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Resistance an issue that must be addressed

November 30, 1999  By Ralph Pearce

Some growing seasons bring the optimism of a new year, with the potential for high yields, low disease or pest pressure and rising commodity prices. Then reality sets in.

In the US Mid-South, growers in Tennessee, Arkansas, Mississippi and Missouri have been forced to deal with the fact that resistance to herbicides, specifically glyphosate, is a problem across that district, and if they are to deal with it effectively, the time to act is now.

Herbicide resistance is a growing problem across all agricultural regions of North America. In Western Canada, kochia has been resistant to ALS inhibitors since the late 1980s. In Ontario, there are now eight different weed species that are resistant to multiple modes of action. And Tennessee saw the first incidence of glyphosate-resistant Canada fleabane (known as marestail in the US) in 2000, a trend that has since spread into Indiana, Ohio and points further north.


But it is the Mid-South region along the Mississippi River that has become a hallmark of sorts. In that part of the US, the primary crops are soybeans and cotton, and almost all of the varieties grown in that region are glyphosate tolerant (Roundup Ready). The resulting reliance on that technology has given root to a biotype of Palmer amaranth (Palmer pigweed) that is now resistant to glyphosate. Reliance on Roundup Ready technology has grown to the point where no amount of glyphosate is sufficient to kill various weed species. That leaves growers to rely on frequent spray applications with other, less-effective herbicides, as well as manual labour to physically remove the plants. In fact, one local farmer spent $250 per acre on weed control in 2009, including manually cutting the weeds out of the field.

Glyphosate still a viable option
In July 2010, Bayer CropScience hosted the “Respect the Rotation” conference and tour in Memphis, Tennessee. The gathering was a follow-up to the company’s Pan-American conference on weed resistance, held in January 2010 in Miami, Florida. The three-day event in Tennessee included presentations, field tours and one-on-one sessions with researchers and extension personnel. The main premise behind the conference was to promote integrated weed management principles while promoting the company’s new LibertyLink soybean system.

The last thing that organizers and speakers wanted to do was criticize or blame anyone for allowing the development of glyphosate resistance. Presenters during the field tours and press conferences repeatedly praised glyphosate, calling it revolutionary and a “one in 100-year event.” Yet the field tour offered a contrast of cautionary tales of what can happen with the extent of glyphosate resistance that has spread across the region, together with a sense of optimism that acknowledges the problem of weed resistance as a first step towards dealing with the issue.

The genesis of the problem
According to Dr. Jason Norsworthy, an associate professor with the University of Arkansas, the first sign of glyphosate resistance in the region was detected in a soybean field in the northeastern part of the state in 2005. It is now to the point where Norsworthy says it is widespread across Arkansas, with the majority of Palmer amaranth in any field being resistant to glyphosate. “You’re not necessarily going to have Palmer amaranth in every acre, and basically, what we’re telling our farmers is, if you learn you have Palmer amaranth, you’d better treat it as if it’s resistant,” says Norsworthy, who was also part of the field tour, held just west of Memphis, near Widener, Arkansas. “Let’s say you make an application on six- to eight-inch Palmer amaranth, and you come back seven to 14 days later and realize that it was resistant. Then you’re looking at an 18- to 24-inch Palmer amaranth. And we really don’t have any option for controlling that.”

The reason for the virtual explosion of Palmer amaranth, adds Norsthworthy, is a blend of factors. Part of it is the relatively aggressive and prolific nature of the plant itself; one plant head can produce as many as half a million seeds. Another contributor is the rotation pattern for the two dominant crops, soybeans and cotton. In Arkansas, there are 1.5 million acres of rice rotated with 3.5 million acres of soybeans, yet only about 500,000 acres of cotton. Most of those cotton acres have been monocultures for the past 30 to 35 years. To add to the challenge, between 1996 and 2000, Roundup Ready cotton became the technology of choice. “It’s quite obvious that we have soybean acreage that is not rotated with any other crop,” notes Norsworthy, adding that some regions are not conducive to growing rice or cotton. “So farmers are going to roll the dice and try their best at getting a crop of soybeans, and these fields have had 10 to 15 years of Roundup Ready soybeans.”

Irrigation of rice fields, poorly managed levies (where Palmer amaranth often grows unchecked), frequent flooding along the Mississippi River basin, and the spreading of seed through cotton gin trash have exacerbated the issue further. Although the US Midwest and southern Canada are not seeing the same spread and degree of glyphosate resistance in the short term, Norsworthy believes it will increase over time.

