Cover Story: Combatting the Menace of Glyphosate-Resistant Weeds
Tuesday, November 2, 2010
Glyphosate-resistant weeds like giant ragweed are becoming increasingly common in Ontario, and farmers are trying a variety of strategies to deal with them. The experts agree on one thing: blaming bad management is too simple an answer
by MARY BAXTER
In the 1990s television series, "Star Trek: The Next Generation," recurrent villains called the Borg were fond of telling the main protagonists that "resistance is futile." The Borg, members of an inter-species collective who communicated telepathically, flew through space in a giant cube and looked like they bathed in buckets of silver paint, delivered this pronouncement with such assurance that it never failed to make the show's central characters doubt their own resources.
Like the crew of the starship Enterprise, farmers in southwestern Ontario are mustering their defenses against an intimidating enemy. Only in this case it's not an intergalactic alien but rather a spectre called glyphosate resistance.
The Willis family, who farm 2,500 acres of corn, soybeans and wheat on rotation near Maidstone in Essex County, were the first in the province to have the resistance confirmed in their fields by University of Guelph researchers. It was found in a flush of giant ragweed in a field the family rented from Windsor International Airport.
Rick Willis, who farms with his father Ron and brothers Rob, Ryan and Ron, says they have grown soybeans continuously in the field for several years. The airport has special cropping restrictions (the field is adjacent to a runway) to discourage birds and pests. The family has grown glyphosate-tolerant soybeans there since 2002.
Willis first noticed something wrong in 2007. A week after it was sprayed with glyphosate, an early flush of ragweed was not yellowing at the growing point although there were burnt leaves. "It looked like it was dying, it looked bad but it just wasn't what glyphosate usually does," Willis says. A week later, it began growing again. Willis contacted his chemical supplier, Essex-based Setterington's Fertilizer Service, and Peter Sikkema, a professor and weed management specialist at the University of Guelph's Ridgetown Campus. Combining another herbicide with glyphosate was recommended, but it had no effect. Some of the weeds went to seed.
The next year the giant ragweed returned in an even bigger flush and seemed to be even more resilient and resistant. The weeds had spread to adjacent fields and to another field further away. The family guesses it was spread either through pollen or a combine.
By 2009, after extensive testing, the university confirmed that the weeds in the first field were resistant. The family no longer farms that field because the airport has reclaimed it. They continue to deal with resistance problems in other fields and are working with the university to find solutions.
"It just grows so fast and the last year that we grew beans in there we're talking a yield loss of 95 per cent in these large flush areas," says Willis.
Worldwide, there are now 19 glyphosate-resistant weeds listed on the industry-funded International Survey of Herbicide Resistant Weeds. Ten of these are in the United States. The acreage they occupy there makes up about 3.3 per cent of the 300 million acres the country harvested in 2007.
A report released this year from the United States' national academies blames the increase in resistant weeds to overuse of glyphosate on crops and warns that repeated, intensive use of the "once-in-a-century" herbicide will diminish the range of weeds it can control. Overuse fosters resistance by creating the conditions for a very small number of weeds with natural resistance to thrive and multiply. Glyphosate-tolerant crops increase the likelihood of the herbicide being used not only year after year but also several times within a growing season.
Popular product
Glyphosate was introduced to Canada in 1976 by its developer, Monsanto. The ability to eliminate a wide variety of weeds at different stages of growth is what has helped it to earn the handle "once-in-a-century" herbicide. Not leaving a toxic residue in soil is another of the herbicide's attributes.
The introduction of glyphosate-tolerant corn and soybeans cemented the herbicide's popularity in agriculture. About 70 per cent of soybeans and 60 per cent of corn grown in Ontario are now glyphosate-tolerant. With the other features that these varieties offer, such as a yield boost, farmers are becoming more dependent on the products.
Philip Shaw, an agricultural economist and writer who farms 860 acres of corn, soybeans, wheat and sweet corn near Dresden in Chatham-Kent, says he introduced glyphosate-tolerant corn into his rotation a few years ago because of the yield boost compared to conventional varieties. And Henry Denotter, who farms more than 1,500 acres of corn, soybeans and wheat in rotation near Kingsville, notes that there are few conventional soybean varieties left other than those for food-grade beans.
François Tardif, an associate professor of weed science in the department of plant agriculture at the University of Guelph, says glyphosate is the most widely used herbicide in the world whose total sales surpass the combined global totals of the 10 next most popular herbicides.
Tardif knew it was only a matter of time before resistance would turn up, although he's surprised it appeared first in giant ragweed, a member of the broadleaf family and cousin to common ragweed. "On our list, Canada fleabane has always been number one in terms of being suspect," he explains. In the United States, glyphosate resistant varieties of the weed have been found in 18 states, including Michigan. The university is also keeping an eye on resistance developing in pigweed and common ragweed.
