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Glyphosate-resistant Downy Brome with Ian Burke

Posted by Blythe Howell | February 3, 2020

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Contact Information:

Contact Ian Burke via email at icburke@wsu.edu.


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Episode Transcription:

Drew Lyon: Hello. 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. In each episode, I speak 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 Dr. 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 the goal of integrating such information into practical and economical methods of managing weeds in the environment. Ian teaches the undergraduate courses in weed science and cropping systems. Hello Ian.

Dr. Ian Burke: Hi Drew.

Drew Lyon: It seems to me every time I have you on we have a story that’s not all that uplifting and today is maybe the same. We’re going to talk a little bit about Roundup® resistant downy brome.

Dr. Ian Burke: I couldn’t agree more. In fact, as I’ve reflected on the last few years as we’ve done more and more testing for herbicide resistance, it does seem to be just a continuous stream of bad news. And a lot of that has to do with how old our crop protection compounds are, their overall durability, how we’ve used them. It seems sort of inevitable that the downy brome was going to evolve resistance to the very limited number of herbicides we had. I didn’t expect glyphosate resistance, although in hindsight I probably should have. We’ve been using glyphosate in the wheat fallow system for years, and there has been a steady shift to at least attempt chem-fallow reduction in mechanical wheat control in that fallow system such that we put a lot of pressure on just glyphosate to control that downy brome and so now we have glyphosate resistant downy brome and that — I think that’s going to fundamentally transform how we farm if it becomes widespread.

Drew Lyon: So tell us a little bit about these downy brome biotypes that are now resistant to glyphosate or Roundup®. Where have you found them, and how resistant are they?

Dr. Ian Burke: We found them in two counties, down in, you know down in the Horse Heaven Hills area and then also in western Whitman County in multiple sites. I’ve walked additional fields since that seem to be infested with, at least, developing populations, you know, car-size populations of downy brome that had survived the chemical fallow treatment. So widespread in geographic region but not necessarily widespread on a field basis. They’re still occupying relatively small areas. The growers who contacted me to test them are painfully aware of what they have. And, at least in one instance, it’s widespread enough that we don’t have any expectation of turning back the clock, of eradicating it from the field through changes. It’s occupied too many acres. Once that happens, you typically expect it to spread even more and given that it’s — we found it in two different places that are not really connected in any way, we suspect that they were developed independently.

Drew Lyon: Okay.

Dr. Ian Burke: Which means that we’ll probably see more develop even if we eradicate these. So as far as how resistant it is, you know our dose response tests — it looks like it’ll survive well more than what you’re legally allowed to apply of glyphosate in the field in any one application. And often we see that it’s capable of resisting somewhere between, you know, eight and 12 pounds of active ingredient per acre. So that’s a nearly complete resistance. We had a graduate student, Pragya Asthana, working on determining the mechanism of resistance. And it appears to be similar to what other weeds have used to escape Roundup® and that it’s duplicated its gene, the gene that codes for the EPSPS enzymes that glyphosate interacts with. And instead of having one copy, it looks like it’s got more than 30. And what that means is the plant is able to make a lot of this enzyme. And the enzyme doesn’t have to be resistant. In fact, it’s a benefit for the plant but it’s not because then it just acts as a sponge so all the glyphosate that gets in the plant is hypothetically absorbed by this extra enzyme.

Drew Lyon: Okay, but there is still enough for it to go ahead and do what it needs to do?

Dr. Ian Burke: It’s still quite capable of growing and reproducing.

Drew Lyon: Okay.

Dr. Ian Burke: So there is a — yeah, in the areas where we’re finding this weed, this glyphosate resistant downy brome, glyphosate is essentially obsolete.

Drew Lyon: Okay. And, you know, glyphosate, for as long as I’ve been in my career since the 80s, it’s what growers have used to control downy brome in fallow because it really doesn’t take a whole — hasn’t taken a whole lot of glyphosate to do the job. How do we manage in a world where we can’t control downy brome with glyphosate anymore?

