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While not yet well-established, 2 types of weedy amaranth, Palmer amaranth and waterhemp, are emerging threats to Prairie field crops. Colloquially known as types of “pigweed,” farmers should be on guard against these herbicide-resistant weeds that are coming northwesterly out of infested field crops such as corn, soybean, and cotton.
But how, and why? And what can be done about it? Dr. Shaun Sharpe of AAFC’s Saskatoon Research and Development Centre is leading the investigation.
Why are they such a threat?
Since these plants are resistant to multiple herbicide groups and can take over fields in a serious way, there is no easy way to tackle them. However, their consequences are serious – in a 2001 research paper studying a corn field in Kansas, yield loss was 91%.
The weed seeds are spread by human activity (such as being accidentally included in feed mixes, flowing along irrigation canals or in contaminated equipment), and through natural causes such as wind, and migrating birds and wildlife. While the spread of these 2 types of amaranths is happening in different areas of the prairies, the movement is generally northwesterly, with both already seen in Manitoba’s Red River Valley. Dr. Sharpe expects Palmer amaranth to arrive in Saskatchewan by 2030 to 2035, if it’s not here now but just undetected. Waterhemp is expected to spread at a similar rate as it shares many dispersal pathways with Palmer amaranth.
Getting piggy with it: identification
Dr. Sharpe’s first piece of advice is to be aware of the threat – that is, to understand it and be able to identify these plants.
However, part of the problem with Palmer amaranth is that, to the untrained eye, it looks like many other types of pigweeds. And to date in Canada, there haven’t been many tools – like plant identification guides or range maps – to share information about these species. While there is an abundance of North American observational data on these 2 species, it has never been evaluated using plant detection apps available to Prairie farmers, nor have their niches (or favoured environments) been modeled and mapped in detail for Prairie systems. Dr. Sharpe’s project aims to change this, starting with an awareness campaign and targeting areas under risk of invasion with outreach and extension efforts.
“Identification is going to be key. Our project seeks to build identification and scouting tools for producers and agronomists and to study effective control practices against these herbicide-resistant plants.”
- Dr. Shaun Sharpe, Research Scientist
There’s also a genetic testing component. Thanks to this project, in the future, producers will be able to submit plant samples for species identification and herbicide-resistance status confirmation testing, which will help support producers, and develop the knowledge base as a whole.
Dr. Sharpe notes that in some ways, this is much like when clubroot was first found south of Edmonton in the early 2000s. While clubroot is a disease of canola and other brassica plants, its spread – that time, mainly southeasterly – is similar to what is happening with Palmer amaranth and waterhemp. And as with clubroot, which now has extensive identification guides, infection maps and cultural control advice, the hope is that knowledge will slow down the spread. Unlike clubroot though, these herbicide-resistant weeds may affect all crops, and herbicide resistance could complicate crop rotation planning and diversification efforts, creating new management challenges.
Potential methods of control
Cultural control is the practice of modifying the environment to reduce a pest’s ability to thrive. For example, a crop could be provided with fewer nutrients if the farmer knew this would cause the weed pest to die off first.
Dr. Sharpe notes that fire (a “cultural control method”) has been shown to work well on established Palmer amaranth plants. Tools could include prescribed burning or the use of weed torches. Other methods include hand pulling the weeds and possibly a foliar spray of herbicides.
Dr. Sharpe is also investigating how commonly grown Prairie field crops (like wheat and peas) may suppress invasive pigweed. The idea is that using competitive crops and higher seeding rates as cultural management strategies could help suppress point introductions of Palmer amaranth and waterhemp via canopy closure and promoting a competitive crop. Given the risk these invasive pigweeds represent, grain amaranths will be used as pseudo-weeds for the cultural management study.
More information will be available as this research project progresses, but in the meantime, Dr. Sharpe reminds farmers to stay vigilant and inform their agronomist, commodity group or research partners if they see any “strange looking pigweed with smooth stems and long petioles.”
Funding for this Agriculture Development Fund (ADF) project was provided in part by the Governments of Canada and Saskatchewan under the Sustainable Canadian Agricultural Partnership, a federal-provincial-territorial initiative. It is also co-funded by the Saskatchewan Pulse Growers, Western Grains Research Foundation, and the Saskatchewan Forage Seed Development Commission.
Photo gallery
Redroot pigweed, a sister plant to the more worrisome Palmer amaranth, seen growing in Saskatchewan.
Dr. Shaun Sharpe stands in front of plots set up to compare mulching and mowing strategies on kochia (another herbicide-resistant weed) management.
Dr. Sharpe is investigating how plants with a dense canopy, like these peas, might help suppress the weed.