If it is applied before waterhemp seed has germinated, each component of this pre-mix (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 tank-mix 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 pre-mixes 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 pre-mix or tank-mix 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 pre-mix or tank-mix ingredient against the weed species of concern.