Waterhemp resistant to protoporphyrinogen oxidase (PPO) herbicides is spreading. It’s been confirmed in seven Minnesota counties — Douglas, Faribault, Jackson (a second location), Mower, Stearns, Watonwan and Yellow Medicine.

“I view these data as an indication that resistance will occur, it’s inevitable, and another reminder that farmers need to develop a weed management strategy that includes scouting, positive identification of weeds and development of a strategy for the crops they intend to grow and the herbicides they intend to use,” says Tom Peters, North Dakota State University and University of Minnesota Extension sugarbeet weed specialist.

SPRAY STRATEGY: A farmer sprays sugarbeets to kill tough-to-control weeds that may become resistant to frequently used herbicides.

Waterhemp populations not effectively controlled by early-summer postemergence applications of PPO-inhibiting herbicides may be resistant to PPO-inhibiting soybean herbicides such as Cobra (lactofen), Flexstar (fomesafen), Marvel (fluthiacet-methyl and fomesafen) and Ultra Blazer (aciflurofen), he says.

“I would be very mindful of three tough-to-control weeds — common ragweed, waterhemp and kochia — and develop a plan across crops that defines how you intend to control these weeds in each crop and defines mixtures with multiple modes of action targeted at these weeds. You can’t use multiple modes of action for all weeds, but you’d better develop a strategy for the tough weeds.”

The challenge for corn and soybean growers will be when to use Enlist and Xtend technologies, Peters points out.

“A farmer will need to declare how and when they intend to use growth regulator herbicides (SOA4) in the crop sequence.”

Small-grains growers who also grow soybean need to think carefully, too.

“I recommend growth regulators [herbicides] only once in the corn-soybean sequence, especially if a farmer is growing Roundup Ready crops. I suggest using Laudis plus atrazine post in corn if a farmer intends to use growth regulators in soybean,” he says.

Example
Peters provided the following example of a weed management strategy for control of waterhemp and common ragweed across corn, soybean and sugarbeet. 

“Note that I use acetamide herbicides across the crop sequence since there are no known resistant weeds from this family of herbicides. Thus, I believe the risk is slight from their repeated use,” Peters says. “I believe it is critical to use three effective herbicides against tough weeds like waterhemp and common ragweed.”

Confirming resistance
Determining whether you have herbicide-resistant waterhemp can be difficult. Bad weather, weed height when they are sprayed, antagonism with another herbicide in the tank or using the wrong adjuvant could all contribute to poor control, Peters says.

Purdue University and the University of Illinois offer molecular herbicide-resistance testing to determine if waterhemp populations have a specific genetic sequence that confers resistance to PPO-inhibiting herbicides.

Learn more about those services at:

* Purdue — ag.purdue.edu/btny/weedscience/Documents/2016HerbicideResistancescreeningform.pdf

* University of Illinois — web.extension.illinois.edu/plantclinic/downloads/WaterhempFlyer.pdf