However, each component of a herbicide premix or tankmix 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 populations, it is not as common or widespread as herbicide resistance in other weed species. Many soil-residual—herbicide premixes 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 premix or tankmix 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 premixes satisfy these criteria when giant ragweed is the target species.
The timing of a herbicide premix or tankmix application can also influence the effectiveness of each component. For example, a farmer may choose to use Harness Xtra (or any other premix 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.
If it is applied before waterhemp seed has germinated, each component of this premix (acetochlor and atrazine) provides effective residual control. However, if the Harness Xtra is not applied until after waterhemp emerges, only the atrazine can effectively control the emerged waterhemp plants.
Similarly, said Hager, "A tankmix of a chloroacetamide herbicide and glyphosate applied after waterhemp emergence does not contain more than one effective mode of action to control emerged waterhemp, nor does it contain more than one effective mode of action for soil-residual control of later-germinating waterhemp."
Illinois waterhemp populations have evolved resistance to herbicides from at least five herbicide site-of-action families, including inhibitors of acetolactate synthase (ALS), photosystem II (PSII), protoporphyrinogen oxidase (PPO), enolpyruvyl shikimate-3-phosphate (EPSPS) and hydroxyphenyl pyruvate dioxygenase (HPPD). "While not every individual waterhemp plant is resistant to one or more herbicides, most field-level waterhemp populations contain one or more types of herbicide resistance," said Hager.
Perhaps even more daunting is the occurrence of multiple herbicide resistances within individual plants or fields. Waterhemp plants and populations demonstrating multiple herbicide resistance are becoming increasingly common, can be more difficult to manage, and few herbicide modes of action are effective for their control.
For example, most Illinois waterhemp populations have developed resistance to ALS-inhibiting herbicides, which is often "stacked" with resistance to herbicides from other mode-of-action families. Soil- and foliar-applied herbicide premixes containing various ALS-inhibiting herbicides are numerous. While many of these products do have multiple modes of action, the ALS inhibitor component cannot control ALS-resistant plants.
In a postemergence scenario, applying a premix or tankmix of a diphenylether (PPO inhibitor) and glyphosate (EPSPS inhibitor) provides two effective modes of action against sensitive waterhemp, one effective mode of action against waterhemp resistant to either PPO inhibitors or glyphosate, and no effective modes of action against waterhemp resistant to both PPO inhibitors and glyphosate.
In summary, using herbicides from multiple mode-of-action groups may help reduce the selection intensity for resistance to a particular active ingredient of a herbicide. However, many factors determine the effectiveness of each premix or tankmix ingredient against the weed species of concern.