Waterhemp Resistance Spreads Across Ontario Farm Fields
Dr. Peter Sikkema of the University of Guelph recently shared insights into the growing challenge of multiple herbicide-resistant water hemp at the 2025 Great Ontario Yield Tour final event in Woodstock Ontario. Dr. Sikkema research highlights both the biology of the weed and practical management strategies in corn, soybean, and wheat rotations.
Waterhemp is a summer annual broadleaf weed, biologically distinct because it is dioecious -- meaning it has separate male and female plants. This leads to significant genetic diversity, which in turn drives rapid adaptation and herbicide resistance.
Dr Sikkema reports that in Ontario, waterhemp has developed resistance to five major herbicide groups: 2, 5, 9, 14, and 27.
Once considered relatively minor, waterhemp has surged in significance over the last decade as biotypes adapted to modern corn and soybean systems. Its ability to emerge from spring through late autumn makes it far more difficult to manage than other annual weeds.
Under ideal conditions, plants can grow more than three and a half metres tall (almost 12 ft), with research showing yield losses averaging 19 percent in corn and 42 percent in soybean and approaching total crop failure in heavily infested fields.
Ontario surveys confirm that waterhemp is now present in at least 17 counties, with some biotypes showing resistance to multiple herbicide modes of action simultaneously. This reality removes up to 80 percent of common herbicide tools from a farmer’s arsenal.
Dr. Sikkema’s trials on commercial farms not research stations underscore that no single product provides consistent control. Instead, a two-pass weed control strategy is critical. In corn, pre-emergence products such as Acuron (Syngenta), Converge XT (Bayer), or Integrity (Syngenta), followed by a post-emergence application like Liberty (BASF) or Shieldex (Gowan), have proven effective.
In soybeans, soil-applied herbicides such as Fierce (Nufarm), Authority Supreme (FMC), or Boundary (Syngenta), followed by post-emergence options tailored to the soybean trait system (Enlist, Xtend, or Liberty), achieve the highest control levels.
The key message, according to Dr. Sikkema, is that waterhemp’s prolific seed production and ability to evolve resistance demand a zero-tolerance approach. Farmers should aim for 100 perent control each year and prevent seed return to the soil. Without strict management, this invasive weed could remain a serious economic threat to Ontario agriculture for decades.
This article was created based on a presentation by Dr. Peter Sikkema during the Great Ontario Yield Tour event in Osgoode, Ontario, watch the video of his presentation on waterhemp below.