Using biological characteristics to manage the weed seed bank with Dr. Ian Burke

May 29, 2023

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Dr. Ian Burke

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Episode transcription:

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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.

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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 at WSU.

Hello, Ian.

Dr. Ian Burke: Hello, Drew.

Drew Lyon: So, let’s talk about weed seeds today. Maybe we can start the conversation off with your definition of what a weed seed bank is. We hear that term quite a bit. What do we mean? Or what does a weed scientist mean when they say a weed seed bank?

Dr. Ian Burke: So, the weed seed bank would be the sum total of all the weed seed deposited over the years in the soil. And it sort of reflects the past successes and failures a farmer has in managing weeds. There are some years where there’s no weed seed deposited into the soil seed bank and there are years where there are a lot of weed seed deposited. And so, the seed bank is really just that accumulation of seed in the in the soil.

Drew Lyon: Okay. I know as a graduate student, I tried to avoid studies working with the weed bank because it was it was a lot of work. So, tell us a little bit about how one studies a weed seed bank.

Dr. Ian Burke: It’s historically been a very laborious process. So, we’d collect hundreds of soil samples and attempt to grow out the weeds by making them germinate. So you put them in a greenhouse and you either keep the core you’ve collected intact or mix it up a little bit, and then you wait for weed seeds to germinate and you assign a whole phalanx of humans to go out and count the weeds that germinate over often a six- to eight- month period.

And so, by the time you get done doing all of that, the total investment in time is enormous just to quantify what we would call the germinable seed bank–the stuff that’s going to germinate in a greenhouse. But that really doesn’t give us all the information we need. And so here in the last few years, we’ve been working on increasing our ability to extract the weed seed bank.

We have an instrument now, it’s an elutriator. We had we’ve always had one, but it will only do one at a time. And this new device now does 48. And so, it’s been an exponential increase in our ability to just extract the weed seed from the seed bank, and now the slow part is just counting them under a microscope–which is no less laborious, but quite a bit more precise when we actually get the numbers of weed seed present.

Drew Lyon: So, explain what an elutriator is for our audience.

Dr. Ian Burke: Yeah, so it’s as simple as some mesh screens that you put the soil in and then run water through the mesh screen so the soil comes out, but the weed seeds don’t. And so you’re left with what usually is a mixture of sometimes clay particles, depending on where we’re sampling, residue from the soil surface that might have been present when we were taking those samples, and the weed seed.

So, then the real process is just sorting through all of that remaining material and figuring out what you’ve got.

Drew Lyon: Okay. So, you know, in a lot of my simple applied field research, I would go out and count how many weeds emerge in different treatments in the field. How is the weed seed bank–can you relate what you get in the field to what’s in the seed bank? And if so, how? How well does that work?

Dr. Ian Burke: So, the weed seed bank data we’re getting is not necessarily relatable to the density of weeds that appear every season above the ground. Something like only 4% of the weed seed bank germinates in any given season because weeds and weed seeds have light-mediated responses to their environment. And so weed seeds have a mechanism to sense when there are plants growing above ground.

That’s why crop canopy is so important, because if you get a good crop canopy quick, weed seeds sort of stop germinating. But what the weed seed bank does tell us is over a long period of time, if you’re able to sample over years, how resilient your system is potentially. So if you see a declining weed seed bank over time and an increase in diversity of different species in the weed seed bank, that’s a real good indicator that you’ve got a very effective rotation and that the weed seed–particularly if they’re not very long lived in the soil, if they’re declining over time–then that you’re also making headway in making your farming system more resilient.

Drew Lyon: Okay, let’s talk about that a little bit. I’ve heard that, you know, the more diverse your weed seed bank is the better off you are. Can you explain why that is?

Dr. Ian Burke: Like all things in farming, it’s complicated. So, you know, whenever you have a very diverse rotation–that would include all the inputs we know that are really impactful to weed management, so chemical controls, good diversity and timing of planting, potentially including a forage crop that would include a mowing, you know, anything that really over the course of multiple seasons selects for weeds in a lot of different ways–there are very few weed species that can really thrive in an environment where they’re being constantly hounded to death. And so, the ones that do often do so at much lower density and with much lower seed production.

And so, what you wind up with is you might have a winter wheat crop where you have Italian ryegrass succeed in that season and make a lot of seed but the next year you maybe throw a later planted spring crop or fallow, some sort of forage crop where you’re actually mowing the Italian ryegrass, and it’s much harder for that Italian ryegrass to set a lot of seed in that situation. And so as you rearrange your rotations, you can select for multiple different species that might succeed to some extent, but no one weed species can really, really succeed and become dominant in the rotation.

Drew Lyon: Okay. And probably change up more than just rotation change up, chemistry change up, tillage change up–basically that prevents any one species or two species from dominating in the system, becoming a problem.

