Tip •  2024-02-16

Why worry about resistant weeds now?

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Two words – weed resistance. While weeds are a general problem, competing for necessary crop nutrients, contributing to yield loss and reduced yield quality, the rise of herbicide-resistant weeds has farmers exploring new approaches to weed control. And the best time to create a plan is now, before the busy growing season or weeds get out of control.

Waterhemp worries

A robust weed control plan in Eastern Canada needs to account for resistant weeds, including waterhemp.

Waterhemp, a member of the pigweed family is the fastest-growing problem weed in Ontario. It was the first weed species in Ontario to evolve resistance to four distinct herbicide groups thanks to its ability to prolifically produce seeds with significant genetic diversity.

When it comes to crop competition, University of Guelph research trials have confirmed poor control of waterhemp can result in over 85 percent yield loss in both corn and soybeans.1

As of December 2023, 18 counties in Ontario have identified herbicide resistant waterhemp. Here’s what you need to know about waterhemp resistant populations in Eastern Canada:

  • Southwestern Ontario is home to the highest concentration of resistant waterhemp
  • Confirmed fields are spreading throughout eastern Ontario
  • 37% of resistant waterhemp is resistance to three modes-of-action
  • 27% of resistant waterhemp is resistance to two modes-of-action
  • 6% of resistant waterhemp is resistance to five modes-of-action
  • Resistance has been confirmed in herbicide groups 2, 5, 14 and 27

For a complete map of waterhemp resistant populations in Ontario, visit the Ontario Ministry of Agriculture, Food and Rural Affairs Crop Protection Hub.

What to watch for

Another member of the pigweed species – Palmer amaranth – is on the radar for Eastern Canadian farmers, agronomists and field specialists. Known as the most troublesome weed in U.S. agriculture, Palmer amaranth made its first Canadian appearance in a southern Manitoba field in 2021 and in Ontario for the first time in 2023.

The greatest concern with the discovery of palmer amaranth is its ability to rapidly generate resistance. Currently, resistance to nine different modes-of-action have been documented, including a population that was resistant to 6 different herbicide modes-of-action.

Farmers are encouraged to scout regularly for weed escapes, including any odd or suspicious looking plants. For help identifying Palmer amaranth, visit this Field Crop News post that includes images and descriptions of the weed.

How to manage resistant weeds

The first step to managing resistant weeds is to create a weed management plan that covers everything from herbicides to tillage and crop rotations to cover crops.

Consult your trusted agronomist or crop advisor for the latest information on herbicide resistant weed populations in your area and be prepared to scout diligently to address any concerns.

Tips for creating a weed management plan:

  • Include multiple effective modes-of-action in every herbicide application
  • Evaluate tillage practices and how they can help manage weeds
  • Consider cover crops to reduce weed pressure and to help manage early spring weed emergence
  • Add multiple herbicide applications at varying crop growth stages (like pre-emergence and post-emergence passes) to manage weeds before they become a problem
  • Rotate crops to introduce a wider choice of herbicides into your weed control programs.

For more information about weed control and making a plan, talk to your local retailer, agronomist or Corteva Agriscience Retail Territory Manager