Q&A: Glyphosate resistance in perspective

As researchers investigate Canada's first potential case, Monsanto Canada's Technology Development Manager for chemistry discusses what it means for growers.

Posted: August 4, 2009

Glyphosate has become arguably the most important herbicide active ingredient for farmers across Canada and many other parts of the world.

But that popularity brings with it a catch 22 - as with any herbicide or single method of weed control, the more it is relied upon, the greater the risk of weeds finding ways to evade control.

As a result, the incidence of a suspected but unconfirmed case of glyphosate-resistant giant ragweed in a soybean field near Windsor, Ont., while of concern, is not an occasion to press the panic button. (For a brief background on the situation, click here to read our recent story on the ongoing situation).

The reality of farming systems today is that weed resistance is not an issue about any single herbicide or method of crop control. Rather, it's a potential risk that needs to be continually managed through knowledge, monitoring and integrated control strategies.

Globally, more instances of resistance to a number of weed contol options, both suspected and confirmed, are anticipated. The challenge for the food production community today is how best to manage and limit those instances, support a long life span for control options, and continually strive toward sustainable weed control systems.

Sean Dilk, Technology Development Manager for chemistry, Monsanto Canada, answered key questions from Canada Sprayer Guide to help put both that broad challenge and the specific Ontario situation in perspective.

When is it best to check for signs of resistance?

Early in the growing season is exactly the right time. Glyphosate is a systemic active ingredient so it's going to take some time to work. Under normal conditions, you should start to see glyphosate activity after seven to 10 days. But this year and even last year some parts of Western Canada weren't under normal growing conditions - so the rule of thumb we're giving growers is that within a two to three week time frame you should start to see it work.

As a result, to assess a field for signs of potential resistance, about a month after application would be a good time. If the herbicide hasn't worked by that point, it's a sign that something has gone wrong. The problem could be many other things than resistance, but that's the first step in determining if there's a potential issue and then narrowing down what that issue might be.

What is your perception of the awareness of the risk of resistance and the management approach needed to address it?

I think awareness is steadily improving. Based on recent surveys, we know that Group 1 and Group 2 resistance is fairly common in Western Canada. That in itself has greatly increased awareness of the resistance issue in general. The suspected case in eastern Canada is likely to have the same effect. But already I would say growers are more aware and doing a better job of reducing risk, scouting fields, and maintaining their field history so it's easier to keep track of what they're doing to help avoid higher risk practices.

What specifically should growers for look when scouting?

If it's a potential case of resistance, what growers are going to see is some weeds in the field that are controlled at the proper labeled rate of the herbicide, along with a patch or patches where there is one weed species of just a few plants.

It's not going to be a patch in the field where there were three or four different weed species present – that would be a case where something was wrong with the application. Either it was a miss, or the wrong rate applied, or weeds were out of stage or there was some sort of antagonism. It's very is important to differentiate between weeds that weren't controlled by the herbicide and a simple mis-application.

Growers may also want to dig through their field records. If they notice they are using a lot of the same product herbicide group on the same field that could be a flag for selection of resistant weeds. They want to be aware of that potential risk and look for ways to reduce the risks. That may be through working with their retailer or their consultants, with a provincial extension person or with their manufacturer. Speaking from my position, we would be more than happy to provide support.

What are the most common misconceptions you run into?

The biggest thing is not to jump to conclusions. There are a number of reasons why a herbicide won't work – resistance is just one of many. To name a few, there are poor environmental conditions, too low of a rate of herbicide, weeds out of stage or in some cases hard water.

A common reason could be as simple as applicator error - just part of the field that was missed. Today, we're also starting to see more tank mixing occurring. And there can be antagonism related to mixing products that aren't compatible. We also have some growers interested in tank mixing micro-nutrients. We know some of those can cause severe antagonism with glyphosate.

There's also a misconception out there where some growers feel that weed resistance is based on a mutation. In other words, you apply the herbicide and it mutates this weed into something that can't be controlled by the active ingredient. That's just not the case. The process is one of selection, not mutation.

Roundup is sometimes viewed as synonymous with glyphosate. Presumably the suspected resistance in Ontario is related to the active ingredient and would be an issue for any glyphosate product?

That's correct. There wouldn't be any difference between products. It's a glyphosate active ingredient issue. Different products have different surfactants and different loads and other elements, but where resistance is concerned at the end of the day it comes back to the active ingredient. All glyphosate products would be in the same situation for this issue.

Are there important crop or region specific differences on the likelihood of glyphosate resistance or the management approach required?

I don't think there would be. It comes back to selection pressure and the need for diversity and integrated weed management, no matter where you are. Those factors would trump any environmental factors such as climate or soil zone.

It will take a while to investigate the suspected case in Ontario and so far it has already been noted the situation was a unique one that may not have broad implications even if confirmed. How can we best keep this situation in perspective?

It's an interesting and important question. When we look at what's in our control, the most important thing is to diversify practices and limit the risk of resistance, whether this ends up being a confirmed case or not.

We know that glyphosate is a low risk herbicide when it comes to developing resistance. At the same time, in the last few years we have had cases internationally, where we're starting to see resistance to glyphosate pop up.

In Canada, there are strong indications that we have been doing a good job of managing things. We have had glyphosate usage for over 30 years and we've had Round-up Ready crops since 1996, and we haven't had a case of confirmed resistance. So a good question is, what are we doing in Canada that's so special?

Again, it really comes back to diversity. In Canada we have different crop rotations that allow us to use different active ingredients. We look at tillage. We look at tank mixes. We're a lot more diverse in how we manage weeds than other world areas. Those factors are certainly part of the reason, and that's an approach we need to continue.