As herbicide-resistant Canada fleabane and other problem weeds continue to frustrate farmers, more creative control solutions are being sought. Planting cover crops, specifically cereal rye is a good tactic, say researchers from the University of Guelph and the Ontario Ministry of Agriculture, Food, and Rural Affairs.
Why it matters: Herbicide-resistant weeds are creating more challenges for farmers to control, so some are looking for non-chemical options.
In a presentation at the Innovative Farmers Association of Ontario annual conference, Mike Cowbrough, weed management and field crops specialist with the provincial agriculture ministry, discussed the effectiveness of common cover crops in suppressing Canada fleabane.
Cowbrough said cereal rye as a cover crop can be a strategic winner in many respects. This conclusion was reached, he says, after seeing repeated evidence of major fleabane suppression in field trials involving cereal rye cover.
In a separate interview, Ted Vanhie, one of Cowbrough’s colleagues and the master’s student currently managing the fleabane-control research initiative, says field trials including cereal rye reduced fleabane vigour and population “in all cases.”
“Cover crops are truly another mode of action in my mind,” says Cowbrough. “Rye is really like a broadleaf herbicide.”
Canada fleabane is a prolific weed that can produce large amounts of minute seeds that easily spread and are difficult to control.
Cereal rye contains allelochemicals, compounds produced by living organisms that can have negative effects on other species. Cereal rye naturally produces 15 different allelochemicals, two of which Cowbrough says have been identified as particularly effective at suppressing Canada fleabane.
This knowledge was unearthed accidentally while performing field trials analyzing the establishment of Canada fleabane — and biomass of established plants — with and without the presence of different tillage methods, herbicides and cover crops. More specifically, the farmer owner of the land on which Cowbrough, Vanhie and colleagues were working — a sand-soil field in Norfolk County — drilled excess cereal rye next to one of their research observation areas.
As the cover grew, they noticed very little fleabane growth within the rye-covered field, as well as a five to eight-inch fleabane-free strip in the area adjacent to the last row. What fleabane was found within the rye itself tended to be quite small.
“Every single herbicide we had worked,” he says.
According to Vanhie, subsequent trials with cereal rye showed similar results. In his research and experience talking to farmers, Cowbrough also indicates others have noticed overall suppressive benefits afforded by cereal rye — including a tendency for significantly stunted fleabane plants to produce fewer seeds or not flower when competing with cereal rye. However, more research needs to be done before definitive conclusions can be drawn.
Control in heavier soils might be tougher, too. This is because it’s more difficult for fleabane seeds to sink below the four centimetre growth threshold, the point below which they can no longer germinate, in harder ground. As well, less drought stress in heavier soil makes cereal rye produce less fleabane-harming allelochemical.
In the future, though, the ability to selectively plant more allelochemically potent (and therefore more effective) varieties of rye might be possible.
Other control strategies
“If you’re growing winter wheat, plant something [afterwards]. That’s the low hanging fruit,” says Cowbrough.
Oats, barley, and red clover all offer some level of fleabane suppression. Regarding the latter, however, fleabane has a tendency to grow wherever holes within the clover cover appear. Applying herbicide to everything earlier in the season is an option, though doing so somewhat defeats the purpose of growing a cover crop for biomass. In their experience, Cowbrough says effective fleabane herbicides include blends of Eragon and Merge applied prior to wheat, or Infinity, Infinity X, and Pixxaro for post-period applications.
Tillage is another control tool, though one with issues of its own.
Cowbrough says several types of tillage have been analyzed. Since fleabane seeds need to stay in the top four centimetres of soil to germinate, methods that better prevent that from happening have an improved suppression effect, albeit an incomplete one, just like the results afforded by herbicides and cover crops.
The issue, however, is tillage also reduces the presence of other, less problematic weeds. This inadvertently reduces competition and gives those fleabane plants that do grow more opportunity. Soil health concerns are also a factor.
No miracle solutions
Overall, both Vanhie and Cowbrough say the research so far shows no complete solution. However, making use of cereal rye and other cover crops in tandem with an effective herbicide strategy — and potentially tillage — does afford farmers more control than one solution alone.
Still, Canada fleabane and its resistant family members remain a wily and frustrating flora. Preparing for a problem, even if a field looks problem free, might be necessary.
“That’s the unpredictable thing of this species,” says Cowbrough.
“Just because you have stuff in the fall doesn’t mean it will be there in the spring. It might. And just because you don’t have anything there on April 20 doesn’t mean, come May 20, you’re [not] in big trouble.”