Mode of action has been talked about for some time in relation to preventing herbicide resistance by weeds, but now site of action is also being stressed by the crop protection industry.

“We want the use of herbicide programs with multiple modes of action, and we also want to make sure that we hit multiple sites of action in the plant, too,” said John Rice, Bayer CropScience technical sales consultant (TSC) in Illinois.

The spread of herbicide-resistant weeds has made selection of herbicide programs more complicated than ever before, and most growers can benefit from assistance in developing a program.

Sam Knott, Crop Production Services (CPS) marketing and sales manager, who also serves as area agronomist for 18 counties in Illinois, said, “A big part of our assistance to farmers involves identifying the problem and finding different ways to resolve it.”

Knott said that growers have to start with clean fields from clean tillage or a herbicide application. After that is control of summer annual weeds with the hot concerns today being waterhemp and Palmer amaranth, both of which are spreading throughout Illinois, he said.

The crop protection industry agrees that a preemergence residual herbicide needs to be applied, and Knott preaches the use of at least two modes of action and prefers three different modes of action for a herbicide combination.

“There are very few products that come ready to use with three modes of action. Agronomists have to use their agronomic knowledge to come up with that third mode of action in many cases, especially to have different sites of action, too,” Knott said.

Follow-up has to be a postemerge herbicide, and mode of action again has to be a consideration. “It is an eye-opener for most farmers when they realize that a soil-applied herbicide can have the same mode of action as a post herbicide,” Knott noted. “One of the fastest ways to develop weed resistance is to use the same mode of action on the same weed species continuously. That is something I spend a lot of time in talking about with agronomists and growers so that there is use of herbicides with both different modes of action and sites of action.”

“Respect the Rotation” is a weed management and stewardship program by Bayer CropScience, which helps representatives educate ag retailers, crop consultants and growers about preventing herbicide resistance and dealing with resistance if it occurs. Mode of action and site of action is a main topic. Respect the Rotation “showcase events,” one-on-one discussions and themed meetings with retailer customers, are one aspect of this program, Rice noted.

Most ag retailer agronomists and crop consultants should have a copy of herbicides listed by mode of action and site of action, Rice said. Having a pdf in a computer or tablet “is a quick, simple way to have that information at their fingertips.”

What’s extremely important in any discussion about dealing with weeds is knowing what ones are a problem in a field. Knott said, “You have to do a thorough job of scouting because if you have a weed like waterhemp or Palmer amaranth, it can grow as much as two to three inches in a 24-hour window.”

Scouting after the herbicide application will provide evidence of problems if weeds are surviving in the field. Rice said confirmation of resistant weeds can be accomplished through local Extension offices, state weed specialists or the local Bayer CropScience rep having weed seed tested.

Rice said, “Our long-term goal is to make sure our customers understand why we have resistance today, how to manage the issues and prevent having problems down the road.”