Integrated Pest Management, Weed Science

Episode 14: Weed Science IPM

Join Emily and Nevin as they discuss the differences between Integrated Weed Management and Integrated Pest management within Weed Science.

This transcript has been edited for clarity.

0:19 Hi everyone, and welcome to another episode of Farm Sci-Ed, the show where we go into the science and education behind farming. I’m Emily Stine, and today we’re continuing the conversation in IPM in different disciplines. Today we’re talking to Nevin about integrated weed management and how it relates to integrated pest management. Nevin, can you describe the difference between integrated pest management and integrated weed management?

0:38 Nevin: Yeah, integrated pest management is something that is, is something that was invented, conceived at first by entomologists. And the idea in – in IPM for weeds, or integrated weed management as some might call it, is basically trying to use a multitude of approaches to reduce herbicide use. And so is there a way we can create different weed control strategies, uhm – such as cultural, mechanical, uhm and biological control in order to reduce the number of herbicides that we – that we’re applying. And the the thought on that is that uh reducing herbicide use may reduce cost, for farmers. uh It reduces exposure of herbicides to the applicator, which sometimes those those can be toxic or carry some sort of health risks. uh There’s environmental benefits too, some herbicides are – can leach into groundwater and cause off-target uhm injury to weeds – or uh to plants – desirable plants if applied improperly. And all these things together is a good reason why we should try to reduce herbicide use if we can. But there’s also a lot of benefits of herbicides as well. I mean, generally they’re gonna be easier to integrate into a system, so if we’re looking to uh incorporate tillage or cover crop, that might require farming a different way, uh by buying a new piece of equipment, adjusting some of your farming practices. So it’s a bit easier to integrate herbicides into the system. uhm It it it’s also could be a cost saving to add approaches other than just using other herbicide practices. So, uh it it’s a trade-off, everything in agriculture has a trade-off. But basically, uhm that’s what integrated weed management is in a nutshell. It’s trying to use diverse uhm management approaches in order to reduce the the usage or the need of different herbicides.

2:46 Emily: You’ve previously mentioned something you call “the critical period” when it comes to weed management. Can you expand on that concept?

Nevin: Yeah, so this is sort of something that I think kind of – it is the closest thing we have to what would be considered an “economic threshold” or an “economic injury level” in entomology, when they talk about IPM. So in entomology, I think what they’re trying to do is figure out at what population threshold a certain insect pest is going to cause enough injury that it’s going to be justified to use an insecticide. And if you’re insect population never gets above that threshold, then there would be no reason to uhm apply that that insecticide. And so a lot of entomology or – a lot of IPM practices based on entomology are formed on the idea of y’know, you’re going to be scouting for the pest, you’re going to be tracking the pest throughout the season, you’re going to know what these thresholds are, and and then do a lot of research to establish these thresholds and sort of guide these thresholds. uhm And those strategies in a lot of ways just don’t work for weed science. And so we uhm – we can’t really uhm, scout for a weed, so it’s in the soil – the seed’s in the soil, and it’s going to come up that next year. There are ways to sample the soil, extract that seed uhm, figure out what that seed is, but those are really difficult to do in a timely fashion and is almost always going to be uneconomical. So it it’s very difficult to scout for weeds in an agronomic setting, and uh as far as thresholds go, uhm there is always going to be enough weeds present in the soil to cause economic injury. All of the time.

There’s never going to be a situation that – I wouldn’t say never, I’ve heard stories of of high-value vegetable producers putting something like a soil sterilant on the ground that kills everything. But that’s very expensive and not very practical. But there’s rarely going to be instances where there’s so few weeds that they won’t have an economic injury.

So uhm you can’t really scout, and you just should expect that every year there’s enough weeds to cause economic injury. So then y’know, how do we sort of model or determine when and when not to control weeds? So this idea was proposed called the “critical period of weed control” which is where you do a series of experiments where you do a series of experiments where sort of half the study you control weeds, maybe for the first uh week, and then the second week, and the third week and the fourth week of the field season. So you’re gonna let uh plots be weed-free for a certain number of weeks, and then you stop controlling weeds. So you might have a treatment where you have a one-week weed-free period, and a two week, a three week and a four week. And so the longer those weed-free periods are, the less yield you’re going to lose – that’s pretty simple, that’s pretty easy to understand. On the other end, of that experiment, what you do is you let the weeds appear all season long, and then later on in the season, you start controlling those weeds. And so you’re going to start controlling those weeds let’s say four weeks after they’ve established all season and then five weeks and six weeks. And so, on the front end, you have different weed-free periods, and then on the back end you have different periods of weed control. And when you link those two values together, you can model then “okay, to preserve – let’s say – 95%, 98% of the yield potential with this crop, you need to control weeds from two weeks from planting until four weeks to harvest and early on in the year, we can let weeds grow and then later in the season we can let weeds grow as well.” So that’s going to prevent at any economic yield loss. And so that’s kind of the methods we can do with yield science as far as modeling when, when not to control weeds and so the threshold really doesn’t have to do with the population of weeds, the threshold has to do with the stage of crop production – or, certain stages of crop production that are more susceptible to yield loss.

6:55 Emily: So in your mind, should integrated weed management be focused on preserving yield, or eliminating weeds?

