If you want to know about herbicide-resistant weeds, Ian Heap is a good man to talk to. An Australian, Heap earned his doctorate on his home soil, before traveling to North America.
“I worked on the first Australian weed identified as herbicide-resistant, rigid ryegrass. That’s probably the worst weed for resistance in the world because it’s become resistant to so many herbicide modes of action.”
After working in Canada at the University of Manitoba, Heap took a job at Oregon State University. He then started his own company, WeedSmart LLC, in 1994. The Oregon-based company focuses on herbicide-resistant weeds.
“Resistance issues are all I work on. Part of our effort involves a resistance survey which I do for industry and the Weed Science Society of America. Primarily, the Herbicide Resistance Action Committee (a group funded primarily by the agrochemical industry), which is composed of global industry, is funding this work.”
A Web site Heap began in 1993 — http://www.weedscience.org — has become the place of record for weed researchers studying herbicide resistance around the world.
The site “was rapidly adopted and usage has increased from year-to-year. It gets a lot more hits than just from weed scientists.
“It’s a bit strange, actually. We’re up around 3 million page views. I’m not sure why that’s the case exactly. I suspect it’s because it’s been around the Web for so long that it’s linked to many other sites.”
Herbicide resistance has been a concern for half a century. And incidents are rising. “In the 1940s, we saw the advent of 2,4-D and the synthetic auxins. Those revolutionized weed control.”
Shortly thereafter, farmers welcomed the arrival of the triazines — like atrazine. Those proved fantastic in corn and other crops.
At the time, weed control was served well by many herbicides. But around 1970, in a Washington nursery crop, the first triazine-resistant weed showed up: common groundsel had become resistant to simizine.
“The common groundsel discovery was a bit of a curiosity. But in short order — from the mid-1970s into the 1980s — we began to see many other triazine-resistant weeds evolving. Those were the first big indications that weed control by herbicides was to be threatened from weed resistance.”
Newer herbicides, like ALS and ACCase inhibitors, were released in the early 1980s. Weeds evolved resistance to those types of chemistries even faster than they did to triazines.
The message to be learned from those early resistance cases “is weeds evolve resistance to different herbicide modes of action at different rates. … Interestingly, it takes a long time for weeds to develop resistance to the first herbicides that came out like the synthetic auxins and 2,4-D.
“So we had quite a collapse of weed control with the ALS inhibitors. Many weeds in the Midwest became resistant to what we termed the sulfonylureas — a big group of ALS inhibitors. Weeds, many species, evolve resistance to them at a very, very rapid rate — sometimes as little as four years.”
Glyphosate as savior
So the search was on for the next big answer to weeds. The advent of Roundup Ready soybeans in 1996 filled that role.
Part of the reason for that is even though Roundup had been used for a long time, at that stage there really weren’t any weeds resistant to it. It was “very clear” that glyphosate was a low-risk herbicide for resistance development.
“But if you begin putting large amounts of Roundup over huge acreages, the selection pressure is very high. Now, we’re beginning to see the appearance of glyphosate-resistant weeds.”
Even in the early days of Roundup Ready crops, Heap says, the dangers of weeds developing resistance to glyphosate were obvious.
“It was clear we’d get resistance to glyphosate. The only people not saying that was Monsanto. But underneath it all, I believe they suspected (resistance) would happen, as well. But we were also fairly sure it would happen at a slower rate than with the ACCase or ALS inhibitors.”
The biggest dilemma Heap now sees is that Roundup Ready crops have changed agricultural economics.
“The economics are now in the tech fees, not so much in the herbicide itself. That has led to a massive collapse of money in the industry to go out and discover new herbicides. It appeared glyphosate was a huge winner, and the competing companies wouldn’t be able to make a lot of money by discovering new modes of action.
“That means we now have a situation where many weeds are resistant to the older herbicides. Glyphosate is still being used, in many cases, successfully.”
But as more weeds become resistant to glyphosate, “it’s unlikely we’ll see a lot of newer herbicides. That’s different than what happened in the past when weeds evolved resistance. There won’t be new herbicides coming to the rescue and that’s a big problem.”
The world picture
How prevalent is weed resistance overseas? Do we really know the full picture about how prevalent resistance it is, or isn’t?
“We don’t have as much detailed data as we need. In many cases, the area infested may be much larger than what’s being reported. The same could be true for the number of weed species with resistance.
“Part of the reason we don’t have (a full picture on the Web site) out of places like China, is we actually require data and scientific rigor before resistant species are listed. So we’re playing a bit of catch-up — maybe a few years behind, in many cases. If researchers don’t actually collect seed, do genetic work that takes a couple of years, it’s unlikely it will get through (the Web site’s gate-keeping).”
The reason for such caution is obvious, because “probably about 30 percent of the cases suspected to be resistance turn out not to be.”
The economic impact of glyphosate resistance around the world at present is very low, but the number of cases is picking up.
“We’re seeing that in South Africa with three glyphosate-resistant weed species and more under investigation.”
It’s also showing up in South America. “That’s where I think we’ll be seeing some major resistance problems appear — especially in Argentina and Brazil. (In those countries) an intensive agriculture has quickly sprung up using Roundup Ready crops. In Argentina … weeds like johnsongrass have become resistant to glyphosate. That’s a major problem.”
With few new herbicides in development, does Heap believe farmers will be forced to adopt new cultivation practices?
“In corn and soybeans there’s still quite a spectrum of herbicides to choose from. Often, the weeds that have become resistant to glyphosate remain susceptible to one, or more, of the old herbicides. Initially, farmers will just be adding herbicides to glyphosate.”
The problem with stacking herbicides is “eventually farmers will ask why they’re still paying a tech fee. Growers are going to face more and more expense.
“There may be some other methods, like cultivation, mixed in. But I think once it gets to a critical enough stage — where companies see 20 or 30 percent of weeds aren’t being controlled by glyphosate — the discovery pipeline will open back up. It may take 10 to 15 years after that before producers start seeing new herbicides. So, growers could be facing 10 to 15 years of pain.”
Asked if there’s a way to pre-empt that period of pain, Heap is upbeat.
“I’d be surprised if there weren’t some of the smarter people in these chemical companies sitting there saying, ‘Hey, in five or 10 years there’ll be a sizable market. We should be into discovery now.’
“That’s probably happening.”
Even so, Heap doesn’t “hear of people jumping up and down admitting they’re doing that. (If they did admit it), maybe they’re worried they’d flag their competition to do the same.”
Currently, Mid-South weed scientists are in the process of extensive lab work on several weed species suspected to be glyphosate-resistant. Heap is well aware of their efforts.
“The big one they’re looking at is Palmer amaranth. That weed is probably the most important glyphosate-resistant weed we have now.”
Pigweeds developing resistance are Heap’s “biggest fear” for the United States. If resistance has happened in Palmer amaranth, “it’s going to happen in water hemp, in redroot pigweed — actually all of the pigweeds across the Midwest and South. That’s very frightening because they’re a key weed already with resistance to triazines and ALS inhibitors. Pigweeds are a major headache.”
Best-case scenario for glyphosate resistance? “Logically, the best-case still includes more glyphosate-resistant weeds developing. But best-case is we’ll see them appear very infrequently — a case here and a case there without widespread resistance across vast areas. Also, companies will produce more herbicide modes of action.”
Worst-case would be an explosion of resistance by pigweeds and other major weeds without alternative controls.
The most probable scenario “lies somewhere in the middle. That would mean that, yes, we’ll see quite a number of glyphosate-resistant weeds. Some, like pigweeds, will be major trouble and we’ll struggle to find some good control alternatives from the past.”