Interest in using multiple modes of herbicide action in weed management programs is increasing, according to Aaron Hager, University of Illinois associate professor of weed science.

However, each component of a herbicide pre-mix or tank-mix with multiple modes of action is not necessarily effective for every weed or under all application conditions.

For example, giant ragweed, a large-seeded, summer annual, broadleaf weed species, can be difficult to control with a single herbicide. Because of its extended germination and emergence characteristics, farmers may have to use more than one herbicide or multiple herbicide applications.

While herbicide resistance has been found in the Illinois giant ragweed population, it is not as common or widespread as herbicide resistance in other weed species. Many soil-residual herbicide pre-mixes containing two or more active ingredients are available to farmers who want to be proactive and use multiple modes of action to reduce the selection for herbicide resistance in giant ragweed.

However, "To effectively reduce the selection intensity for resistance to a particular active ingredient, each component of the pre-mix or tank-mix should have similar efficacy against the target weed species," Hager said. "In addition, each component should demonstrate similar soil persistence." Few commercial soil-applied herbicide pre-mixes satisfy these criteria when giant ragweed is the target species.

The timing of a herbicide pre-mix or tank-mix application can also influence the effectiveness of each component. For example, a farmer may choose to use Harness Xtra (or any other pre-mix of a chloroacetamide and atrazine) to control waterhemp, a small-seeded summer annual broadleaf weed species that has developed resistance and is becoming a serious problem in Illinois fields.