“We need to be very diligent with the management of our herbicides,” said Kevin Carlson, Federated’s agronomy manager, as he described how growers are “down to using [herbicides with] multiple modes of action” to fight the toughest weeds, especially waterhemp.

Waterhemp has shown itself good at adapting to herbicide modes of action. Most recently, in Illinois, it was documented to be resistant to Group 15 herbicides, “which is particularly alarming,” said Carlson. This new development means the list is getting shorter for herbicides that effectively fight weeds such as waterhemp.

Learn from others.

Although there is no predicting when the resistance will hit Minnesota corn and soybean fields, growers need to learn from what’s happening in nearby states. “It if happens somewhere else, it can happen here,” said Carlson – especially relating to waterhemp. To date, he noted, waterhemp is resistant to seven different sites of action.

“It’s up to us to be stewards of the sites of action,” said Carlson. “We need to do everything we can to stave off resistance in [waterhemp].” That stewardship begins with pre-emerge herbicides. “Use a pre-emerge with multiple modes of action if there is an established or significant waterhemp issue,” he said.

Then, post-emerge applications need at least two modes of action, and those modes must be effective modes. “Consider what is effective” when choosing herbicides, said Carlson. For example, Xtendimax® (dicamba) plus glyphosate offers only one effective mode of action, not two, on waterhemp. Alternatively, Acuron (see related article) for corn has four different ingredients and three different modes of action that work to kill weeds and manage resistance.

Timing matters.

The timing of herbicide applications for weed resistance management is also important. “Pre-emerge herbicides have to take priority,” said Carlson. Custom application requires another level of timing, and growers need to get plans in place to accommodate that.

The Federated Agronomists are ready to help with those plans – from field mapping to what’s planted to herbicide choices. “That planning can be done now,” said Carlson, by mapping out a Plan A and a Plan B.

Once planting begins, it “all comes down to communication” between growers, agronomists, and applicators as weather and other factors determine which plan goes into effect. But remember: Both Plan A or B should include good pre-emerge herbicides.