Solving the Resistance Riddle
By Jason Jenkins, Mill Creek Communications
In the 1993 blockbuster film, “Jurassic Park,” director Steven Spielberg brings to life a world where once-extinct dinosaurs now walk among humans at an ill-fated theme park. Set on a fictional tropical island in the Pacific Ocean, the movie introduces audiences to Ian Malcolm, a mathematician and chaos theorist.
While on a tour of the park’s laboratories, Malcolm and the other visiting scientists learn that to prevent “unauthorized breeding,” all the dinosaurs at Jurassic Park have been genetically engineered to be female. Upon learning this, Malcolm is the first to share his opinion.
“The kind of control you’re attempting is, uh, it’s not possible. Listen, if there’s one thing the history of evolution has taught us, it’s that life will not be contained. Life breaks free, expands to new territories and crashes through barriers, painfully maybe even dangerously,” he says. “Life finds a way.”
Kevin Bradley would agree. Life does seem to find a way, but he’s not worried about being trampled by a T-Rex or eviscerated by a Velociraptor. Instead, the University of Missouri Extension weed scientist is concerned about pigweed that prevails despite a producer’s chemical-control efforts.
For nearly 75 years, farmers around the globe have enlisted herbicides in their war on weeds. While these chemicals have revolutionized worldwide food production and helped usher in today’s modern agricultural system, they haven’t eliminated the enemy. Herbicide-resistant weeds are prevalent, and their strength and numbers seem to increase with each growing season.
“Given the way we do agriculture in the United States, resistance is pretty much inevitable with any herbicide,” Bradley says. “It’s so intense, it’s so massive and we rely so much on herbicides. So, it’s apart of life, and life does find a way.”
The world’s first successful chemical herbicide — 2,4-D — was commercially released in 1946, a result of research conducted in both the U.K. and the U.S. during World War II. The selective chemistry provided control of broadleaf plants in cereal crops such as wheat, corn and rice. Producers around the world quickly adopted the technology, and other selective and nonselective herbicides would follow, including atrazine in 1958 and glyphosate in 1974.
But nearly as quickly as companies could commercialize new herbicides, other scientists were identifying plants that wouldn’t succumb to weed killers. In 1957, just 11 years after 2,4-D was introduced, occurrences of resistance to the herbicide were documented in populations of both Queen Anne’s lace and spreading day flower. In 1970, atrazine resistance would be confirmed in common groundsel and in 1984, a population of field bindweed was found to be resistant to glyphosate.
“We don’t create resistant weeds,” Bradley says. “They are out there, and we select them out by applying the herbicide. You could have a brand-new herbicide, and chances are, there’s already a weed out there resistant to it. We get into trouble when we start applying that same herbicide over and over, selecting out that one plant and allowing it to live and produce seed. Then next year, there’s more of them.”
Research supports the existence of naturally occurring resistance. In 2013, a group of French scientists reported on a project in which DNA analysis
had been conducted on more than 700 herbarium specimens from three European collections. One particular specimen, a slender meadow foxtail collected in 1888, contained in its DNA the genetic mutation that would have provided resistance to Group 1 herbicides, the ACCase inhibitors — even though the first of these herbicides was not discovered until the 1970s.
“That just tells you, resistance is something that’s present in the population, whether we can see it or not,” Bradley says. “We often never see those survivors. In a good management system, you’ll spray, and you might have a survivor or two. But then you come back in and take care of those survivors. You won the battle.”
At the beginning of 2020, there were 510 unique cases of herbicide-resistant weeds worldwide, according to the International Survey of Herbicide Resistant Weeds. Resistance was reported in 262 different species and in 93 crops across 70 countries. Of the 26 known herbicide sites of action, weeds have evolved resistance to 23 of them.
In Missouri, the International Survey recognizes 10 different weed species resistant to at least one herbicide group. The most concerning of these for the state’s soybean producers is common waterhemp, Bradley says, followed by Palmer amaranth, especially in the Bootheel where it’s more prevalent. Other herbicide-resistant weeds of concern include marestail and giant ragweed.
“All pigweeds now, in Missouri at least, are pretty much resistant to Roundup,” he says. “We started using another class of herbicides, mostly the Group 14 herbicides such as Cobra and Flexstar, but we used those so much we’ve now got wide-scale resistance to them.”
The MU Extension weed scientist has conducted regular surveys of waterhemp populations across Missouri during thepast 16 years. He said that today, almost every population has two-, three- or even four-way herbicide resistance.
“We’ve found some that have five-way resistance, and in 2018, we even found one that can’t be controlled by six different classes of herbicide,” adds Bradley, referring to a population identified in Randolph County that survived application of 2,4-D, atrazine, chlorimuron, fomesafen, glyphosate and mesotrione. “That’s pretty scary.”
