chart

The message from weed scientists to producers to rotate and include multiple modes of action and sites of action in their corn and soybean herbicide programs has intensified with the increasing number of acres infested with herbicide resistant weeds. Along with that message have come many tools to help farmers ensure the proper rotation of herbicides, including the Corn and Soybean Herbicide Chart from the glyphosate weeds and crops working group. This chart has been distributed by university weed scientists to aid producers in determining herbicide programs with optimal site of action rotation. The Purdue Weed Science program has noticed that many producers would not use the chart if not properly instructed on how to use the chart. We emphasized instruction on how to use the chart at meetings this past winter. The following is an extension of this effort to explain the chart layout and how Purdue weed scientists are encouraging producers to use the chart when planning their weed management program.

Mode of Action vs. Site of Action

The first explanation to end some confusion is the difference between “mode of action” and “site of action”. Sometimes the terms are thrown around and interchanged and can become confusing as to what is what.

  • Mode of action: refers to the way in which the herbicide effects plant growth (visual symptoms on the plant) and eventual death at effective doses •
  • Site of action: refers to the specific enzyme site or pathway that the herbicide binds or inhibits to create the plant growth effects

In simplistic terms the sites of action are subsets of the broader modes of action. Several modes of action only have one site of action while others have two or three sites of action. To ultimately increase the number of chemicals available for use in a rotation, producers should focus on “Sites of Action” rather than “Modes of Action”. The Corn and Soybean Herbicide Chart was designed around the Sites of action and WSSA assigned Site of Action group.

Chart Layout

The chart actually contains two charts, “By Mode of Action” and “By Premix”, which are linked by a color-coding system. The “By Mode of Action” chart would be the large chart under the large black header and the “By Premix” chart being the smaller chart on the far right.

“By Mode of Action”

The “By Mode of Action” table is a grouping of active ingredients and products containing single active ingredients into chemical families, sites of action and lastly modes of action. The single ingredient products and active ingredients are listed individually on the right of the chart and progress in their groupings to the left. Each column of the chart from the left to the right is explained below:

  • Mode of Action: As explained previously, this is the broad grouping of herbicides by their effect on plant growth. Within this chart the Modes of action are separated by brackets and different colors 
  • Site of Action Group: The site of action group is a numerical value that has been assigned to each site of action by the Weed Science Society of America. The site of action group numbering system was designed for quick and simplistic recognition rather than using the complicated scientific names that can be cumbersome and confusing for producers. A colored box that corresponds to the mode of action that each site of action belongs encloses and represents each site of action group number. Modes of action with multiple sites of action have different shades of the mode of action color representing each site of action (i.e. Site of action groups 5, 6, and 7 are all Photosynthesisinhibitors and are represented by three shades of green.)
  • Site of Action: The site of action as explained above is the site or physiological pathway that the herbicide binds or inhibits. Again the sites of action are subsets of the mode of action and should be the focus of herbicide program rotations. Each site of action will have a site of action number as described above.
  • Number of resistant weed species in U.S. The numbers encircled in black dots represent the number of weed species that are resistant to each herbicide site of action.
  • Chemical Family: The grouping of herbicides within each site of action by their chemical structures.
  • Active Ingredient: The accepted common chemical name of the actual component that is responsible for growth effects, injury, and death of susceptible plants. This will be listed on the front of every herbicide label on the front panel under active ingredients.
  • Product Examples (Trade Name®): The marketed or trade name of products that only contain only one active ingredient that is listed to the immediate left.
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