The Agronomists, Ep 98: Battling herbicide-resistant beasts with Drs. Charles Geddes and François Tardif


The advent of herbicide-resistant weeds was bad enough, but the expansion of type, area, and mode-of-action resistance has left nearly no field untouched across the all regions of Canada.

What can we do and how did we get here? To tackle that question, host Lyndsey Smith is joined by Dr. Charles Geddes, Agriculture and Agri-Food Canada at Lethbridge, and Dr. Francois Tardif, University of Guelph, to discuss resistance occurrence, development, and management.

Tune in every Monday, 8 pm E for a new episode of The Agronomists! 


  • We start with herbicide-resistance highlights of Tardif’s career — never assume!
  • Field-level testing must be utilized to be sure if resistance is present or not
  • Crop rotation is a key area of managing herbicide resistance
  • Need fall-seeded and perennials in the mix to get the full benefit
  • Increasing crop diversity decreases likelihood of resistant weeds
  • Lack of herbicide diversity also increases likelihood of resistant weeds
  • Broadleaf vs. grass weed resistance — is one worse than the other?
  • Weed-type and resistance rates is location specific
  • Resistant weed rankings, what has the greatest impact?
  • Controlling threats during off-season rotations, i.e. fleabane and fall rye
  • Cultivar and season effects
  • Clip from March 2020 Kelvin Heppner and Tammy Jones addressing herbicide-resistant kochia in Manitoba, Soothsaying?
  • Populations with low level resistance, overlooked in the past due to the expectation of some weeds being left over following herbicide application
  • Current state of herbicide-resistant kochia
  • Maps of Alta. over time showing progression of  herbicide-resistant kochia
  • As time progresses larger proportions of weed populations are herbicide-resistant and more species are developing resistance
  • Managing herbicide-resistant populations, are we missing a step?
  • Seed destructors on combines, still not a perfect solution
  • Back to kochia!
  • Methodology has remained the same for detection of resistance but different results are clear: resistance is spreading
  • What contributes to spread of herbicide-resistant kochia
  • Combine cleaning: who’s job is it? Is custom work spreading weeds?
  • High rate vs. low rate application explanation
  • Cutting rate saves money but results in lower control rate of weeds, speeds resistance
  • Higher rate application will eliminate weeds with low grade resistance mechanisms
  • Cross pollinated vs. self pollinated, effects herbicide application rate decisions
  • Resistance building is on an exponential scale, one season there may be slight resistance and next year the resistance rates rapidly increase
  • Cross pollination vs. self pollination: how we get stacked resistance
  • Are grasses less resistant due to higher self pollination rates?
  • Other methods of control: Tillage and plowing, zappers!
  • You can’t spray your way our of resistance but multiple modes of effective action may stave off resistance development
  • Spot spraying, see and spray technologies, an exciting frontier but not the silver bullet
  • Now that herbicides are failing, there is opportunity for innovation in other fields
  • The next challenge is collaboration across disciplines to work out new methods of weed management
  • How long would it take for weeds to develop resistance to a novel MOA? A wide range
  • Rate of resistance development is driven by many factors
  • Can weeds “forget” resistance to herbicides? Removing selection pressure may result in loss of resistance, but sometimes for a completely different reason (plant fitness)

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