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Giant ragweed in southwestern Manitoba?

Photo courtesy of Scott Chalmers, MAFRD.

A weed more common to warmer climates in the United States and a few places in southern Ontario and Quebec — giant ragweed — made an unorthodox appearance in southwestern Manitoba in 2012. A few random metre-high plants showed up in research plots — a very unexpected sight.

“We were surprised to even find giant ragweed this far west in Manitoba,” Scott Chalmers, diversification specialist with Manitoba Agriculture Food and Rural Development (MAFRD) says. “Giant ragweed can occasionally be found in southeastern Manitoba along ditches around Morris or Steinbach, but this far into the southwest region is unexpected. This weed has primarily been a problem in the southern U.S. but has moved into places like North Dakota along the Red River. We expect this Manitoba population may have come from seeds that had floated north [from the States] during the 2011 flood along the Souris River.””

Giant ragweed (Ambrosia trifida) is an annual plant in the aster family and native to many places across North America. Giant ragweed reproduces from seed, is wind pollinated and can grow up to six metres under the right conditions, although two to three metres is more typical. In many places in the U.S., giant ragweed has become a super weed resistant to glyphosate. And in 2008, giant ragweed was the first glyphosate-resistant weed confirmed in Ontario. Its smaller relative, common ragweed (Ambrosia artemisiifolia), is a widespread, annual weed across North America, and herbicide resistant populations have also been reported, including glyphosate resistance in Ontario in 2011.

“The first patch of giant ragweed showed up in 2012 randomly in one of our plots, and was also found within the rest of the field locally,” Chalmers explains. “In 2013, Melita [Manitoba] experienced another minor flood along the Souris River and many acres in the valley were not planted. We found even more plants in the same field, but generally there were, on average, a couple plants per acre. We conducted glyphosate tolerance tests in the field (in vivo) on the few plants that were growing there, comparing different application rates.”

Overall, the glyphosate treatments did inflict severe damage on the plants, although in some plants, the seed-containing raceme did appear to survive while all leaves had browned off. One plant required a second application using a tank mix for full control.

“In 2014, we continued to monitor the population in our plots as well as a larger patch discovered further upstream,” Chalmers says. “About two weeks after the producer sprayed the patch with glyphosate, we noticed that quite a few of the plants stayed green. We worked with the producer to closely monitor the patch and discovered common ragweed as well. This patch had two bad weeds in the same place at the same time, both notorious for glyphosate resistance. There was considerable variation in the control of both common and giant ragweed plants, ranging from completely dead to completely alive and all sizes as well. This is characteristic of glyphosate resistance, so we took seed samples from dead plants and those that survived for testing from both species.”

Chalmers is working with University of Manitoba weed scientist Rob Gulden, who has sent the seeds to Ontario for resistance testing.

“With the recent flooding and excess moisture, we are getting cycles of weeds and weed seed banks that we don’t normally have to deal with,” Chalmers explains. “In North Dakota, common crop rotations of winter wheat, corn and soybeans may encourage weeds like giant ragweed, which can easily spread by seed through flooding events. As climate change potentially increases our temperatures to more what North Dakota has, these types of weed issues may creep north along with these crops. With cropping practices, increased precipitation, climate change and flooding concerns, it is almost like a perfect storm of pressure for weed problems like giant ragweed. So far our colder climate and more diverse crop rotations in Manitoba have reduced the risk of larger populations of giant ragweed spreading quickly.”

Giant ragweed has a large seed, similar in size to a wheat kernel. The seeds have two main strategies for reproduction. Seeds that remain on the soil surface can germinate within a few weeks in the same year or in the following spring. Seeds that get buried in the soil typically remain dormant for a year and germinate the next. Seeds can remain viable for several years.

“These strategies may be a product of glyphosate resistance in ragweeds adapting to current agricultural practices,” Chalmers says. “Those seeds that germinate early are banking on a continuous rotation of glyphosate-tolerant crops, whereas those that skip a year or two might be banking on wheat (for example) that would have herbicides that would have controlled or suppressed the glyphosate-resistant ragweeds.”

Producers in western Manitoba are encouraged to maintain fields with a diverse crop and herbicide rotation. Many other weed species exist in Manitoba with various levels of resistance to other herbicides such as Group 1 or 2 herbicides, some with multiple forms of resistance. Tank mixes of different chemical groups are a more effective way than increasing rates of a single chemical. Hand pulling small patches of suspected resistance is always the best control option. Use of unmanned aerial vehicles (UAVs) could prove beneficial in identifying these smaller patches, reducing overuse of herbicides.

“We will continue to monitor the river flats where there are more plants than usual including giant ragweed,” Chalmers says. “I think giant ragweed will be a slow advancing problem in our area because we tend to grow many other crops with a broad mix of herbicides and are not as reliant on just soybeans, corn and winter wheat. However, we are also concerned about the potential of glyphosate-resistant kochia that is well suited to our area and may also have moved into our area as a result of the flooding.

“Everyone is encouraged to be vigilant and monitor weed patches closely so any resistance concerns can be managed quickly and reduce the risk of widespread problems,” he adds. “Weeds like giant ragweed and others are likely here to stay.”

March 18, 2015  By Donna Fleury