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ON THE GROW: Herbicide resistance is spreading quickly

05/24/2018

Manitoba Agriculture is warning about new and looming threats when it comes to resistant weeds


Manitoba’s provincial weed specialist is urging a harder line on herbicide resistance.

“Zero tolerance is really where you need to be,” Tammy Jones of Manitoba Agriculture said. “Controlling your escapes. Control them with mowing; control them with tillage. If you really want to, control them with hand weeding, but if you have resistant wild oats or resistant whatever weed and there’s just a few of them, controlling them now is going to set you up for future success. If you leave them to set seed, you have years of headache ahead of you.”

Resistance is a growing concern in weeds, including glyphosate resistance. Glyphosate-resistant kochia was found in the RM of Grasslands last year, bringing the number of municipalities with confirmed resistant populations to six. A sample from the RM of Rhineland has also been sent for testing. Also concerning is the rising incidence of herbicide resistance in wild oats.

The 2016 Manitoba weed survey of herbicide-resistant weeds found more than 5.4 million infested acres, about 1.4 million more than the last survey in 2008.

Management important

Jones and other weed experts are recommending management practices that incorporate both chemicals and cultural practices. Those include higher seeding rates, rotated seeding date, crop rotations, and more scouting — including harvest scouting, which she says can identify gaps in a weed management program — and herbicide layering, although pre-season applications may be too late for this year.

“Glyphosate alone, to me, is not the option,” she said. “I know it’s an economical weed control choice, but if you have glyphosate resistance, it’s a non-effective weed control choice. You need to make sure that you have a mode of effective action in there for those herbicide-resistance weeds.”

Layering normally uses sequential passes of multiple modes, both before and after seeding.

Adjusting water volume and calibrating nozzles may also boost chemical control, she added.

Lionel Kaskiw, Manitoba Agriculture production specialist in Souris, stressed pre-seed scouting and weed management.

“Don’t forget about the burn-off, because it definitely does something to help you later on in your herbicide choices when you go back in crop,” he said.

There were few weeds in the field as of early May, he said, although record-breaking temperatures on May 7 likely jump-started growth.

Incoming threats

Waterhemp, yellow foxtail, biennial wormwood and giant ragweed might be the next things to watch out for in the field.

Yellow foxtail made its way onto the province’s top 20 weeds for the first time in 2016, jumping from 30th in 2002 to sixth.

The Manitoba’s herbicide-resistant weed survey in 2016 found 549,400 acres infested with Group 1-resistant yellow foxtail, 218,700 acres resistant to Group 2 and 199,000 acres resistant to both. The last survey in 2008 did not note herbicide-resistant yellow foxtail.

Waterhemp and giant ragweed are both rare in Manitoba right now, although Jones warns that may change in the future.

Manitoba Agriculture expected waterhemp to make an appearance for years, after watching its northward climb through North Dakota. Those fears became reality in 2016, when a field in the RM of Tache reported waterhemp. Another incidence was later confirmed the next year in Rhineland and Jones says the weed has since been found near Melita.

“It moves in a lot of ways,” Jones said. “It’s in soybeans, which is one of the crops that we grow fairly frequently. It’s very likely that we’re going to see it unless someone builds a wall between us and the rest of the world.”

The plant has become a thorn in the side of farmers in the U.S. and Ontario. Waterhemp, like other members of the amaranth family, is well known for prolific seed production, and one plant can produce over a million seeds.

The weed’s penchant for herbicide resistance has added to the problem. Group 2- and glyphosate-resistant waterhemp are common both south of the international border and in established populations in Canada. In 2017, a study by the University of Guelph noted an Ontario soybean field survey that found 39 glyphosate-resistant populations in 2015. Further greenhouse experiments found widespread resistance to Group 2 and Group 5, while 61 per cent of the 49 samples had triple resistance to Group 2, Group 5 and Group 9 herbicides.

Biennial wormwood is another weed farmers might be seeing more of and is tolerant to Group 2 herbicides, Jones warns.

The weed misses most post-emergence herbicide passes, since it typically breaks ground weeks after pigweed, lamb’s quarters or foxtail, she said.

“This is something that we need to be relatively cognizant of because we’re going to need to look at different strategies to control late-emerging or late-flushing weeds, similar to annual sow thistle that always seems to escape our control because it comes out after we’ve sprayed,” she said.

Biennial wormwood jumped 23 places on Manitoba’s weed abundance index from 2002 to 2016, bringing it into the province’s top 20 weeds.

Giant ragweed, meanwhile, has also been found in the province, including surprise cases in southwest Manitoba in 2012. The plant can spread through waterways and Group 2- and Group 9-resistant populations have been reported in both Ontario and Minnesota.

Jones predicts that any giant ragweed in Manitoba will share that resistance.

“It does take a long time to get it to germinate for the resistance testing,” she said. “It likes to be soaked in water for almost a year before it wants to germinate, so it’s taking a long time to have that testing done. When you see it full grown, it’s between 1.5 and three metres, so ‘10 feet tall and bulletproof’ is sort of what we’re looking at, at that point in time.”

Early scouting and control will be critical to curb the giant weed, she said.

Wild oats also made Jones’ list of weeds to watch in 2018. Wild oats slipped from Manitoba’s second most abundant weed to fourth in 2016, but Jones noted that more acres tested positive for Group 1 and Group 2 resistance in 2016.


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