Now that they have confirmed glyphosate-resistant giant ragweed, Tardif, Sikkema and Joe Vink, one of their graduate students, are studying how far it has spread and how best to control it. Last year, Vink collected samples from 60 locations in Essex County, grew them at the university's main campus in Guelph and sprayed them with glyphosate to determine whether they were resistant. Results of the survey were to be determined by the end of October. He's added locations in Chatham-Kent and Lambton County to this year's survey list.
For his part, Sikkema anticipates resistance eventually won't be limited to giant ragweed but points out that it's unlikely all of the resistant varieties found in the United States will be able to survive Ontario's climate. Studies have begun on another weed species, but he won't say what it is. Confirming resistance takes time, he explains. He would prefer to proceed slowly to ensure the correct information is in place before releasing conclusions rather than retracting it "just because we didn't have all the experiments in place to say that conclusively."
Already it looks like the problem weed is more widespread than first thought.
Shaw says he first encountered glyphosate-resistant giant ragweed in 2005, a time when he did not grow glyphosate-tolerant crops. He noticed it when he was burning down his fields. "It would burn to the ground and come right back up." The University of Guelph researchers confirmed its presence last spring.
The weed is appearing along the edge of a field that borders four other farms. He's told neighbours about it and they're watching for it, too. "It wouldn't surprise me if southwestern Ontario has a problem with it," he says.
Pollen flow a factor?
Chris Snip, an Agris Co-operative crops sales specialist who serves western Essex County, says the problem isn't spreading as much as "farmers are becoming aware that it's possibly out there." His company has fielded calls on the problem. "Farmers are concerned. They want to know what they can do to help minimize the potential for themselves to run into the situation."
He first noticed glyphosate-resistant giant ragweed about three years ago on two different farms in his service area. At first, he thought the weeds survived because of herbicide mistiming. "Giant ragweed is a very tough to control to begin with and there's a lot of herbicides that don't work well on it," he says. "Some of the stuff we just assumed was beyond the stage of easy control."
There's a perception that the problem has to do with poor management, but Snip says the resistant weed has been spotted at locations where glyphosate wasn't overused.
Tardif agrees that bad management may be too simplistic an explanation. Although there are lots of Roundup Ready soybeans used in the area, "we suspect pollen flow could be an issue," he says, pointing out that the weed is abundant in southwestern Ontario and grows in ditches.
Hugh Beckie, an Agriculture and Agri-Food Canada research scientist who specializes in weed management in Western Canada, says he's seen herbicide resistance (other than glyphosate) develop on well-managed farms with a diverse crop rotation. Those who do find it on their farm often become cautious about advertising the discovery. "It's a bit like saying that you have cancer, a bit of a guilt thing." It can negatively affect land values, he points out. He urges farmers suspecting glyphosate-resistant weeds to contact researchers as soon as possible.
Willis says that it's important to be forthright so people can work together to stop "problems like this." By shying away from the subject because "you think people are going to judge you on how you manage your farm, you're just hurting the industry more."
He says the problem does not seem to be apparent on fields where the family has introduced rotation. In the airport-owned fields they continue to work, and where much of the problem is evident, they are considering adding wheat – not only to introduce rotation but also to gain access to 2,4-D, which is licensed for wheat crops. The airport has relaxed some of its restrictions, he says.
Shaw, who rotates his crops, has tried different herbicides to tackle the resistance and says this approach is working. In his fields, glyphosate still appears to be effective if it's applied a month earlier or later than when he would traditionally apply it, around mid-May. A few years ago, he heard a U.S. expert propose that insects interacting with the plant might boost its resistance and wonders if this might be taking place in his field. "I think there are mitigating factors in this."
Denotter says he hasn't seen any indication of the weed in his fields. Nevertheless, he applies glyphosate sparingly. If he uses it for a pre-seeding, burn-down application, he will choose another herbicide to apply before the crop emerges. That way, glyphosate becomes the backup if another round of spraying is needed once his glyphosate-tolerant soybeans have broken the ground.
Denotter, who is president of the Ontario Soil and Crop Improvement Association's St. Clair regional association, sees rotation as a key strategy to keeping resistant weeds at bay. He says many Essex growers don't rotate their crops.
Snowball effect
Brian Taylor, vice-president and owner of Setterington's, says a significant portion of the county's roughly 270,000 acres of cash crops is a monoculture of glyphosate-resistant soybeans. "We've never been a big corn county." Convincing growers to introduce rotation is not realistic, he says. Many are older and do not want to invest in the storage and drying equipment that accompanies corn production.
He estimates there are now about 30,000 acres infested with giant ragweed. Most of it is concentrated near Windsor and close to the lakes. "I think we have resistance on a lot more than giant ragweed," he adds, noting that common ragweed, velvetleaf and smartweed are becoming tougher to eradicate. "We have this whole snowball effect here."
To handle the problem, the company first tried mixing other herbicides with glyphosate. Because of the lack of rotation, he worries weeds will build resistance to these chemicals, too. In the Willis' case, he has recommended a switch to glufosinate-tolerant soybeans. It seems to have worked, but he questions how long it will be before weeds will build resistance to that herbicide.
