Preemergence Herbicides for Grass Control with Dr. Ian Burke

Italian Ryegrass.

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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 podcast app and leave us a review so others can find the show too.

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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 also teaches the undergraduate course in weed science. Hello, Ian.

Dr. Ian Burke: Hello, Drew.

Drew Lyon: So, we’ve had you on before, often talking about troublesome annual grass weeds and herbicide resistance. We seem to be seeing more and more of that all the time, and as we see more of that, there appears to be kind of a wholesale shift back to preemergence herbicides for annual grass control in wheat. How has our situation changed since the 1970s, which is when that’s all we had to now?

Dr. Ian Burke: In a lot of ways, we’re right back to where we were in the 1970s. The options that we have are very similar to what farmers would have used back in the 1970s for weed control. So those who have institutional knowledge of how we did it then likely probably have a little bit of a leg up about what might work in their situation now. The you know, the herbicides that come to mind that would have been widely used in the in the late 1970s would have been Metribuzin. It would have been potentially been an old product called Hoelon, which was Diclofop. That came along in the mid-1970s. And it would have also probably included interesting things like Atrazine, which can be sort of fraught with Peril. And so there’s these old products now available, still available, and then we’ve now really just combined them with new products. And so, I think it’s really incumbent upon farmers to know and understand how to use those primers and herbicides as a foundation and then augment them with postemergence herbicides that might still work, depending on their situation. And that’s really true for all annual grasses. It really doesn’t matter: cheatgrass, Italian ryegrass, rattail fescue. Those are all pests that really respond to preemergence programs.

Drew Lyon: Okay, I don’t know that you even have to go back to the seventies. When I started my career in the nineties in western Nebraska, there were no postemergence selective herbicides. So it was all those products you talked about, I think even Trifluralin trying to do — and they all require different things to make them effective or safe wheat use and it was really quite a challenging time and then all of a sudden the Group 2’s came along, I think Maverick was the first one around 2001 or so, and things really changed and made grass control in wheat quite a bit easier. But those days may be limited. So yes, the preemergence herbicides are coming back, but what factors affect the efficacy of preemergent herbicides because it isn’t just go out and spray them. You need to kind of know several things before you use them. What are some of those? Can you discuss those first things?

Dr. Ian Burke: The things I think about what I’m using pre-merger herbicides are largely related to water availability and you need a lot of water to activate certain herbicides that we’re using now. Pyroxasulfone, so Zidua or Anthem Flex, most farmers will know those names, is really sensitive to the amount of rainfall that occurs after application for activation and so that’s unfortunately something we don’t have a lot of here these last few farming seasons where we just haven’t had a lot of fall moisture until very late and that means that we’re likely not having good activation to manage things like fall emerging cheatgrass without herbicide or it’s a hit or miss proposition. And so, you kind of have to be very careful with your note taking, understand when and how much rain occurred and that likely can help you diagnose when you see gaps in your preemergence program emerge because of just because of a lack of activation. The other thing we’re really seeing with that particular herbicide is a tie up or it’s not necessarily tie up, so it’s not active in residues sitting on the soil surface. And that’s really a bit of a frustration. So, I’ve watched cheatgrass live its entire lifecycle in the residue of the, you know, in these cropping systems. And so, if it’s in the residue, it’s likely not being exposed to the to the herbicides in the soil. And you missed the weed because of that. And so, you’ve got a there is a little bit of mixture of residue management. I’m not talking about, you know, getting rid of your residue. I’m talking about understanding how to make sure you have a thick enough residue layer to promote soil conservation, but not enough residue that it might allow cheatgrass to live its entire lifecycle. And so, you just got to evaluate what your current status is. Think about how and when that cheatgrass might germinate and emerge, and then choose an effective residue management system that would facilitate success with the herbicide in such a way that maybe you can solve the problem enough that you don’t need to necessarily do residue management in the future. So, there’s a there’s a bit of a recipe there for success you’ve just got to experiment with.

Drew Lyon: Okay. You mentioned combining preemergence herbicides with the postemergence herbicides. Can you give some idea of how that might work? Are you putting on those post emergence in the fall as those weeds come up or do you wait if you use a preemergence, wait to the spring to put them on and how do you know? — I guess most growers will know which postemergence’ s herbicides won’t work. But what if you don’t have any postemergence herbicide that works? Do you then mix a couple different preemergence herbicides?

Dr. Ian Burke: Yeah, that’s a I think that’s the I guess hundred-million-dollar question, right. So, how do you design these systems based on, you know, increasing prevalence of herbicide resistance particular to the postemergence herbicides, knowing that we can’t just keep using something like Zidua or Anthem Flex year after year after year and every crop on the entire farm and not expect the same result? So, it’s really important to get your weeds tested for herbicide resistance or at least know what works and what doesn’t on your farm. You can do that on your own, but you also have to –usually I like to see old Metribuzin used with Zero or Anthem Flex at some point in the cropping season that usually goes out early postemergence to late postemergence in the spring. It always seems to bring a little bit of benefit no matter what the situation is, which we were talking about. It’s a completely different mode of action. You know, picking postemergence programs that are effective, understand that preemergence herbicides will almost always make post emergence systems more effective because if they’re used well, the preemergence herbicides will make the weeds germinate later in the season so that they’ll be much smaller when you use your postemergent herbicides to control them. It gives farmers more time to get out and be more timely when the weeds a little bit smaller, we know that smaller weeds respond, usually little bit better to our postemergence herbicides. So again, there’s a recipe there for success. You use your pre’s, not only just to manage the weeds all fall into the early spring, but also to make sure that what does germinate germinates later and it gives you more time to be timely to treat those weeds with a postemergence herbicide.

