Here in the 2020s, herbicide resistance seems just part of the reality of managing weeds. It’s certainly not a new topic. We faced resistance to atrazine in the 1970s, then Group 2 ALS inhibitors in the 1990s and a more recent, widespread resistance to glyphosate. But of course the resistance threat keeps getting more complicated: In the state of Illinois, some waterhemp plants today can shrug off seven different modes of action.
And it’s even more complicated than that, because of the discovery of metabolic or metabolism-based resistance in weed species.
Some researchers are sounding alarms about the discovery of metabolic resistance, and scientists are only beginning to unpick the complexities of the processes involved. Dr. Bill Johnson of Purdue University and Dr. Patrick Tranel from the University of Illinois have been studying the phenomenon and making presentations at conferences and annual meetings for the past couple of years.
Certified crop advisors (CCAs) and agronomists do seem to be paying attention, but there’s also a lot of inertia in today’s agriculture, stoked by a “farming for today” mindset among many farmers.
How it works
Since waterhemp is resistant to so many modes of action, it’s the subject of study for Tranel, Johnson and many others. Target-site resistance occurs because the weed has a mutation that affects that site that the herbicide wants to attack. This changes the sequence of amino acids in a protein and prevents the herbicide from binding to it. Often, the alteration of just one amino acid is enough.
Non-target site resistance is different — and metabolic-based resistance, in particular. Here, the resistance occurs somewhere else in the plant and affects the translocation or isolation of the herbicide so the herbicide can’t get to the target site.
The scientific details are complex. It’s described as a three-stage process which starts with the modification of the herbicide molecule, priming it for further modifications. The second stage sees a combination of the modified herbicide with sugar or some other compound. The third stage involves “transport enzymes” which move the herbicide molecule to a plant cell’s vacuole, where it’s discharged.
“There are specific mechanisms, different genes that can be contributing, so it makes it much harder to understand how it evolves,” says Tranel, professor and associate head of crop sciences at the University of Illinois. “Resistance is an evolutionary process and in order to manage it we have to play by the rules of evolution. But if we don’t understand precisely how it’s evolving, we can’t come up with recommendations on how to delay that process.”
According to Tranel, the first case of metabolic resistance in waterhemp was to atrazine, but it was already known to have target-site resistance, so discovering there was metabolic resistance wasn’t going to change how to manage it.
But since 2013, all new mechanisms of resistance that scientists have uncovered in waterhemp have been non-target-site — a complete turnaround.
“When it really became important was with HPPD resistance — the Group 27 herbicides — because we did not have any target-site resistance to that herbicide at that point,” he adds. “We didn’t have any resistance to HPPD in any weed; waterhemp was the first example and it was metabolic resistance. That’s really what caught the attention and why metabolic resistance became important.”
It’s a considerable problem for Tranel and Johnson, not knowing what’s causing resistance or selecting for it. Unlike target-site resistance, they can’t tell a farmer what to do to prevent herbicide resistance or at least delay it, or how to use herbicides to prevent metabolic resistance because no one understands how it’s evolving.
“But the truly scary thing about metabolic resistance is that you can have resistance to herbicides that may not have been used before,” says Johnson, professor and weed control specialist with Purdue University.
Tranel notes that he is starting to see metabolic resistance in the Group 15 chloroacetamide products (shoot growth inhibitors), often in the same field where there’s resistance to HPPD inhibitors (Group 27). It might have been selection that had been occurring for years with chloroacetamides that actually provided some of the selection of the Group 27 herbicides.
The issue of cross-resistance has also entered the discussion. Weed populations that are resistant to a particular herbicide also may be resistant to different herbicides, including experimental products still being tested. Tranel isn’t aware of any specific example in the U.S. where it’s known with certainty that one enzyme is metabolizing herbicides from two different sites of action. In Australia, however, scientists have found a specific enzyme that can metabolize ALS-inhibiting herbicides and ACC-ase-inhibiting herbicides (Groups 1 and 2). There’s a more recent report of a grass species commonly found in rice where one enzyme can metabolize herbicides spanning multiple sites of action.
“In the U.S., I don’t know that we’ve actually confirmed that yet,” says Tranel. “We’ve had some of these issues where we don’t know — for example the HPPD and the chloroacetamides — we don’t know if it’s one enzyme metabolizing both herbicides or not. It could be but we’re not sure.”
As for industry reactions to this, Johnson says on one hand, they’re very concerned because this affects the long-term viability of a number of products. In the short-term, however, it’s as though their focus is on making money this year, not worrying about years to come.
“In terms of discovery, it ought to spur those who write the cheques that we need to invest more in herbicide discovery and look at some different sites of action,” says Johnson. “But I’m not sure we have enough metabolic resistance on enough acres now that it’s spurring that level of concern.”
He also wonders if there’s too much reliance on the development of digital tools and service provision to farmers instead of investing in research and development of new herbicide sites of action.
Tranel agrees with Johnson’s take but also points out the time, investment and complexity involved in developing new herbicides. Discovery of a new molecule (that has the resilience or efficacy of a glyphosate) takes years, as well as navigating through regulatory channels, which are becoming more stringent in this day and age. And there is still so much to be learned about metabolic-based resistance.
“In theory, if we really understood metabolic resistance, we could know which herbicide mixtures would be the best for delaying metabolic resistance,” says Tranel. “If we know this weed can metabolize herbicide ‘A’ and ‘B,’ then we’d know that you can’t use herbicide ‘A’ and ‘B’ as an effective tank mix, just like we talk about effective tank mixes for target-site resistance. We’re not there yet but maybe in a few years we’ll have a better understanding of metabolic resistance so we can come up with better chemical-specific strategies to delay and manage it the way we delay and manage target-site resistance.”
The worst outlook is that farmers keep to current practices until the chemical companies and retailers run out of products to sell.
“Right now, the only thing we have as a post-emerge, where we don’t have any resistance in waterhemp is with glufosinate,” adds Tranel. “We already have some dicamba and 2,4-D resistance that are starting to show up. With glufosinate, there’s already been enough selection pressure that I’d think we’ll burn through that within three years if used extensively.”
In the meantime
Another response from chemical companies is a growing interest in cover crops and non-chemical weed control methods, possibly as a way of entering into those two markets, says Johnson. The equipment sector is also seeing a significant change, particularly in price and demand. Johnson and his son have decided to buy more equipment to work their land, and have found the market for rotary hoes and cultivators to be ramping up, now two to three times the prices they were a few years ago.
The addition of a third crop in rotations is another discussion point but Johnson comes up short of defaulting to winter wheat, as is the custom in Ontario. He believes if they could figure out a way to grow a third crop on 80 per cent of the acres, a lot of the weed resistance issues would be significantly delayed for several years. The challenge for Illinois and Indiana, however, is that 60 per cent of the land is wrapped up in absentee ownership, putting pressure on the renters to make money while they can.
“In a lot of cases, that’s what drives the lack of diversity — the rent prices and the share-crop arrangements,” says Johnson. “There’s no money to be made on small grains most years when you can gross $800 or $1,000 an acre on a corn or soybean crop. But not having that third crop is such a big deal, just in terms of forcing selection pressure with glyphosate, glufosinate, triazines and now the auxins.”
One of the other things that he’s been encouraging growers to do in the last four or five years is to consider adding “one thing” to their weed management practice that they normally wouldn’t do.
“Whether that’s cleaning out your combine from a dirty field to a clean field, managing your field borders, tillage in fields that are not highly erodible, or being smarter about where your cover crop seed is coming from,” says Johnson, “whatever that happens to be, try to do one thing, and if we can get a lot of people to do one thing, we would be a leg up on this.”