Herbicide-Resistant Weeds with Ian Burke

Diversity of Russian Thistle

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Contact Ian Burke by email at icburke@wsu.edu or by phone at 509-335-2858.

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Episode Transcription:

[ Music ]

Drew Lyon: Hello, and welcome to the WSU Wheat Beat Podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. We have weekly discussions with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production. If you enjoy the WSU Wheat Beat Podcast, do us a favor and subscribe on iTunes or your favorite podcasting app. And leave us a review while you’re there so others can find the show too.

[ Music ]

Drew Lyon: My guest today is Ian Burke. Ian is a professor in weed science at Washington State University. His research program is focused on basic aspects of weed biology and ecology, with a goal of integrating such information into practical and economical methods of managing weeds in the environment. Ian teaches the undergraduate courses in wheat science and cropping systems. Hello, Ian.

Ian Burke: Hello, Drew.

Drew Lyon: So one of the topics we’ve discussed some in the past is herbicide resistance and how that’s going to affect how we manage weeds, particularly in fallow. I wonder if you can update us on Russian-thistle resistance, which is one of the things driving this question of, how do we control weeds in fallow?

Ian Burke: Sure. In the last couple of years, we’ve identified several Russian-thistle biotypes that we are confident are resistant to glyphosate. And when I say resistant, they’ll live through an application of a gallon of 4-1/2-pound glyphosate product. So a full, 4-1/2-pounds of Roundup of glyphosate, itself, was not sufficient to kill the Russian thistle. And that has a real profound impact on some of the decisions you — the growers might be making to manage Russian thistle. You have to be early. We know that sometimes growers like to wait a little bit and let it get all the weeds up that might be possible to get germinated before they treat that. I think that’s a potentially disastrous approach, particularly for Russian-thistle. As far as the resistance mechanism, that remains undetermined, although Russian-thistle is in the Amaranth family, and many other species have been confirmed resistant in that family to glyphosate, including Palmer Amaranth and kochia. And in each of those cases it’s been because there’s been a modification in the genome that allows those weeds to have multiple copies of the target enzyme for glyphosate, that plants act as a sponge. And so we’re not going to spray our way, or increase a rate to control these weeds. We really come up — need to come up with some alternative methods for managing them.

Drew Lyon: So you know, back in the ’80s we had Russian-thistle become resistant to glean, sulfonylurea, and that seemed to happen really quickly. Are we going to see something similar like that with glyphosate, do you think, or do growers have a while to figure out how to deal with this problem? Or are they — it’s going to be knocking on their door here, in another year or two?

Ian Burke: You know, Russian-thistle has a very efficient means of dispersal. It’s a tumbleweed. One of my favorite papers of all time in weed science was some work Don Till did, where they painted Russian-thistle with spray paint and followed the carcasses with helicopters. And those carcasses can travel many miles to the next fence row or the next road, or the next bridge. And they’ll drop their seed along the way, and wherever they get to. And then the next year the process repeats, and they go another few miles. So I have every expectation that we’re going to see this spread quite quickly. It also outcrosses so the pollen can spread between plants to a certain extent, and that, we also know, that’s quite efficient moving the resistance. I expect a quick turnaround, turnover in the population, just like what we saw with the glean resistant Russian-thistle.

Drew Lyon: Okay, so we always better be preparing. It won’t be long. If they don’t have it yet, they probably will shortly.

Ian Burke: They need to be exploring and learning how to use other herbicides.

Drew Lyon: Yeah. So glyphosate has been used quite a bit in fallow, particularly once it went generic, got cheap. A lot of people just went out and started using glyphosate. Are there other weeds besides Russian-thistle that growers should be concerned about resistance with if all they’re going to do is rely on glyphosate for their fallow weed control?

Ian Burke: Yeah, I have a few weed species that I’m really concerned with. We have several collections of downy brome in the lab now, we’re working with, that appear to be quite resistant, similar level of resistance to what we’re seeing in Russian-thistle. I don’t expect that particular problem to spread as fast, because downy brome seeds essentially shatter and fall to the ground. And unless there’s a lot of tractor movement that moves seed, or they wind up getting into the combine and moving around with the combine, they don’t usually travel as far. The other weed that I’m always a little worried about is all the kochia we have in the basin. We know glyphosate-resistant kochia has occurred all throughout the Midwest in the dryland areas, fallow areas there. And there’s every likelihood we’ll have the same problem here. We’re fortunate in that kochia doesn’t seem to be as well-adapted to the PMW, which it’s not a common weed on a broad basis, although it’s — if you look, you can find it in roadsides, and particularly around old grain bins. So it’s one that we’re keeping an eye on, too.