Mindset is also a factor
Another of the contributors to this situation is the continued belief that the chemical industry will be there to bail out farmers, regardless of reliance, or in this case, over-reliance, on glyphosate-resistant technologies. By all accounts, glyphosate’s development dramatically changed the whole approach to weed management, with a corresponding sense of trust that scientists and researchers will find something as good as glyphosate, despite repeated assurances that no such “silver bullet” is in the pipeline. “We’ve never faced losing a technology that’s revolutionized farming like this one has, and the LibertyLink technology is very good in a lot of ways,” explains Ford Baldwin, a weed management consultant with Riceland, a farmer co-operative based in Stuttgart, Arkansas. “But we’ve abused the Roundup Ready technology to the point it’s going to force us to abuse the LibertyLink system, and so we are just going to start chasing traits now instead of chasing herbicide modes of action.”

During the winter of 2004-2005, Baldwin wrote continuously of the impending “train wreck” of Roundup Ready resistance, and while he concedes he might have done more to inform farmers and the industry, he also points out that people have to be willing to listen. “Some of that complacency’s wearing off,” says Baldwin, who was also with the recent tour in Widener, Arkansas. “The difference between our farmer awareness from 2009 to 2010 has been a huge step forward, and I’m hoping that we’ll get the snowball effect now, and they are finally going to see that ‘Hey, we’ve got to do something, we’ve got to listen, we can’t depend on somebody now to swoop in with a magic bullet,’ because that’s not going to happen.”

Canadian growers have time, for now
There is little room for debate when the topic of herbicide-resistant weeds is raised, even in Canada. Despite colder climates (which Norsworthy contends is not as much of a limiting factor), it is the longer rotations together with either separation of technologies or rotation of different technologies that has done more to delay the onset of resistance. That growers in Eastern Canada can interrupt their Roundup Ready reliance with wheat or a forage crop is a tremendous advantage, and farmers in Western Canada can seed wheat and rotate four different canola technologies (InVigor/LibertyLink, Roundup, Nexera and Clearfield) to break the cycle. “We need to diversify the system as much as we can, and Canada is a good example of that,” says Dr. Stephen Powles, director of the Herbicide Resistance Initiative at the University of Western Australia, and considered one of the foremost authorities on the subject of herbicide resistance.

Reliance on glyphosate has led to an inability to control Palmer amaranth in the US Mid-South, but Powles posed  one particular question during the field tour at Widener: If we put our reliance in LibertyLink now, what is to prevent a similar gathering in the same field, five years from now, with everyone asking, “How do we counter resistance to LibertyLink?”

“Thus far, there are no examples where Roundup Ready resistance is evolving on the Prairies, or if there are, they‘re very few. And why is that? Because there’s diversity in the system, and glyphosate is only being used in part of the system.”

As diverse a cropping system as it may be, some, like Mike Cowbrough, maintain that resistance is only a matter of time, thus the need for vigilance now. As the weed management lead with the Ontario Ministry of Agriculture, Food and Rural Affairs, Cowbrough is author of and contributor to several factsheets and circulars detailing the “obnoxious eight” weeds in Ontario that have shown resistance to multiple modes of actions. “Weeds are incredibly adaptive and they’ve lasted millions of years, so it’s foolish of us to think that we can harness and control them,” notes Cowbrough. “We’ve been trying to do that in agriculture for hundreds of years, and we’re unsuccessful.”

On the plus side, however, Cowbrough points to Ontario researchers such as Drs. Peter Sikkema, Clarence Swanton and Francois Tardiff, all from the University of Guelph, whom he says are very responsive to the onset of resistance.

“Look at giant ragweed, for example. Peter’s been looking at that for two or three years, and eventually, it will become more widespread,” details Cowbrough. “When that does happen, do we know how to manage it?  If you’re complacent, then you don’t have a chance, but we also have a very good culture that exists in the province, where agronomists and growers are sharing issues and are really helpful in determining how we move forward.”

In the final analysis, it is not the technology itself that provides the answers. Kate Barrie, technical support representative with Bayer CropScience in Guelph, Ontario, acknowledges that LibertyLink is part of the answer, but so too is partnering with the best people in an effort to get the necessary information into the hands of farmers. “A movement or an event like “Respect the Rotation” is a tool and an idea to help growers be aware of what’s going on,” she says. “Saying ‘Respect the Rotation’ over and over isn’t going to get guys to change from Roundup to something else; that’s not the point. The point is to educate and let them know that it could happen if they’re not aware.”


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