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Resistance Management

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Mike Cowbrough thinks resistance prevention is an oxymoron.  “If you are using chemical weed control there is no such thing as resistance prevention, growers need to focus on resistance management,” says Cowbrough, the provincial weed specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs.

Cowbrough says the agricultural industry has been talking about resistance management in weeds for years.  “And what conclusions have we come to?” he asks.  There are many cases of resistant weeds, but most growers only think about it when the problem happens on their farm. “I understand why growers make decisions the way they do- it’s necessary to deal with what is in front of them at the time,” says Cowbrough.  “But what they need to remember is what we have learned historically – a lot can change in five or six years.

Cowbrough encourages growers to think strategically about the reality of weed resistance developing on their farm.  Imagine you have rented a new field for IP soybeans and have no history of the weed issues- how would you approach weed control in that field?  You would need to develop a management strategy to cover all variables for weed control – plan for the worst (lots of resistant weeds) and hope for the best (none).  Developing an approach now will reduce the risk of resistance development down the road.  

Cowbrough recommends:
Rotate chemical groups. The best strategy, bar none, for delaying the development of resistant weeds is to alternate the use of herbicides with different modes of action, as needed throughout the growing season.
Scout for resistant weeds.  After herbicide applications, scout fields for signs of any persistent weeds.  If you find any suspect weeds, identify them (using web tools listed below)  and consult your local CCA for further management strategies.
Use the tools.  There are a number of online tools available to help identify weeds and provide resistance management strategies.  Use these tools to help if you are unable to identify the weed or need further management information.
www.weedinfo.ca – information and tools for making informed risk-reducing weed control decisions.
www.weedpro75.com – online herbicide selection tool focused on profitability and environmental stewardship.
www.mixitup.ca – learn how resistant weed populations develop and key strategies to reduce the risk on your farm.

Cowbrough isn’t sure what it will take to bring the practice of resistance management to the forefront of growers’ minds.  It’s human nature to believe it can’t happen to you…and then be surprised when it does.  Cowbrough continues to look for new angles to talk to growers about weed resistance, in the hopes that it will soon start to stick.

If You Suspect That You Have a Resistant Pest

If you think that you may have a resistant pest, contact a crop specialist in your area. See Chapter 2 - Integrated Pest Management for ways to contact IPM and Crop Specialists. These specialists will confirm the resistance. They know which pests in your region are resistant to which Groups of pesticides and they can advise you on the best Integrated Pest Management (IPM) strategy to use to manage the resistant pest.

Genetic Testing Sample Project for Weed Species

A total of 20 genetic tests are available to identify herbicide resistance in 13 weed species with results provided to you within the same growing season. Also, two genetic tests for species identification (Amaranthus and Brassica spp.) are available. If you suspect you have a herbicide resistant weed and would like to get it tested or if you need to identify a pigweed species, visit the website of Harvest Genomics for information about how to collect and submit a leaf tissue sample to their lab in Guelph. Sample collection kits can be obtained from kristen.obeid@ontario.ca. Participating in the genetic testing sample project is free, through project funding.

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