Managing herbicide resistance on the Prairies increasingly comes down to production decisions made well before the sprayer hits the field, says Dr. Charles Geddes, research scientist with Agriculture and Agri-Food Canada, on this episode of RealAgriculture’s Canola School.
Geddes says variable spring weather makes early-season weed control a challenge for many growers. Dry conditions can delay weed emergence, allowing some weeds to escape pre-seed burn-off applications and placing more pressure on in-crop herbicide timing. That shift matters because “that’s really the window where we’re primarily selecting for resistance right now,” he says.
Long-term Prairie weed surveys show overall weed densities declined for decades, but more recent data point to a gradual increase in both weed pressure and herbicide resistance. New resistant species continue to be identified across the region, highlighting the risk of weed management programs that rely entirely on herbicides.
Research from Geddes’ program consistently shows that competitive crops are one of the most effective non-chemical tools available to growers. Increasing seeding rates and improving seedbed utilization can help crops suppress weeds, reduce weed seed production, and improve the effectiveness of herbicides. While higher seeding rates may be difficult to justify in canola due to input costs, small grain cereals can be used strategically ahead of canola to reduce weed pressure and limit seed return to the soil.
Row spacing also plays a role. Narrower rows, higher plant populations, or a combination of both can significantly reduce weed seed production, even in resistant populations. Geddes says that large changes are not always required; incremental adjustments can still contribute to resistance management.
Going into spring, Geddes emphasizes the importance of planning. Pre-seed weed control, diversified crop rotations, and rotating herbicide modes of action all work together to reduce selection pressure. As resistance continues to stack in species like kochia, proactive, integrated production decisions remain essential to protecting herbicide tools for the long term.
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I'm Amber Bell and this is Real Agriculture here today for an episode of Canola School with Dr. Charles Geddes of Agriculture and Agri Food Canada. We're going to be talking herbicide resistance and what has been going on out in the. So welcome. It's great to see you, It's a.
Pleasure to be here.
Okay, so let's dive right into it. What have we seen in herbicide resistance?
Yeah, so I mean it's been a really interesting year because primarily because weather was very highly variable depending on where you were on the prairies. Right. So it's a pretty hard question to answer, but in a lot of the areas where we were drier in the spring, we saw later emergence of a lot of the weed issues that we were dealing with, which meant that a lot of those weeds weren't being caught with the pre plant burn down herbicide application. Right. So that creates greater pressure on your in crop weed management window. Right. Which is really the window where we're primarily selecting for resistance right now.
What have some of those long term trends been towards resistance and how much of a risk is it for growers?
So long term trends, I mean really when it comes to uncontrolled weeds in fields, I mean surveys across the prairies have been looking at this since the 1970s. What we have observed is that the overall densities of weeds have declined over time and the recent few rounds of surveys. So basically in the last decade we started to see those numbers coming back up a little bit, whether that's due to weather or herbicide resistance. Now when it comes to resistance, what we're finding is that essentially there's a linear increase in the number of resistant weeds that we're dealing with and also in the discovery of new herbicide resistant weeds that are in the region. So sitting at about one new herbicide resistant weed every year, that seems like a lot. It's something that. It's a big issue for farmers, of course. Right. But there's a lot of selection pressure out there, especially if we're talking about weed management programmes that are 100% herbicide based.
Now you have a number of projects going on. Tell me a little bit about your work when it comes to herbicide resistance in Canola.
Yeah, so my research programme focuses on the discovery, characterization, monitoring and management of herbicide resistant weeds. So we kind of COVID a fairly wide portfolio, you could say of Weed research. And so, I mean, I lead the Prairie Herbicide Resistant Weed surveys under the Prairie Weed Monitoring Network, looking at the abundance of these weeds across the region. We also look at on the molecular side, what are the mechanisms that cause herbicide resistance and what kind of management practises can we pull from that sort of more basic biology side? And then on the other end, we also do small plot research looking at integrated weed management programmes. So essentially layering in some of those non chemical practises with a herbicide programme to try and get ahead of these issues.