Currently, Bradley is conducting another waterhemp survey. This time around, he’s focused on identifying any resistance issues that may be occurring with glufosinate and dicamba, the herbicides employed in the LibertyLink and Xtend soybean systems, respectively.
“We’re seeing some failures at the field level, and we’re worried about resistance,” he says. “We’re worried about losing Liberty because we use Liberty a lot.”
In the greenhouse, waterhemp plants are grown until they reach a height
of 3 inches, at which time, they are sprayed with the labeled rate of either glufosinate or dicamba. Bradley notes that he and his team are seeing things that concern them.
“We don’t have any final results, but there’s great variability when you run 75 different waterhemp populations beside one another in the greenhouse,” he says.
“You have some populations where only 50 percent of the plants die, and the rest survive. Then, you’ll have another population where all the plants die. So, those are the ones we’re going to look into more thoroughly and see what’s going on.”
While resistance to an ever-growing list of post-emergence herbicides has been the story line for waterhemp, a major plot twist occurred in 2019. Weed scientists in Illinois announced they had identified two populations of the weed that were resistant to Group 15 soil-residual herbicides. This group includes metolachlor, the active ingredient in Dual Magnum and commonly used in many premixes. The discovery represented the first confirmed case in the world of a broadleaf weed species being resistant to a Group 15 herbicide and the first known to be associated with corn and soybean production.
“That is a terrifying finding because we are relying so much on soil-residual herbicides right now, and particularly metolachlor and the Group 15 herbicides,” says Bradley, noting that shortly after the Illinois announcement, researchers in Arkansas confirmed Group 15 resistance in Palmeramaranth.
“If you were to ask me what’s the most scary new type of resistance that we could have in waterhemp, that would probably be on the top of the list. I don’t know of any in Missouri. I don’t want any.”
If a future scenario were to develop in which waterhemp couldn’t be controlled with herbicides, it could have far-reaching consequences for soybean producers in the Show-Me State and beyond.
The weed grows quickly — as much as 1.25 inches per day — and multiple flushes can emerge throughout the growing season. A single plant produces roughly 250,000 seeds, which can remain viable in the soil for about four years. Overall, season-long competition with soybeans can reduce yield by 44 percent.
While Bradley would like to paint a more positive picture for the future of chemical weed control, it’s hard to overlook the history of herbicide resistance.
“We’ve been on this herbicide treadmill, and when you see the whole history, it doesn’t take too long before you ask, ‘Why do we think that herbicides are going to solve all these problems?’” he asks. “We’re down to fewer and fewer options.”
A dwindling list of effective herbicides means that more pressure gets placed on those active ingredients that are still working, Bradley notes. This increases the likelihood of future resistance. He says any living weed that isn’t killed by herbicide has the possibility of becoming a resistant weed.
“All that’s required is the weed to be present and the herbicide to be sprayed,” Bradley says. “If you look at it that way, it’s just a matter of time.”
He adds that as soybeans with new traits are released and adopted over a wide geography in a short period of time, more pressure gets placed on a particular class of herbicides.
“These traits are marketed as the answer for resistance, and growers
are obviously planting them as the answer,” he says. “We have a history of the industry bringing something new that bails growers out and solves a given resistant weed problem. So, growers believe there will always be something that will solve the next resistance problem — some new trait, some new mode of action. That’s where historically we have gotten ourselves in trouble.”
We’ve been on this herbicide treadmill, and when you see the whole history, it doesn’t take too long before you ask, ‘Why do we think that herbicides are going to solve all these problems?’ We’re down to fewer and fewer options.
Recognizing that a day may come when the agrochemical industry isn’t able to deliver the “next” answer for herbicide- resistant weeds, Bradley is among the group of weed scientists around the globe researching solutions for weed control that go beyond something sprayed. Cultural techniques such as tillage, row spacing, cover crops and harvest weed seed management are all part of his research program, which is funded in part by the Missouri Soybean Merchandising Council.
“Some of these things aren’t new, but we’re trying to get them back in front
of growers and get them integrated with the herbicide program to make an overall effective program,” Bradley says. “In the future, it’s going to take more than herbicides. Just doing herbicides is what’s gotten us into trouble.”
Photos courtesy of University of Missouri College of Agriculture, Food and Natural Resources and the United Soybean Board
This is the first article in a yearlong series examining the past, present and future of weed control in the production of soybeans in the Show-Me State. This issue, we address the topic of herbicide resistance — from its origins to its future implications.
Find the rest of the issue here.