Tardif says studies show that adding wheat to a rotation of glyphosate-tolerant beans and corn will slow the spread of resistance for perhaps a couple of years, but it won't eradicate it. He explains that one giant ragweed can produce thousands of seeds, though only a few will germinate in the first year. The remainder germinates in subsequent years. Mathematical modelling indicates that mixing herbicides with different types of action is more efficient than rotation.
Adding more herbicides can mean greater expense for farmers. (Willis estimates it costs $10 to $20 extra per acre to add other herbicides to glyphosate). And higher herbicide costs to combat glyphosate resistance in weeds can lead to significant changes in tillage practices, suggests a 2005 study of the North Delta area of the southern United States.
The study indicated that 30 per cent of all farmers and 50 per cent of cotton growers who practised conservation tillage abandoned the practice because of increased herbicide costs.
Tardif acknowledges that mechanical control is the strategy being adopted in places such as areas in Australia where glyphosate resistance has become a serious problem. But, in Ontario, where other tools are available to control the weeds, he doubts there's an imminent threat to conservation tillage.
Although resistance can happen quickly, it's localized right now. "It's not like 90 per cent of the fields in the township have them," he says. But if it reaches a point where there are species resistant to glyphosate and in the same field there is also resistance to herbicides such as triazolpyramidines, classified as Group 2 herbicides, "then the number of options get so low that you may be forced to using tillage again."
Willis suspects that the weeds near the airport are also resistant to Group 2 herbicides, because spraying with a product from that category had little effect. No decisions have been made on whether to change the family's minimal tillage practices. He's heard that giant ragweed seeds remaining on the surface in a no-till system would germinate more quickly, making it easier to address the problem with pre-planting herbicide applications. But he's also heard that burying the seeds using deep tillage would delay their emergence so that, when outbreaks happen, they are smaller and therefore easier to deal with.
"I think they (experts) are mixed on it, too," he says. "It hasn't been around for enough time to really get a hold on what we've got to do."
Web-based tool
In the meantime, Sikkema has begun work on obtaining a license to apply 2,4-D on fields before soybean planting. He doubts that increasing weed resistance to glyphosate will spell the end of the use of the chemical, glyphosate-tolerant crops or conservation tillage. Glyphosate still controls the majority of weed species in the field and reduced tillage offers "huge advantages," he says. "I think that, through the use of a diverse crop rotation and multiple herbicide modes of action in the tank, over time we should be able to manage that problem without having to resort to increased tillage."
Mark Lawton, Monsanto's Eastern Canada technology development lead, says researchers are also working on a web-based tool that will help farmers to judge the risk of weed resistance in their fields. The tool, already available to farmers in Western Canada and the United States, analyzes responses to an interactive survey and can be found online at www.weedtool.com. "We certainly will drive growers to that as a way for them to assess their own situation," Lawton says.
The company is also helping to fund research at the University of Guelph but won't disclose the amount. "It's really a project between us and the University of Guelph, so we don't typically disclose support that we're giving to this project or other projects," he says. The company supports the use of other herbicides along with applications of its product. But there are no plans to introduce a rebate program, such as the one it offers U.S. cotton growers to fight glyphosate-resistant Palmer amaranth. The program offers rebates of up to US$12.50 per acre for adding a residual herbicide application for those who buy the company's glyphosate products.
"The situations are very different, north and south," says Lawton. "We're nowhere near that. We've got one field and they have a lot more than that in different regions."
Shaw, for one, isn't wasting sleep over the issue. "When soybean aphids came along back in 2001, we all thought the apocalypse was here, including me. Well this year, we didn't have any. In fact, we haven't had a significant amount since then," he says. "So things change and you manage things differently." BF
Some preventive measures you can take
Here are some tips from the experts and those who are already dealing with the problem.
• Maintain a diverse crop rotation.
• Introduce different modes of herbicide action.
• Scout crops before and after all herbicide applications to measure effectiveness.
• Apply the rates listed on the label and exercise care with spraying equipment so that it is set up to tackle the size of weed in question.
• Take into account weed size and don't apply too low a rate.
• Contact a chemical representative, dealer or seed agronomist as soon as possible if there are concerns about glyphosate or another herbicide's performance. Don't wait for it to become a bigger problem.
• If glyphosate resistance is found, clean out combines to avoid spreading it.
• Tarp trucks when moving from field to field.
• Avoid contaminated crop seed when planting. BF
What is giant ragweed?
Giant ragweed is an annual that's part of the broadleaf family and is related to common ragweed. Farmers and researchers describe it as a tough weed to kill, especially as it gets larger. (It can reach 12 feet in ideal conditions). Glyphosate resistance in this weed poses the greatest risk to soybean crops because there are few alternative herbicides that can be used in conjunction with the crop to control it. BF