Lessons to learn from the “Weed Wars”

Lessons to learn from the “Weed Wars”

Where we stand with hard-to-kill and resistant weeds in the Northern Plains and Western Provinces.

New fronts are opening in the annual weed wars on the Northern Plains and Canadian Prairies, with hard-to-kill and herbicide-resistant weeds on the move. That’s not good news, say U.S. and Canadian weed scientists.

As always, the best offense is to make smart choices in crop chemicals and crop rotations while scouting diligently for signs of weeds not responding to treatment. Here’s a rundown on what to look for in your area.



South Dakota State University extension weed management coordinator Paul Johnson, at Brookings, has been watching the weed wars for about 35 years. Five broadleaf weeds in South Dakota are confirmed as glyphosate-resistant. He says waterhemp ‘obviously’ is the worst.

Glyphosate-resistant tall waterhemp was confirmed in 2010. It infests corn and soybean fields. There’s evidence that some may be resistant to other Group 9 herbicides.

“When I first worked in South Dakota, waterhemp was pretty much confined to a small area in the southeast. Twenty years ago, all of a sudden, it adapted, changed, and moved throughout the state,” Johnson says.

“I’m guessing we hit some type of mutation. The plant became more winter-hardy and was able to move into a harsher climate. I think we’re seeing that with Palmer amaranth. It has shown the ability to survive in harsher climates than it used to,” he says.

Waterhemp can be found anywhere in the eastern third of South Dakota today, and any of it could be glyphosate-resistant.

“It all depends on the farm history. Generally, we’re seeing resistance occurring in areas that went maybe five or six years with no other chemicals but glyphosate being used,” he says.

Now, waterhemp is in the Red River Valley. It’s in North Dakota, and it’s an even bigger concern in Minnesota than it is in South Dakota, he says. Waterhemp with resistance to both glyphosate and the ALS inhibitors (Group 2) also was confirmed in Minnesota in 2007 and in southern Ontario in 2014.

In fact, waterhemp with resistance to four sites of action was confirmed in Iowa in 2011. Resistant waterhemp has not been identified yet in western Canada.

“Waterhemp probably is moving site-to-site with mechanical means, like combines that haven’t been cleaned enough,” says Johnson. “It is widespread enough now that it is pretty hard to trace it to any particular method.”


Kochia problems

Other glyphosate-resistant weeds in South Dakota today include ragweed, kochia, Palmer amaranth and horseweed (marestail).

One of three Palmer-amaranth sites in South Dakota is confirmed to have glyphosate resistance. It was caught early, in an area where livestock trailers were being cleaned out on a ranch in south-central South Dakota. The three sites are being hand-rogued, Johnson says.

The ragweed and marestail incidents have been “very spotty” and seem to be confined so far.

Kochia is a problem. It is found throughout the Dakotas, western Minnesota and eastern Montana.

“We see glyphosate-resistant kochia in the eastern half of the state and west of the Missouri River in isolated areas. We’re seeing that weed in no-till settings, where there’s history of a lot of glyphosate use,” Johnson says.

Waterhemp and kochia have the ability to flush all summer long, especially after a heavy rain while the canopy is open.

“To control the waterhemp, and the kochia, you need a soil-residual herbicide,” Johnson says. “That’s where we’re looking – using the pre-emergence products that have some residual to buy time. Whatever the crop is -- corn, soybeans or wheat -- canopy closure usually eliminates any more weeds from germinating.”


Good news in wheat fields

For wheat, most weed control is being done post-emergence.

“The good news is, most broadleaf post-emergence products are a great rotation to use because they will control those weeds that are resistant and break the cycle of using glyphosate,” Johnson says.

A second good thing, he says, is that guys are switching the burndown away from glyphosate, if they’re using no-till and planting spring wheat.

“They are using older products like bromoxynil or paraquat every other year, to avoid using the same thing over and over. We like to see that. Every two years the crops are grown, they are using four different chemistries, to control weeds and avoid future problems.”

