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Glyphosate-resistant weeds in Ontario

There are four glyphosate-resistant weeds in Ontario. Glyphosate-resistant giant ragweed was first found in 2008, and is now found in the six southwestern counties in Ontario – Essex, Kent, Lambton, Elgin, Middlesex and Huron – as well as Lennox and Addington county.

July 11, 2016  By Peter Sikkema professor University of Guelph Ridgetown Campus.

The second glyphosate-resistant weed in Ontario is glyphosate-resistant Canada fleabane, confirmed from seed collected in 2010. Glyphosate-resistant common ragweed was first found in 2011 and surprisingly, it really hasn’t moved much in the province. To the best of our knowledge, it’s only on five farms and all of those farms are in Essex County.


The fourth and most recent weed found in 2014 in Ontario is glyphosate-resistant waterhemp, which has been found in four fields in both Essex and Lambton counties.

Glyphosate-resistant Canada fleabane is by far the biggest challenge for Ontario growers. A field survey in 2010 found eight fields in Essex County with glyphosate-resistant Canada fleabane. By 2015, it was found across the province of Ontario and is occurring at densities that cause substantial yield loses in both corn and soybean.

However, the problem has become worse in Ontario. There now is multiple resistant Canada fleabane to glyphosate (Group 9) and FirstRate (cloransulam-methyl; Group 2). Twenty-two counties in Ontario now have multiple resistant Canada fleabane.

Why did this weed move so quickly across the province? The first reason is Canada fleabane produces a large number of seeds per plant. In an uncompetitive environment, Canada fleabane can produce up to 200,000 seeds per plant and secondly, those are very small seeds that move easily under field conditions.

Some research papers indicate 99 per cent of Canada fleabane seeds fall within 100 metres of the mother plant. A study in Arkansas found Canada fleabane seed could move up to 500 kilometres in one year on the wind. A study at Pennsylvania State University showed Canada fleabane seed was found in the planetary boundary layer – nobody knows how far it can move in one season.

In terms of management implications, a single field or single grower management is not a viable management option for glyphosate-resistant Canada fleabane. We could have the absolute best farmers in Ontario using the best integrated weed management practices and those farmers can still get glyphosate-resistant Canada fleabane due to wind blown seeds.

We do have good management options to control glyphosate-resistant Canada fleabane in corn. Dicamba-based herbicides, Banvel and Marksman, as well as Callisto + atrazine and Integrity applied pre-plant are good. Post-emergence control is good with Banvel, Distinct, Marksman or Pardner + atrazine.

In winter wheat, Infinity herbicide is the herbicide of choice because it gives commercially acceptable control of glyphosate-resistant Canada fleabane and has a wide margin of crop safety.

The real challenge is glyphosate-resistant Canada fleabane in soybean. However, our research has found that this is a manageable weed.

Our research found 29 tank mixes with glyphosate that did not provide acceptable control with preplant, soil-applied residual or post-emergent herbicides.

Mark Loux at Ohio Sate University says, “The bottom line in Canada fleabane management is that we are trying to avoid having to control it with post-emergence herbicides, since they largely do not work.” That’s exactly what we found in Ontario.

We concluded that Roundup + Eragon, (Roundup + Heat in Western Canada) is the foundation for glyphosate-resistant Canada fleabane control, but the big question is consistency of control. In 25 studies in farmers’ fields in southwestern Ontario we only obtained 90 per cent control 56 per cent of the time. Essentially one out of two times the control was unacceptable.

So, we looked at a number of factors that could explain why Roundup + Eragon gave such variable control. We looked at density of the Canada fleabane and the size of the fleabane at the time of application. We did biologically effective rate trials.

We eventually concluded we needed a three-way tank mix with Roundup + Eragon. In another study, Roundup + Eragon provided 84 per cent control. When we added a third tank-mix partner, either 2,4-D, Gramoxone or Sencor, we had greater than 90 per cent control. If a farmer calls me with glyphosate-resistant Canada fleabane, I am going to recommend Roundup + Eragon + Sencor. That’s about 400 grams of metribuzin per hectare. That tank mix is going to cost him almost $30 per acre, so instead of spending $6 on his pre-plant burn down, he’s going to spend $30. The first few bushels of soybeans that he produces on that acre are just going to pay to control that one weed: glyphosate-resistant Canada fleabane.

We’ve also looked at some of the new technologies including the use of Xtendimax in Roundup Ready Xtend soybean. Xtendimax provides excellent control depending on rate. When dicamba was applied at 300 grams per hectare, control ranged from 83 to 87 per cent. The 600 grams per hectare rate, either as a split application, half down pre-plant and the remainder post-emergent, or just half a litre pre-plant, provided over 95 per cent control of glyphosate-resistant Canada fleabane.

Michael Owen at Iowa State University said, “The evolution of glyphosate-resistant weeds is not a problem with the herbicide, glyphosate, or the Roundup Ready technology. Glyphosate resistance developed because of the way we used glyphosate and the Roundup Ready technology.”

For farmers to continue to get benefit from this technology in the future, it has to be used less frequently or it has to be used differently than it was in the past.

I always finish my extension talks with eight questions for growers to ponder. <num>

  1. 1. Would you consider adding a non-Roundup Ready crop to your rotation or adding an additional non-Roundup Ready crop to your diversified crop rotation? We have lots of options in terms of including non-Roundup Ready crops: conventional corn, identity preserved soybean, cereals, either winter wheat or spring cereals, dry beans or forages.
  2. 2. Would you consider applying multiple herbicide modes-of-action on every acre, every year by using a two-pass weed control program of a pre residual followed by a post?  
  3. 3. Would you consider applying multiple herbicides modes-of-action on every acre, every year by adding a tank-mix partner to Roundup applied post?
  4. 4. Would you consider strategically incorporating some of the alternate or new technologies when they become available? Liberty Link and Enlist corn, Liberty Link and Roundup Ready Xtend soybean. Enlist soybean is expected to be available in the future.
  5. 5. Would you consider including tillage at strategic points in your diversified crop rotation?
  6. 6. Would you consider seeding a cover crop after winter wheat harvest to reduce Canada fleabane emergence? We’re doing research with 17 different cover crop blends.
  7. 7. Would you consider inter-seeding a cover crop in your corn to reduce Canada fleabane emergence? At the six-week stage of corn, some farmers will seed annual rye. It’s a pretty nice cover crop when they harvest the corn.
  8. 8. Would you consider making near-perfect weed control your objective in your corn-soybean-wheat rotation? What we really want to do is reduce the weed seed return to the soil. </num>

My hope is Ontario farmers will implement weed management practices that limit the selection of additional glyphosate-resistant weeds. This will ensure the usefulness of glyphosate and Roundup Ready crops for many years into the future. If you do not have glyphosate resistance on your farm yet, adopt integrated weed management now. Use glyphosate judiciously at strategic points in your long-term crop rotation.





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