Dr. Ian Burke: I’ve been wrestling with the question since before we found it. You know, it’s sort of — the writing is on the wall not just for downy brome but for weeds like Italian ryegrass and, of course, Russian thistle, kochia. Those are all weeds that are common in eastern Washington area and have evolved glyphosate resistance in other areas if not here. And so how do you go about changing perception of value associated with those weeds? That’s a challenge that I haven’t yet figured out but there are a number of good — best management practices that you can employ. There is a very nice publications that came out, produced by the WSSA, in the last decade that itemized those BMPs and we went ahead and put them together into a BMP document that’s going to be distributed on a fairly widespread basis just as a poster to hang up in a corner somewhere, just so everyone can kind of see what they look like. And it’s really meant to be a reminder of all the little things that go into wheat management that really can often pay big dividends if practiced effectively and proactively to minimize the impact of weeds. You know, it starts with keeping things clean, staying clean, prioritizing areas of fields to visit last because they’re pretty weedy over areas that are not. It’s just good, sort of common sense BMPs. I think the most important BMP though on the list is the incorporation of multiple modes of action into whatever herbicide plan you’re using and that’s actually tricky to do with a weed like downy brome where we might not be able to control it anymore with any post-emergent herbicides. And certainly on our mind is sort of the last post-emergence mode of action that really is effective which would be the ACCase inhibitors. For growers who are thinking forward and looking at how to manage annual grass weeds in a fallow system, you know, rotating would be ideally the best to a crop or they can use a diversity of additional herbicides that have grass activity that would be very different from what they would use in their wheat crop but absent of that ability because of some monetary constraint, I recognize that happens a lot, then incorporating additional modes of action even in the fallow but also in wheat to really manage those really troublesome weed species is really critical.

Drew Lyon: Yeah. And downy brome, it gets a little difficult if we can’t use glyphosate in our fallow and now we have, especially down in the Walla Walla area a lot of biotypes that can’t be controlled by any of the Group 2 herbicides. We’re starting to run out of things to rotate with. What does somebody do there? I guess we have the Group 15s maybe and maybe ACCase, the CoAxiumTM wheat system but as we have fewer and fewer tools, the ability to rotate becomes more and more difficult.

Dr. Ian Burke: I think the most alarming thing is going to be that the dollars and cents are going to wind up not toward chemical weed management but toward mechanical weed management. I think when you start adding together all the pre-emergent herbicide costs and you couple that with essentially no return investment in a fallow system for the fallow year, it becomes really difficult to make additional herbicide inputs work. And in that situation, mechanical weed management is going to be likely the next opportunity and that’s going to take us backwards. We know that many of these pre-emergent herbicides, if they’re using — if growers are using those in wheat, they really need to be effective on downy brome to manage residue in a way that allows the herbicide to have good soil contact. Downy brome can live its entire lifecycle in the duff layer and not root into the soil where the herbicide is. And so we really had a lot of mixed results using pre-emergent herbicides for downy brome control because of that.

Drew Lyon: Okay.

Dr. Ian Burke: So herbicide like Zidua® or Anthem Flex® has really good activity if there is no residue. And you can have really mixed results when there is, there is a lot of residue. You could have — it’s always really important to — there is old herbicides like Metribuzin® that still have activity that maybe cause potential wheat injury or important to begin to relearn how to use those. That’s an important and different mode of action that we could utilize to manage downy brome. And together, we can build a program that is effective but management of residue becomes critical. We know that the heavy harrow is capable of transporting the seed all around the field and so how you — the particular mechanical weed management that might be deployed or residue management that might be deployed, planter systems that might be deployed should be carefully considered based on how they transport potential weed seed. And if you’ve got one that moves a lot of weed seed around the field, now is the time to really start thinking about trying to find one that maybe doesn’t.

