Ontario Agronomists Warn of Rising Waterhemp Pressure and Emerging Palmer Amaranth Risk

Key Takeaways

• Waterhemp now dominates fields in Essex County and Chatham‑Kent
• Average waterhemp yield loss reaches 43% across crops
• Palmer amaranth first found in Wellington County, 2023
• Ontario Palmer amaranth already resistant to herbicide groups 2,5,9
• Early detection and manual removal key, per North Dakota success

Pulse Analysis

Herbicide‑resistant weeds have become a headline issue for Ontario’s grain belt, with waterhemp leading the charge. Once confined to isolated pockets, the species now blankets much of southwestern Ontario, delivering average yield penalties of 43 percent and, in extreme cases, total crop loss. Researchers at the University of Guelph quantified the impact, noting corn yields can fall 19 percent while soybean fields may see up to 100 percent loss when waterhemp escapes control. These figures underscore the economic urgency for growers to move beyond reliance on post‑emergence herbicides, which many still favor despite diminishing efficacy.

The appearance of Palmer amaranth in Wellington County last year adds a new layer of complexity. Known in the United States for its aggressive growth and prolific seed production, the pigweed has already shown resistance to multiple herbicide modes of action—groups 2, 5, and 9—in Ontario’s sole documented population. This mirrors the challenges faced by U.S. producers for three decades, where Palmer amaranth has driven up production costs and forced the adoption of integrated weed‑management plans. Ontario agronomists point to North Dakota’s grassroots campaign, which combined field scouting, rapid removal, and grower cooperation, as a practical template for curbing the weed before it establishes a seed bank.

Effective mitigation will hinge on three pillars: accurate early identification, diversified control tactics, and coordinated industry action. Training programs that help agronomists and growers distinguish waterhemp and Palmer amaranth from look‑alike species are already being discussed, as misidentification remains a major barrier. Diversifying tactics—rotating herbicide modes, integrating cover crops, and employing mechanical or manual removal—reduces selection pressure and preserves herbicide efficacy. Finally, a collective response, modeled after the North Dakota effort, could involve shared scouting data, rapid‑response teams, and incentives for growers who proactively eradicate infestations. Implementing these steps now can protect Ontario’s agricultural productivity and limit the long‑term economic fallout of resistant pigweeds.