Resistance Management School: Watch out for waterhemp

Earlier this year, Dr. Peter Sikkema, professor with the University of Guelph and his research team confirmed Group 14 resistance in waterhemp at multiple locations in Ontario.

Sikkema’s research team, including graduate student Lauren Benoit, has also confirmed waterhemp with cross-resistance to Groups 2, 5, and 9, as well as 14. In this episode of the Resistance Management School, Benoit takes a close look at waterhemp and notes how the small-seeded summer annual broadleaf weed is closely related to redroot and green pigweed. She offers tips on how growers can distinguish waterhemp – it is completely hairless while redroot and green pigweed both have hairs on the stem.

Since 2015, Sikkema’s team has surveyed 76 sites across Ontario and Quebec. The sites with confirmed glyphosate resistance are located in Essex, Chatham-Kent, and Lambton counties, and the southwest corner of Middlesex county, as well as in Quebec. That footprint is expected to grow as new reports of the weed’s presence are investigated and confirmed.

“This development continues to limit the control options for waterhemp that farmers have, and reiterates the need to use integrated weed management strategies including proper herbicide stewardship practices,” says Sikkema. “Tank-mixing and rotating effective modes of action is strongly recommended to reduce the development and slow the spread of resistance.”

In the video below, Benoit discusses why farmers need to be concerned about waterhemp, which emerges throughout the growing season and is a prolific seed producer – up to 300,000 seeds per plant. She notes that equipment serves as the primary dispersal mechanism for the weed so it is critically important for growers to be diligent when purchasing used combines and other equipment that may have been exposed to contaminated fields.

Lauren Benoit shares waterhemp management tips at the University of Guelph’s Cottam, Ontario, research location. Story continues after video.

Benoit also reviews the most effective options for controlling resistant waterhemp. Based on Sikkema’s research, the Group 27 herbicides continue to be a strong option for Ontario corn growers: Acuron and Lumax applied pre-emergence provide 91% and 87% control respectively. Callisto plus Atrazine, and Marksman applied post-emergence to 10 cm-tall waterhemp provided 92% and 87% control, respectively.

Multiple-resistant waterhemp will be a larger problem for soybean producers. Group 15 herbicides (Dual, Frontier, or Zidua) plus metribuzin applied pre-emergence provide good control. Post-emergence, Reflex and Liberty (in LibertyLink soybeans) are the best options, but should be used in a two-pass program. Research from Mike Schryver shows 95% control of waterhemp using Boundary followed by Liberty in a two-pass program.

Click here for more Resistance Management School episodes.

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