Drew Lyon: You know, I think one of the real weakness — biological weaknesses of downy brome and other annual grasses, winter annual grasses in particular is the seed isn’t really very long-lived in the soil so crop rotation can be quite useful. In the Great Plains, we introduce summer crops into that rotation. It’s a little more difficult here in the Pacific Northwest where it doesn’t rain but if you can get two years between winter wheat crops rather than one, it makes a big difference but then the economics comes into play again because winter wheat tends to be the moneymaking crop in a lot of years.

Dr. Ian Burke: It’s definitely a conundrum. You know, we’ve dealt with managing downy brome without herbicides from the 1930s to the 1970s. We really didn’t get very effective herbicides until then. What it looked like was you would grow downy brome until it got really bad in your winter wheat and then you would rotate to spring wheat for some period of time and take the yield loss. And then when you felt like the seed bank was depleted enough, then you’d rotate back to winter wheat. And we know that doesn’t eradicate the seed bank. I think the seed bank for downy brome actually lasts longer here than it does in western Nebraska. Everything else seems to because of our Mediterranean climate. We don’t have a lot of moisture in the summer to break the seed down and then they’re frozen all winter. And I suspect, like our herbicides, that downy brome can carry over for longer. And, you know, you put that all together and it does make for a bit of an intractable situation. So the value of the weed changes. So this is something that can really — can be quite limiting for our production system. We might have to place a different value on that downy brome and prioritize it for, you know, something like a zero tolerance system where we just don’t tolerate any downy brome at all and that could take years to get through a system like that but once you get on the other side, what that looks like, how you manage in the absence of downy brome infesting your fields, it might be very different from where we currently sit.

Drew Lyon: Okay. Your statement earlier about not having effective herbicides — I remember in the 1990s, I told all my growers we’re never going to have a herbicide that will selectively take downy brome out of winter wheat. We need to use these cultural practices and then Maverick® showed up. And for the last 20 years, we’ve been spoiled because we’ve been able to do it, but it looks like we’ve overused that technology and maybe have to go back to some of those techniques we used to know about.

Dr. Ian Burke: The financial reality of low external input dryland farming is that we seldom can afford the number of inputs we likely need to deal with some of these pests. And we’re often just farming for the next great year and hoping that the limited inputs we can afford to apply continue to work. And eventually those sorts of systems fail and the Aussie’s call it, “hitting the wall” where the cropping system’s practice can no longer continue. And I’m afraid that we’re on the verge of hitting the wall in a good part of our production system.

Drew Lyon: Well, I’d hate to end on that upbeat note [ laughter ] but what are some of the solutions do you think? What do we need to be doing differently than we’ve done in the past?

Dr. Ian Burke:I think it’s critical to be painfully aware of the populations you have in your field to begin to really pay close attention to what they’re doing, how they’re moving, how they’re evolving in response to what you’re doing and begin to prioritize around management of those populations as best you can. I recognize there are a whole host of other demands on the farm, but take a good close look at these BMPs. I think there’s a lot of solutions here. I think there is a wide variety of practices we could employ that maybe would not necessarily be a financial burden but might be a time burden and that we can kind of trade one for the other and find ways that we can successfully overcome its potential challenge.

Drew Lyon: All right. That’s a little better note to end on. [Ian laughs] Thank you Ian.

Dr. Ian Burke: Thank you Drew.

[ Music ]

Drew Lyon: Thanks for joining us and listening to the WSU Wheat Beat podcast. If you like what you hear don’t forget to subscribe and leave a review on iTunes or your favorite podcasting app. If you have questions or topics, you’d like to hear on future episodes please email me at drew.lyon — that’s lyon@wsu.edu –(drew.lyon@wsu.edu). You can find us online at smallgrains.wsu.edu and on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat podcast is a production of CAHNRS Communications and the College of Agricultural, Human and Natural Resource Sciences at Washington State University. I’m Drew Lyon, we’ll see you next time.

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