Watch for expansion of waterhemp, Palmer amaranth and kochia in 2020

Serious weed threats continue to advance and grow, farmers urged to test for resistance

For growers in Manitoba, waterhemp is a serious threat in 2020, reports Tammy Jones, a Manitoba Agriculture weed specialist. This weed was present last year in both eastern and central Manitoba, and Jones’ “biggest fear” is that more will be found in the province this year.

And, as reported by Canola Council of Canada (CCC), Palmer amaranth, another aggressive pigweed with populations resistant to many herbicides, is moving north from North Dakota toward Manitoba and Saskatchewan as well.

Each waterhemp plant can produce up to 200,000 very small seeds, which can be spread by contaminated equipment, waterfowl and more. Like Palmer amaranth, waterhemp is present in both North Dakota and Ontario. As reported late last fall, researchers have found waterhemp populations in two areas of Ontario with resistance to glyphosate, independent of resistant populations in the United States.

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Clark Brenzil, a provincial weed control specialist with Saskatchewan Agriculture, has heard rumours that waterhemp is present in a couple of counties in North Dakota just south of the border between Saskatchewan and Manitoba. He will be looking for it in southeastern Saskatchewan this coming summer and encourages any producers who think they have it (it’s similar to a pigweed with narrow leaves) to contact him by email ([email protected]) or phone (306-787-4673).

Under Manitoba’s Noxious Weeds Act, waterhemp and Palmer amaranth are categorized as Tier 1 Noxious and must be destroyed — but should be tested for resistance first. Farmers can be fined if they do not comply with notices to destroy it, and any equipment that’s been in a field containing waterhemp must be thoroughly cleaned. Contact Jones by email ([email protected]) or phone (204-750-1235) to report the presence of waterhemp. She can help arrange for herbicide resistance testing using green leaf material as well as a weed destruction plan.

Palmer amaranth.
photo: File

“There are seven modes of action that this weed is resistant to, and resistance to Group 2 and 9 are so far confirmed by PCR testing in Manitoba,” says Jones. “We really need to be proactive this year with surveillance and do a lot of testing to optimize management. It costs about $200 to test 10 plants from a field. Compared to the cost of the herbicide and of spraying it — and the impact of the weed growing out of control because the herbicide doesn’t work anyway — $200 is a good investment.”

Alberta is at low risk for waterhemp to get a foothold, says Chris Neeser, a weed research scientist at Alberta Agriculture and Forestry, because it doesn’t do as well in the cool nighttime temperatures of the western Prairies. However, it may arrive in future.

There are two other weed species on Jones’ radar. “Woolly cupgrass in North Dakota also may be a threat in the future,” she says. “I keep expecting resistant Canada fleabane to take off in Manitoba but it doesn’t seem to be happening as it has in Ontario. I’d be very happy to have it stay that way.”

Herbicide-resistant kochia

Kochia is another weed found across the Prairies that has resistance mostly to Group 2 herbicides and with some resistance to Group 4. There has also been a sharp increase in resistance to Group 9.

Jones believes this is the tip of the iceberg. “A lot of farmers are using glyphosate and in many populations it will now control 80 per cent of kochia, but in two years, with continued reliance on glyphosate alone, it will not be effective at all,” she says. “Kochia has a short seed life of one to three years and we should be able to get rid of it, but it’s more resilient and adaptable than I had thought.” She reports that growers can get a green leaf material test for glyphosate resistance through the Pest Surveillance Initiative (mbpestlab.ca) with results in one to two weeks, however, seed testing takes longer.

In terms of spread, CCC reports in Canola Watch that kochia produces 15,000 to 25,000 seeds per plant, and mature weeds, once they break from their stems and start to tumble with the wind, can spread these seeds over a fairly wide area. In addition, kochia often takes over saline or other marginal areas. Farmers should consider seeding kochia-infested areas to salt-tolerant perennial forage rather than continue to throw inputs at it, the publication states.

In Alberta, Neeser reports normal and glyphosate-resistant kochia is present south of Highway 1 and north of it up to Red Deer county. “Use rotation and other integrated weed control practices as much as possible in addition to alternative herbicide options,” he says.

Alberta growers who wish to test for kochia they suspect may be glyphosate-resistant should contact Charles Geddes, lead investigator in weed ecology and cropping systems at Agriculture and Agri-Food Canada’s Prairie Herbicide Resistance Research Lab in Lethbridge (403-359-6967 or [email protected]). Geddes can direct farmers to the appropriate lab. His lab also tests other weeds for glyphosate resistance and carries out tests for dicamba/fluroxypyr resistance.

About the author

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Treena Hein is a freelance writer specializing in science, tech and business trends in agriculture and more.

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