Drew Lyon: Okay, many preemergent herbicides, because they need to have some residual to be effective, can have effects on following rotational crops. So how do you go about using those in a system where you plan to rotate two things other than small grains?

Dr. Ian Burke: Rotation restrictions are probably the biggest frustration for us in the PNW. It seems like our herbicides last twice as long here as anywhere else I’ve ever worked. And it’s the nature of our cold winters and our dry summers that just sets us up for persistence for certain herbicides. So, the the way to manage that is to make sure you know what the limitations are and you stick to your rotations, stick to the plan. It’s really, it’s a frustration not to be able to perhaps chase commodity prices depending on what the crops are doing. But, you know, sticking to a weed management plan, sticking to the rotation so you don’t potentially cause yourself injury because of the use of certain herbicides and then rotating to the wrong crops. The other consideration I think of when I think about rotation and herbicide choice is not using the same herbicide over and over again, which you can do with Zidua and Anthem Flex. You see, they’re labeled on most all our crops now. It’s important that if you have options that are as effective for annual grass control, that you exercise and use those options. So, there are a number of other herbicides in the same mode of action as Zidua or Anthem Flex. Group-15s. I’m thinking things like Dual, Outlook. They can be used effectively in our pulse crops. They’re not labeled in our wheat crops and there’s not been a lot of evidence that we get cross resistance when we when we go from one Group 15 to another, they’re just different enough that that’s a little bit less of a concern, particularly when you’re using post emergence programs too. And so, it’s really just about designing that program and sticking with it, knowing what each herbicide brings to the table in terms of its strengths and weaknesses, where they fit in that rotational system, and thinking about the system in its entirety in that year to year, that sort of mitigates that issue for rotational issues.

Drew Lyon: Okay. I was going to ask you, you know, what are some of the herbicides we can use in other crops that will help us with our winter annual grass problems? You mentioned a couple of them. Any others that come to mind?

Dr. Ian Burke: You know, there’s a number of older products, you know, Prowl, Triathlon, of course, Triathlon requires some sort of rainfall for incorporation or mechanical incorporation to be used. There’s old Fargo, which seems to be of interest now, particularly where we’re seeing a wild oat resurge in certain areas. And so, there’s a lot of different several different products that could be deployed to control this annual grasses. And your results may vary. It just depends on what you’re dealing with in the situation around but knowing what each one of them can do for you is really important these days.

Drew Lyon: And you mentioned rainfall and preemergence herbicides. I think a lot of them are also affected by soil texture, organic matter. How does a grower manage those sorts of things?

Dr. Ian Burke: You know, organic matter is usually strongly associated with reduced longevity. So, increasing organic matter will reduce the overall persistence of residual herbicides. So that’s particularly true once you get up above about 2.5 and 3% organic matter, I start to really see efficacy drop off quickly, certainly with — and that’s clearly reflected in the label often by increasing rates recommended on the label in response to increasing organic matter. And so that’s the basis that just the organic matter just ties things up. On the other side of that, you can have too low organic matter in products like old Lenox or Lorox. Once you get below about 1%, organic matter can potentially cause crop injury if you’re used to high of a rate. And so, you’ve got to be aware that you can fine tune these rates based on something like organic matter. pH another driver for those rates and has a very similar effect. So certain herbicides persist for much longer when pH is low, which is a chronic condition in our area, particularly the sulphonylurea’s and the imidazolinone’s. And so, there’s a but that’s also a herbicide-by-herbicide dependent. And so, you’ve got to really read those labels carefully, understand what kind of tillage system you’re in and what that might mean for persistence, understand pH and organic matter and that can kind of help you fine tune those rates for the maximum benefit.

Drew Lyon: Okay. It seems like we’ve been kind of spoiled in the last 20 years. These postemergence herbicides were quite easy to use and effective. Those days are seem to be disappearing a bit on us. And as we go to preemergence herbicides, I think the message is read the label, understand that they’re not quite so simple. They can be effective, particularly if used in association with postemergence herbicide. So, Ian, I’m sure we’ll have you back. I don’t think our herbicide resistant issues are going away any time soon, if ever. And so, we’ll catch up with you at another time. Thanks for sharing your information on preemergence herbicides for the control of annual grasses in wheat.

Dr. Ian Burke: Thank you, Drew. Great to be here.

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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 podcast app. If you have questions or topics, you’d like to hear on future episodes please email me at drew.lyon — that’s — ( You can find us online at 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.

The views, thoughts, and opinions expressed by guests of this podcast are their own and does not imply Washington State University’s endorsement.