A third good thing is that the grassy weed seedbank in some parts of South Dakota has been depleted.

“A lot of areas do not worry about grasses at all because we’ve had so many years of great grass control,” Johnson says. “Grass seeds don’t have a long life. In five years, most germinate or no longer are viable seed.”

Where winter wheat continues to be grown in South Dakota, however, downy brome continues to be an issue.  Both Pre-Pare and Everest 2.0 herbicides control downy brome.

“We’re seeing those Arysta LifeScience products used a lot in winter wheat areas. In those areas, they have a niche and do a good job,” he says.

Southward expansion

Agriculture and Agri-Food Canada weed scientists at Saskatoon monitor the changing weed population in Canada’s Prairie Provinces. Saskatchewan was surveyed in 2014-15. 

“The top three weeds in Saskatchewan have been the same since the 1970s – green foxtail, wild oat and wild buckwheat,” says AAFC monitoring biologist Julia Leeson. Two, cleavers and narrow-leaf hawk’s-beard, increased their range by moving south.

Spiny annual sow-thistle climbed 28 spots in rank, gaining sixth-place in Saskatchewan’s weed survey. It also jumped 22 spots in Manitoba’s last survey (2002), making spiny annual sow-thistle one of the ‘top 20’ and a weed to watch.

“That’s definitely a surprise. And, it does have resistance issues with Group 2 ALS inhibitors,” Leeson says.


Resistant weeds in Canada

Canada has more than 60 herbicide-resistant weed biotypes, according to Hugh Beckie, AAFC resistant weeds specialist.

Wild oat and green foxtail are the main annual grasses with resistance. There are populations of each with either Group 1 or Group 2 resistance.

Kochia is foremost among broadleaf weeds, with virtually 100 percent resistance to Group 2 herbicides and increasing glyphosate resistance across western Canada, Beckie says. Wild mustard and cleavers are other broadleaf weeds with Group 2 resistance.

“We estimate that over half the cultivated acres on the Prairies now have a resistant weed species. Usually, it is wild oat,” Beckie says. “Already almost a decade ago, the majority of cultivated land in Manitoba had a resistant weed. About half the land in Alberta had resistance, and about 40 percent of Saskatchewan had resistant weeds.”

Resistance on the Prairies could be worse, though.

Beckie says, “Compared to other places, crop diversification is quite high in western Canada. And, we have two unique in-crop modes of action for weed control in canola, one of our most common crops:  glyphosate and glufosinate.”

However, Beckie has identified emerging trends that are worrisome.

The most worrying is a recent confirmation from Alberta. Five-way resistance was confirmed in wild oat populations there, even though the weeds have never been sprayed with products from two of the five groups. One population was found resistant to Group 1, Group 2, Group 8, Group 14 and Group 15 products.

“What do you do with five-way resistant wild oat? Fortunately, it’s still susceptible to glyphosate and glufosinate. We need to do further research, but we suspect that this cross resistance may be more widespread,” Beckie says.

“Mainly, this is restricted to grass weeds, and we think it’s due to metabolism, but you could have resistance to herbicides that have never been used in a field. That should concern industry as well as producers,” he says.

Other trends include:

  • Kochia with Group 2 resistance has moved north. It is showing more resistance to glyphosate treatments and starting to show resistance to Group 4 herbicides like dicamba and fluroxypyr.
  • Cleavers is becoming much more abundant across the Prairies, and it is increasingly developing Group 2 resistance. 
  • Climate change and a longer growing season, Beckie believes, are allowing kochia and waterhemp to increase their range northward. He expects a number of weeds to move north into Canada.
  • Finally, there is increasing desperation to find alternatives when grass weeds survive Group 1 herbicide treatments.

Beckie says, “They continually ask, if I have resistance to one Group 1 product, will another one work? We don’t recommend that, but some growers are desperate. There are only a few options available, and it’s certainly understandable.”

Canada has more than 60 herbicide-resistant weed biotypes, according to Hugh Beckie, AAFC resistant weeds specialist.