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Cover crops and tillage for glyphosate resistant Canada fleabane management

Principal Investigator: Francois Tardif

Research Institution: University of Guelph

Timeline: May 2018 – October 2019


  • Understand the influence of cereal rye cover crop and fall tillage on the population biology of fleabane as to optimise fleabane control.


  • The determination of the best timing and intensity for light fall tillage with or without cereal rye cover crop will allow farmers to optimise management of glyphosate-resistant Canada fleabane in soybeans and reduce the requirement for pre-emergence or in-crop treatments the following spring.

Scientific Summary:

Glyphosate-resistant Canada fleabane is one of the most important issues facing growers in Ontario. This weed is very difficult to manage in soybeans and there are limited options to control it with herbicides. Some pre-emergence options exist but can be unreliable, and new post-emergence technologies, such as Xtend and Enlist, while effective, are not appealing to all growers. Tillage has been promoted by many as an effective way to control fleabane, but there is a lack of reliable data on this. In addition, heavy tillage is discouraged, and the efficacy of lighter tillage is still unknown. During preliminary trials, we observed that light fall tillage is very effective at reducing fleabane density, but plants that survived that treatment grew taller the following spring, making them more difficult to control with herbicides. That preliminary trial site had an estimated 3.5 million fleabane seeds per acre in the soil. Yet there were no seedlings established where rye had been planted as a cover crop in the previous fall, which was completely unexpected. The mechanism underlying this very unique observation is not known. Although pre- and post-harvest tillage, use of cover crops, and herbicide selection all play a role in the management of glyphosate-resistant Canada fleabane, Ontario grain farmers have still experienced late seedling recruitment of the weed that cannot be addressed by any of those three strategies.

Through this project, researchers examined the effect of rye cover crop and tillage on the population dynamics of Canada fleabane. They planted rye cover crop with and without tillage and monitored the seed bank and population of seedlings in the fall and spring. This was done to allow a determination of the best cultural/physical control methods to reduce the impact of this weed and provide recommendations to growers managing this difficult weed.

The researchers were able to show that a natural chemical from rye, benzoxazinone acid, is affecting the germination of fleabane seeds. The results showed that the rye cover crop as a sole treatment managed to reduce the number of weeds that emerged by 30% and 40% in 2018 and 2019 respectively. Furthermore, rye decreased the biomass of the fleabane’s population by 80% and 98% compared to the untreated check, between the two seasons.

Shallow tillage managed to decrease the biomass of the fleabane by 95% in the first season and 65% in the second season. Herbicide treatments showed that only Banvel II in 2018 and Eragon and 2-4-D in 2019 provided over 90% visual control. When the cereal rye was paired with one other management tactic, either herbicides or tillage, visual control of Canada fleabane exceeded 95% in several cases.

At a practical level it is recommended that cereal rye be included in rotations as it helps reduce the negative impact of glyphosate-resistant Canada fleabane, especially if it is combined with either shallow tillage or herbicides. This will provide choice to farmers and help maintain the sustainability of farming operations.

The results of this trial are promising; they show new management strategies in which multiple modes of selection work in symphony to improve the efficacy and consistency of control of Canada fleabane. Finally, this research gives farmers choices in terms of management solutions that best fit their operation and helps diversify the methods used to control this weed to prevent further resistance.

External Funding Partners:

This project was funded by the Canadian Agricultural Partnership, a five-year federal-provincial-territorial initiative.

Project Related Publications:

Cowbrough, M. Pest Patrol: Cover crops versus glyphosate-resistant weeds. Country Guide, September 18, 2018.

Cowbrough, M. Managing glyphosate resistant Canada fleabane with cover crops and tillage. Crop Talk, 18(4) November 1, 2018.

Cowbrough, M. Managing glyphosate resistant Canada fleabane with cover crops and tillage., November 20, 2018.