Once a persistent but manageable roadside weed, Canada fleabane has become a serious management issue for Ontario farmers. Over the past six years, herbicide resistant Canada fleabane has spread quickly over a large area of the province and, depending on the crop, has been a challenge to eliminate.
“We first confirmed glyphosate resistant Canada fleabane in Essex County in 2010, which is in the far southwest area of Ontario near the Michigan border,” says Peter Sikkema, professor of weed management with the University of Guelph, Ridgetown Campus. And in five years since that time it has moved more than 800 kilometres and has been confirmed in 28 counties — as far east as the Quebec border.
Canada fleabane is a common winter and summer annual weed whose seeds are spread through the air. It germinates mostly in the fall while a small portion of seeds will germinate in the spring. Emergence has sometimes been observed during the winter months during warm spells, which shows this plant’s adaptability.
Fall-emerging Canada fleabane grows to a rosette stage before overwintering, then resumes its growth in the spring when it bolts and flowers. Spring emerged individuals go straight to the flowering stage. Canada fleabane, growing in an uncompetitive environment, can produce more than 200,000 seeds per plant, which are blown through the air, much like a dandelion, for an almost endless supply of seed. Plants can grow up to two metres in height, higher than many crops, and compete with the crop for moisture, light and nutrients.
“Canada fleabane has always been around but it somehow flew under the radar,” says François Tardif, professor of plant agriculture at the University of Guelph. “You would see it in old fields and in pastures. It wasn’t until no-till caught on that we started seeing it a lot more in field crops, but even then we just sprayed it with glyphosate and it was gone,” he says. “When glyphosate resistant Canada fleabane was confirmed in soybean in the U.S. in 2001, we knew there was a possibility for resistance in Ontario and we started to look at it more closely.”
While Tardif says he has also seen Group 2 resistant Canada fleabane, (see table on next page for modes of action) he has yet to confirm it independent of Group 9 glyphosate resistance in Ontario. Unfortunately, this secondary resistance adds another level of complexity because it takes Group 2 chemistries out of the mix for possible weed management solutions.
Tardif’s research looks at the mechanism of resistance in Canada fleabane. “We have been looking at the biology of Canada fleabane to determine the genetic diversity within the population to see how it is selecting for resistance,” he says. “We are looking to see if there are ways you can eliminate resistance within the weed by methods such as spraying glyphosate when the weather is cold. Anecdotally, we have heard that could be part of the solution but I would be skeptical to say that works unless we can prove it.”
Tardif says there has been a lot of research into the development of glyphosate resistance in fleabane in the U.S. as researchers there look for alternative ways to control the weed, and his research looks at applying those ideas within a Canadian context. But, he says, all are in the early stages of finding solutions.
“One of the things we are looking at is if you can apply products that are normally injurious to the crop in a way that would be safe for the crop and still kill the resistant fleabane,” he says. “An example would be the use of Wick Weeders. These applicators “paint” the surface of the weed with herbicide which is then translocated down throughout the roots without harming the crop. This could be one way to offer growers more weed control options on resistant species.”
MATCHING THE CROP TO THE SOLUTION
Resistant fleabane can be found in any crop in Ontario, but it is particularly problematic in soybean. In winter wheat, growers are able to use a post-emergent application of Infinity herbicide for safe and effective control of the weed. In corn, glyphosate has historically been the chemistry of choice for a pre-seeding burndown of fleabane. But with the spread of glyphosate resistant fleabane, the yield losses can be significant if no weed management tactics are employed.
“In fields where we have conducted research on glyphosate resistant Canada fleabane, the yield losses have been substantial,” says Sikkema. “This is not in every corn field. I want to stress that the fields where we conduct our research tend to have the most severe densities of resistant weeds. In those corn fields, average yield loss is 65 per cent due to glyphosate resistant Canada fleabane interference, and in fields with the heaviest weed pressure, corn yield can be reduced by 99 per cent.”
Sikkema says that his research focused on both pre-plant and post-emergent control and he was able to find solutions control, we studied which herbicides provided commercially acceptable control when combined with glyphosate,” he says. “We found four products — Callisto + atrazine, Integrity, Banvel and Marksman — which all provided greater than 90 per cent control of glyphosate resistant Canada fleabane applied pre-plant in corn.”
For post-emergent control in corn, he found some of the better options were the dicamba-based herbicides including Banvel, Distinct and Marksman. In addition, a tank-mix of Pardner with atrazine also provided acceptable control.
Soybean has presented the greatest challenge for managing glyphosate resistant Canada fleabane, since removing a broadleaf weed in a broadleaf crop can be very difficult at the best of times. In fields with high densities of glyphosate resistant Canada fleabane, Sikkema says they documented yield losses anywhere from 70 to 99 per cent.
Unfortunately for growers who have glyphosate resistant Canada fleabane in their soy, Sikkema and his team trialed all registered broadleaf herbicides in soybean, and none of them provided consistent control of glyphosate resistant Canada fleabane.
“We conducted three different experiments with glyphosate tank-mixes for the control of glyphosate resistant Canada fleabane,” he says. This included products aimed at effective control at burndown, those applied at burndown with full season residual control, and those applied post-emergence. “Not one of the registered broadleaf herbicides in soybean applied in a tank mix with glyphosate consistently provided 90 per cent control, which is the minimum growers consider acceptable.”
However, all hope is not lost. Sikkema’s team was able to achieve greater than 90 per cent control consistently when using a three-way tank mix. The top performers were glyphosate mixed with Eragon and 2-4-D at 92 per cent control, and glyphosate with Eragon and Gramoxone at 95 per cent control. The top performing option was glyphosate with Eragon and metribuzin which provided 97 per cent Canada fleabane control.
KEEPING YOUR OPTIONS OPEN
While tillage is one component of an overall integrated weed management strategy for the the control of glyphosate resistant Canada fleabane, Sikkema cautions against the use of aggressive tillage. “We would really be taking a step backward by relying on tillage for weed management as we would quickly deplete our topsoil resource,” he says. “However, by incorporating tillage in a long-term diversified crop rotation, it could help reduce the incidence of the weed.”
While Tardif also has concerns with wide-spread tillage, he is researching the minimal level of disturbance that will best preserve the soil while still killing the fleabane. “We are also looking at the timing of that low level of tillage,” he says. “Perhaps a grower can till the soil less in the fall than in the spring, and still kill the fleabane. Our research is looking to find answers.”
Sikkema says that growers need to consider an integrated weed management approach on their farms to reduce the incidence of herbicide resistance. “To keep our herbicides working longer we have to diversify our crop rotation and we need to make sure we are using multiple modes of action on every acre,” he says.
“We should be using more cover crops to prevent weeds from becoming established and possibly incorporate tillage at strategic points in a long-term diversified crop rotation,” adds Skikkema. “In addition, growers should aim for near perfect weed control in order to reduce weed seed return to the soil. By using all these strategies, hopefully the selection for herbicide resistance will be reduced.”