Nevin: That’s a good question. And uh, one of the problems with the critical period of weed control is it it’s focused on preserving yield; so that’s an economic benefit. But you’re producing all of those weed seeds, and so those weeds that you decide you can let grow for these periods, especially on the tail end, are going to produce more seeds. And so they will be contributing more weed seeds in the soil, which might create more problems in the future and it also sort of ignores all the other problems with weeds. And so we have direct costs of weeds, which is generally competition and yield loss, but there’s also indirect costs of weeds.

So, we have potential for weeds to contaminate: weed seeds to contaminate grain that we’re harvesting, or lower quality, and those may not be factored into this equation, so we might say that if you left y’know, weeds until the end of the season, you’re not having that much yield loss, but you also might be sucking a bunch of palmer amaranth seed, let’s say into your combine, which is then gonna create problems. Weeds can also be hosts for insects and pathogens, and so that model also doesn’t really take into account those issues. And so for – although the critical period of weed control is a way to model when or when not to control weeds, in a lot of ways it’s a bit too simplistic.

Additionally, this idea of waiting until y’know, two or three weeks into a season to control weeds, uhm while that may work, it ignores how a lot of our herbicides are used. And generally, in weed science, the best time to apply herbicides are going to be before the weeds ever germinate – or at least before they emerge from the soil. And so we can put on soil-active herbicides early in the year, to prevent weeds from emerging from the soil and it’s easier, generally to control a weed before they emerge, they’re gonna have less yield impact because they never actually come out of the soil, and these models sort of ignore that. And so, for a large part, this critical period idea ignores some of these issues and is simplistic for what farmers actually need to do.

9:01 Emily: So then how does integrated weed management differ from resistance management?

Nevin: So, uh one of – one of the problems with integrated weed management is that the idea is that you’re trying to reduce pesticide use – or, herbicide use. So, okay, if you throw in a cover crop, and you do this sort of tillage strategy, and maybe you plant your crop a little earlier or a little later or at a higher density, you can reduce your reliance on herbicides. And what that generally means is that you’re going to be using less herbicides. And not less herbicide, but less number of herbicides.

So, a farmer might be using let’s say, in a corn field, four to five active ingredients sometimes, so different types of chemicals to control weeds. And so with this integrated weed management does if it’s successful at reducing herbicide use, what that means is you’re going to reduce it down to two or one herbicide. And a big one that would be reduced, especially since – the the herbicide that would be then the herbicide of choice, especially after the introduction of herbicide resistant crops through transgenic technologies and gene insertion would be glyphosate. And so what you’re doing now is you’re reducing the need for these other herbicides, and you can just get by with one herbicide, let’s say, or two herbicides in large part, that means glyphosate. So what that does then is it puts a tremendous amount of selection pressure on the weeds to evolve resistance to glyphosate. And so, sometimes what integrated weed management does is it makes it – it reduces your number of herbicides you’re using so that one herbicide or two herbicides that are left are more likely to get resistance developed. And that’s a problem.

And so, in the long term, thinking about all these different strategies, you know, is the goal to reduce herbicide practices, or is the goal to preserve all these different tools? So as soon as we have resistance developed to a certain herbicide, that herbicide might not be able to be used again in that field for a very long time, potentially forever, because of those resistances development. And so, integrated weed management also seems to reflect that as well. And so, resistance management, which is a different idea, is you use whatever tools are available to spread out that selection pressure so it’s not just focused on a single herbicide. Or it’s going to be focused on several herbicides.

So the idea of herbicide resistance management is you are going to use cover crops, you are going to use y’know, tillage where appropriate, you are gonna use cultural practices, but that’s in addition to applying any – as many herbicides that are useful. And trying not to rely on a single chemical or a few chemicals a year, year after year after year. That way, you’re spreading off the selection pressure and the idea is, basically, the more diverse your management practices are, the harder it is for a particular weed species to adapt to that and become very difficult to manage year after year after year.

12:05 Emily: Do you think integrated weed management is sustainable?

Nevin: So that really depends on what you mean by “sustainable.” And uh that’s that’s a tricky term. Y’know, I like to think of sustainable meaning “is this practice gonna be something that farmers can keep doing for thirty years?” y’know – “can they keep using that?” And one of the problems, like I said, with integrated weed management, is you tend to narrow the number of herbicides you use and that’s going to place a big burden on those weeds to become resistant to that. And so in some points, I would say it’s probably not the best strategy. Using herbicide resistance management, where you are using a diversity of herbicides, along with multiple approaches, to really broaden that selection pressure probably is the best management practices for some folks. Although, it’s very complicated to implement that, because it’s very complex. And how do we take a very diverse herbicide management program, that may be costly because you’re using multiple herbicides, multiple cultural approaches, and then you have to add in other practices – your fertility, and disease, insect management, uhm it just gets to be actually quite complicated. But in some ways, I feel like integrated weed management or IPM for weeds; this idea of narrowing down the use of herbicides, over time may not be the best strategy.

13:24 Emily: Well folks, there you have it. Today we talked to Dr. Nevin Lawrence about integrated weed management and how it differs from integrated pest management within weed science. Join us next time as we talk to Bob about how integrated pest management relates to plant pathology, and we continue our exploration into integrated pest management. Find us on twitter @TheFarmSciEd or visit our website at for transcripts and other episodes. Have a good one!