When glyphosate-resistant waterhemp was first discovered in Lambton County in 2014, University of Guelph weed scientist Peter Sikkema never imagined the course the weed would take across Ontario, or the speed it would travel.
In 2020, Sikkema and his team collected seeds in Leeds & Grenville County. “So that means glyphosate-resistant waterhemp has moved 700 kilometers in six growing seasons, which really surprises me.” The invading resistant weed, often misidentified as redroot, green, or smooth pigweed, has now been found in 14 counties across the province.
Sikkema shared these insights during a Ridgetown/Simcoe virtual agribusiness breakfast meeting earlier this month. Even more concerning he says is the proliferation of multiple resistant waterhemp. In eight of the counties, four-way resistance has been confirmed, making the weeds resistant to Group 2, 5, 9 and 14 herbicides.
The course and speed of the spread has left Sikkema and his research team scratching their collective heads. “The rapid movement of Canada Fleabane across Ontario is quite understandable — it’s windblown seed,” he says. “But I don’t fully understand how the waterhemp seed is moving across the the province as quickly as it is.”
In soybeans, Sikkema’s research reveals that the average yield impact of waterhemp is 43 per cent, but he has documented losses of up to 93 per cent in the highest populations observed in Ontario. Similar yield losses have been observed in corn.
Another mystery Sikkema is wrestling with is genetic differences of waterhemp found in the province. He notes that the profile of resistant seeds found in different fields in Leeds & Grenville County is quite distinct and different.
“That would suggest that there are multiple different sources of waterhemp seed in the county, 700 kilometres away from where it was first identified in the province,” notes Sikkema. “Obviously it’s coming from different sources.”
So where are all these waterhemp biotypes coming from? That’s a loaded question that’s difficult to answer, says Sikkema. He notes that University of Toronto research looking at the biotype of waterhemp found in Essex County indicates it’s almost identical to waterhemp found in Mississippi.
How did it end up in southwestern Ontario? That too is a puzzling question, says Sikkema, noting that seed, contaminated equipment, or migrating waterfowl are all possible sources.
Another twist to this story is the profile of the waterhemp first found in Lambton County — it’s almost genetically identical to the waterhemp that’s been found in Ontario for the last 150 years. That makes it distinctly different from the resistant weeds found in nearby Essex.
Sikkema is also trying to wrap his head around the movement of native Ontario waterhemp that’s resistant in Lambton County. “Historically, that waterhemp was only found along open water courses in the province. But something has changed in the last 20 years and it’s adapted to our corn and soybean production systems and now it’s found in commercial fields.”
How can farmers tackle the yield robber? In corn, based on Sikkema’s research, the Group 27 herbicides really are the backbone for managing multiple resistant waterhemp. Integrity (Group 14/15) has also provided good control.
In soybeans, the best soil-applied herbicide is Fierce, says Sikkema. However, he notes that while this product delivers the best level of control, the margin of crop safety can be quite narrow for soybeans in stressed environments. For more on waterhemp control options visit Field Crop News.