Now, what have your findings been or what are you seeing as you determine what might be most helpful for growers on the ground that are trying to manage these weeds?
Yeah, so, I mean, one of the things that is the most consistent across research is increasing seeding rates. So increasing seeding rates, promoting a competitive crop, that goes a long way to helping to actually improve the performance of your herbicide. But also when your herbicide's missing those resistant weeds, it helps to reduce the amount of seed that they produce, the amount of seed that goes back into the seed bank. Talking about canola, though, with the price of seed that farmers are dealing with, Right. Canola may not be the best window, the best crop to do that. Right. But maybe some of those small grain cereals where your seed costs are lower, having those before your canola and upping the seeding rates there, having a nice competitive crop before your canola sets you, really sets you up really well for that canola year.
Now, there's been a lot of conversation around row spacing. I know, I have talked to a lot of people that have been doing intercropping. What have, have you looked into that? What are some of the findings around that?
Yeah, so row spacing I've looked into as a part of our research. And so what we found is essentially it all comes down to seedbed utilisation. Right. So trying to have a dense process plant population there. Right. So whether that's done through narrowing up those rows or increasing those seeding rates, or ideally a combination of the two. Right. So we found, for example, with herbicide resistant Kochia, looking at narrowing up the rows and increasing those seeding rates, in our research anyway, near Lethbridge, it provided up to an 80% decline in the number of seeds that those Kocher plants are producing just by halving the row spacing and doubling a seeding rate. Practically speaking, farmers are probably not going to implement a double seeding rate in half their row spacing, but it is a contribution Right. So just even marginally decreasing your row spacing or marginally increasing your seed rate can really help.
Does the different weed species, does that make a difference and how does that play into the whole weed management strategy?
Yeah, so it does vary depending on the weed species. Right. So but really, across the board, a competitive crop is a competitive crop. Right. So really, when we're talking about plant population, it is fairly consistent across weed species. It's also fairly consistent across environments. Right. So there can be some slight differences in there as well, obviously. But really that's one of the most consistent non chemical practises that have been observed in research across the prairies. So one thing I'll add is with row spacing there has been, I think, an idea that if you're dealing with a really dry year, upping that seeding rate could actually produce more biomass and it could actually hurt the yield of that crop. But really, unless you're talking about really, really high seeding rates, that hasn't been observed in a lot of the weed management research that's been conducted in the region.
That's really good to note. Now, going into spring, what are some things that growers should be considering when it comes to the possibility of herbicide resistance in their fields?
Yeah. So I mean, going into spring, I mean, we're thinking about pre emergence pre seed weed management. Right. So ideally you're setting that up in the fall, if you can. Right. And getting down a residual product in the fall that may be less product prone to variable moisture in the spring. Right. But if you didn't get it down in the fall, the spring is still a really good window, but it may depend on the rainfall as well. So keeping an eye on those forecasts and trying to plan things that way. But of course, planning ahead rotation is a big thing. Right. So using crop rotation to help manage those weeds. We talked a little bit about using those small grain cereals to set up canola or conversely, also setting up your pulse crops as well. Right. So that could be a really, really good tool.
And how important is the pre seed burn off?
The pre seed burn off is really important. So I mean, especially if you're, if you're in no till. Right. It's critical. Right. So with glyphosate resistance in Kochia, we're seeing, I mean, glyphosate being the primary product that farmers are using for that burned down window. Right. We're really seeing the need to add in alternative modes of action. And unfortunately what we're also seeing is some of those alternatives. We're starting to see resistance there as well. In Kochia, for example, you start seeing the stacking of glyphosate resistance with Group 14 resistance, and it really starts to limit that window, unfortunately.
And we talk about crop rotation. How important is herbicide rotation?
Herbicide rotation, I would say, is equally as important, actually, kind of come hand in hand. Right, because your herbicide options vary among crops. But really, that's why planning ahead is super important when it comes to weed management.
That's great. Well, I want to thank you so much for your time. And that was Dr. Charles Geddes on real agriculture.