Drew Lyon: Yeah, it seems to be expanding, maybe outside the basin. It’s — I think it started in the basin, but I’m starting to see it in some off the dryland adjacent to the basin, which is a little worse. And there was a major weed in the Great Plains, but I, you know, Russian-thistle is big weed here, but I think kochia might be coming long. And perhaps it’s the glyphosate used in fallow that’s allowing that, I’m not sure.

Ian Burke: It’s capable of adapting. I think that it just hasn’t quite found the right mixture of genes to allow it to survive in our Mediterranean climate, quite yet.

Drew Lyon: Okay, so kochia, Russian-thistle — prickly lettuce is a species that we get a lot of questions about. It has some resistance that’s used. It can also just be a tough weed to control. Do you have any insights on how we manage prickly lettuce?

Ian Burke: Prickly lettuce is one of the first weeds I began to work on when I arrived in the PMW. We — we’ve done a lot of work with that, primarily around 2,4-D resistance. And we know that there’s a lot of 2,4-D-resistant prickly lettuce in the state. I’ve had trials on it in Walla Walla, all the way up here to the PNW, and points north. We added 2,4-D to glyphosate in the ’70s, primarily because 2,4-D was absolutely outstanding for managing prickly lettuce at relatively low doses. And that practice has persisted. But I think it actually causes us more problems than it actually solves nowadays. When we add glyphosate and 2,4-D together for fallow weed management it tends to make the prickly lettuce a lot more difficult to control. It swells the stems. It might burn the leaves off. Certainly when you’re scouting early it looks like it’s going to die, but it doesn’t. It just hardens off really bad. And that is a really, really difficult prickly lettuce to manage. We’ve had several trials where we’ve tried all different manner of mixtures, you know, labeled and unlabeled, and settled on Sharpen as sort of the go-to for that hardened-off prickly lettuce. I’ve had complaints about Sharpen from many growers, a little concern, maybe, about the prickly lettuce’s ability to grow back even from that treatment. And so I’ve countered with some additional tips. Prickly lettuce is known as a compass plant, too. And we call it the compass plant, because it orients its leaves north and south, and then it holds its leaves nearly vertically, perpendicular to the ground. It’s a drought-avoidance mechanism the plant has. And so if you’re carrying your spray boom with a single row of nozzles directed straight down at the ground, you’re essentially trying to treat a leaf edge. And coverage is a real big — a really important factor for managing prickly lettuce, particularly if it’s been hardened off, because it’s not growing quite right. You really need to coat that plant with herbicide. So these days I recommend carrying an angle boom, potentially even spraying east to west, and if you can, investing in a good set of twin-jet nozzles that actually have different angles. And of course, when you’re talking about a contact type herbicide, you really need to carry more water. Higher carry volume is really important. That will get you a pretty significant response from that hardened-off lettuce, but sometimes it doesn’t completely kill it, and so in additional applications, really the goal becomes to keep the prickly lettuce from setting seed. It’s not going to be using a lot of moisture, because most of the leaves have been burned off of it, but keeping it from setting seed is the primary goal right through planting time. And usually by the time you get around to planting, you’re doing some sort of pre-plant tillage, or you have a little bit more aggressive drill, and you can really get it that way. But if you minimize that seed production you likely will have less of a problem next year.

Drew Lyon: Okay. Yeah, the higher a gallonage, the issue I think is a change in mindset. Because with glyphosate, glyphosate tends — you want to use lower gallonage to limit, I guess, the antagonism problem. And because glyphosate’s translocated, it seems to work. But if you want to accomplish coverage you need more gallons, and that’s a different way of thinking for a lot of growers. You mentioned 2,4-D resistance. Are other auxin imitators similarly problematic as 2,4-D? In other words, dicamba, clopyralid, some of those products?