Dr. Ian Burke: That’s exactly right.

Drew Lyon: Yeah. So, you also mentioned longevity, seed longevity. And you know, some weed seeds don’t stay very long in the soil viable. Others can stay quite a long time. So, what’s the spread and are there certain weeds that tend to have longer lived seeds than others?

Dr. Ian Burke: These days I like to say that like everything else in the PNW, it seems weed seed last longer here than other places. And so, our environment is not very conducive to the breakdown of seeds as rapidly as what’s been reported in other areas. So, we’re really focused on trying to figure out what’s going on here.

We know, for example, when we look at Italian ryegrass, that’s really sort of an out of control population. We’ll see 40, 50,000 seed per square yard, which is an enormous population. When you calculate that over an acre, you know, it’s tens of millions of seed. But if we are effective at managing that population so it’s not setting a lot of seed or no seed over a very short period of time, ryegrass disappears from the seed bank. The seed are not very long lived and we think of that in terms of maybe 3 to 5 years, although we’re still trying to figure out exactly the amount of time.

Downy brome, we suspect that there are some small amount of downy brome seed that are very persistent with a very large proportion of the seed bank of downy brome likely also declines at the same rate as Italian ryegrass.

So, a very quick drop off, but there’s some amount that’s sort of difficult, recalcitrant we would say. That compares rather interestingly that something like a common lambsquarters, which appears to have a very long-lived seed, often measured in decades. And so, you can have an outstanding success at managing something like common lambsquarters over a farming career 30, 40 years and one failure can completely reset that seed bank and last another 30 to 50 years, maybe longer.

And so, your management of seed bank is really about understanding those biological characteristics in such a way that it allows you to think critically about how to prioritize your farming system–if weed management is very critical in it for targeting the weeds that don’t last long in the seed bank and understanding there are certain weeds that that you’ll never really get rid of.

Drew Lyon: Okay, so what are some of the management strategies for managing a weed seed bank? I think you just started down that road. Let’s go down a little farther if we can.

Dr. Ian Burke: So, anything you can do to manipulate the seed bank to the farmers benefit, they’re often quite obvious. So, we typically start with a we would call a stale seedbed. So, you might do a little bit of shallow tillage to stimulate seed that are responsive to light for germination and then over the next three to five weeks, potentially use herbicides to kill anything that emerges, and then plant.

So that would be a stale seedbed technique. And we see that repeated commonly in multiple different systems. It assumes you have moisture and some I know don’t. But that’s definitely a technique where you can manipulate planting time with a mechanical input and use pesticides to arrive at a situation where there’s a lot lower germinal seed bank at the soil surface.

You can also manipulate seed position in the seed bank by perhaps inversion tillage where you would periodically–maybe five, seven, ten years as you accumulated a weed seed bank on the soil surface through no till operations then sort of consciously in that single inversion event–put them as deep underground as you can get them and keep them there. So it used to be a single inversion event and then you wouldn’t go back to it for often a decade or more.

By putting the seed very deep into the seed bank, into the soil profile, they’re often placed into a situation where they either go through a terminal germination—they can germinate, but they won’t emerge, they’re too deep–or there’s not a stimuli to get them to germinate and the seed actually eventually rot.

And so those are both pretty positive outcomes. Both of those scenarios involve tillage, but they’re good examples. The other, of course, big technology we’re seeing adopted here in the Pacific Northwest is harvest weed seed control, where you actually manage the weeds seed before they set off, before they leave the head, and we’re interested in seeing what kind of impact that has on our seed banks as well.

Drew Lyon: Okay. And I like your focus on those weeds that turn over fairly quickly because the ones that don’t are pretty difficult to manage.

Dr. Ian Burke: Those are actually a no till solution. So, the ones that are difficult to manage and they’re recalcitrant and don’t germinate and last a long time, and so no till actually makes a lot of sense for those kind of weeds where you exhaust the seed bank at the surface and don’t allow anymore weed seed to set. That’s an ideal outcome.

Drew Lyon: Okay, very good. So, a knowledge of what weeds you have in your seed bank—are we coming to a day when that might be a service that’s provided where people, you know, they go out and sample for nitrogen and pH and things could also do a sample to kind of find out what their weed seed bank is? I would see this may be more useful for somebody who’s taking over some land that they haven’t farmed before or something. Any thoughts on that?

Dr. Ian Burke: You know, I could see it becoming a service eventually, even though we’ve increased our throughput significantly, it’s still pretty arduous. Certainly, it’s something we can do as a built-in component of our research program in very large numbers. But I do think that that could be a service provided in the future, particularly if we can bring more technology to bear.

Drew Lyon: All right. So, anything else you’d like to share with us today while we have you here, Ian?

Dr. Ian Burke: No, this has been a great conversation, Drew. Thank you.

Drew Lyon: Thanks, Ian, as always.

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