Ian Burke: Yeah, they appear to be. We’ve tested the two already-resistant biotype we were working on. It entailed all the other growth regulators we could get our hands on. And there didn’t seem to be any difference. The 2,4-D resistance appears to confer a cross-resistance to dicamba and MCPA. You know, the other herbicide that I’d — I also like to mention is Paraquat. It’s also a contact-type herbicide. Growers are a little reluctant to use it, because it is relatively more toxic, requires some preparation and management, but it’s also pretty effective, particularly in temperatures get out above 90 degrees for a long period of time. Those prickly lettuce just aren’t moving a lot of water. They’re not growing very fast, and so those systemic herbicides don’t work as well. And a contact-type herbicide where you’re really burning the tissue is a better approach.

Drew Lyon: Okay, so I guess this message you’re sending is that, you know, growers have relied on glyphosate for fallow management, and that may not be the strategy going forward. So what are some of the new strategies growers should be looking at for fallow weed management into the future?

Ian Burke: I think technology’s going to be a real important aspect of fallow weed management. You know, we’ve just purchased a nice little weed-sensing sprayer that works on reflectance. There are other technologies coming along that might actually be able to differentiate our crops from the weeds and selectively spray them. Regardless of what system you eventually engage, I think growers are going to be using this technology. It’s really valuable in fallow, particularly later in the fallow when much of the ground is weed-free. And instead of broadcasting the Roundup over the entire field, that — these new sensor sprayers allow us to put a pretty significantly greater rate of herbicide specifically on the weeds, themselves. The label interpretation actually changes, depending on which herbicide you’re looking at. When we go from this broadcast treatment to a spot treatment, we can do a lot more herbicide per weed than we have been doing when we broadcast those applications out. That could help long term for resistance management. The other thing it changes is timing. Instead of driving out when all the weeds are — maybe most of them have germinated, now the biggest consideration maybe would be diesel cost, and how much it costs to cross that field. The timing changes such that you’re out on much smaller weeds, and we know that’s a far more effective way to manage weeds. The smaller, the better. They’re a lot more responsive to those herbicide doses that way. So I’m sure there are many other nuances and subtle changes that will happen when we adopt that sort of technology, but that’s where I would encourage growers to look at — take a good look at what’s on the market, what might be coming. Pay close attention, because that’s going to be where we’re really going to see change.

Drew Lyon: I know in the Great Plains with the glyphosate-resistant kochia, one of the strategies is to use more pre-emergence, try and get some soil residual. I wonder if — whether that might be part of the answer, to use that, and then in addition, with the weed-seeking-type technology, maybe that’s the way we need to go. But I think we’re going to have to do something very different than what we’ve been doing for the last so-many years.

Ian Burke: I agree. I don’t know that I have an intuitive feel for how pre-emergent herbicides would fit into a system like that, but certainly, based on my experience with pre-emergent herbicides and sensing sprayers, any time you reduce seed germination, you’re going to be reducing the amount of time that sensor is actually on. And so that’s a — all of that sort of plays into an integrated weed management strategy in that fallow, where you’re combining multiple different tactics and giving yourself a lot of latitude, where maybe economics would’ve limited you by — when you were making those broadcast applications. All off those things are now, sort of, up for reconsideration. What our — what that new system looks like, I think, will be an evolution here in the next 10 years.

Drew Lyon: Yeah, I can see where some of these newer chemistries that tend to be a little pricier, and really not a consideration for fallow, may become a consideration with this type of technology. All right. Anything else on fallow weed control and the age of glyphosate-resistant weeds that you’d like to talk about?

Ian Burke: Not that I can think of, Drew. This has been a great conversation.

Drew Lyon: Thank you very much, Ian.

[ Music ]

Drew Lyon: Thanks for joining us and listening to the WSU Wheat Beat Podcast. If you like what you hear, you can subscribe on iTunes or your favorite podcasting app so you never miss an episode. And leave us a review while you’re there. If you have questions for us that you’d like to hear addressed on future episodes, please email me at drew.lyon@wsu.edu. You can find us online at smallgrains.wsu.edu. You can also reach out on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat Podcast is a production of CAHNRS communications in the College of Agricultural, Human, and Natural Resource Sciences at Washington State University. I’m Drew Lyon. We’ll see you next week.