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The first weed with characteristics of herbicide resistance was discovered in 1967. To date, there are currently around 61 herbicide resistant weed varieties in Canada and 150 in the U.S, and many more on a global level. But how exactly are these weeds becoming resistant to herbicides? How can a herbicide be completely effective at killing your weeds one seeding season, but the next season that same weed is not only able to survive but thrive in the presence of that herbicide?

The map above represents the cases of herbicide resistant weeds globally.  It ranges from 1 herbicide resistant weed to 150 by country. Source

The answer comes down to genetics and the ability for weeds to adapt to a herbicide over time. For instance, it only takes one weed to have a natural resistance to your herbicide to breed more resistant copies of itself. Each year you apply the same herbicide application it allows more of the second generation of those weeds to “step out of line” as plants with a bred resistance passed down from their parent.

 

 

There are some things you can do to lower your risk of having a herbicide resistant variety of weed on your field. The Weed Resistance Education and Action Program refers to a technique called Integrated Weed Management (IWM) strategy. This technique is intended to “outsmart” the weed, and prevent it from adapting to the herbicide.

Follow along with the diagram to the right as the naturally resistant parent plant passes on its resistance to its offspring. Source

 

Make Your Crop More Competitive

• Seed at the highest rate possible to help your crop out-compete your weeds.
• Place seed in a way that allows it to emerge earlier than the weeds i.e. narrow row spacing, shallow seeding, or side-banding fertilizer.
• Grow crops that are known to be very competitive within your rotation i.e. fall rye, winter wheat, spring barley, or annual forage.

Be Choosy About Your Crop Seed

• Select for or rotate between varieties of seed with herbicide resistant traits, this gives you a wider range of herbicide options for controlling weeds.
• Buy only a certified seed as these tend to have less risk of weed seeds and have more vigorous seedlings than bin-run seeds.

Keep Good Records

• Know which herbicides groups you used and when, and remember to rotate between these groups to prevent the weed from adapting to any one herbicide.

Create Your Own Spraying Technique

• Use tank-mixes i.e. control for each weed type by combining two or more herbicide groups for extra protection.
• Look at your fields, let the quantity and frequency of weeds determine the optimal time for spraying.

 

HERBICIDE RESISTANT WEEDS IN CANADA.

Below is a list of weeds in each Canadian province that have some form of herbicide resistance.

Alberta	Saskatchewan	Manitoba	Ontario	Quebec
Common Chickweed	Kochia	Wild Oat	Redroot Pigweed	Birdsrape Mustard
Kochia	Russian-thistle	Green Foxtail	Giant Ragweed	Common Ragweed
Wild Oat	Wild Oat	Redrrot Pigweed	Eastern Black Nightshade	Redroot Pigweed
Green Foxtail	Green Foxtail	Field Pennycress	Horseweed
Spiny Sowthistle	Wild Mustard	False Cleavers	Large Crabgrass
Common Hempnettle	Persian Darnel	Common Chickweed	Common Ragweed
Ball Mustard	Common Chickweed	Powell Amaranth	Tall Waterhemp
Field Pennycress	False Cleavers	Pale Smartweed	Witchgrass
Wild Buckwheat	Shepherd’s-purse	Kochia	Yellow Foxtail
Russian-thistle	Field Pennycress	Wild Mustard	Wild Mustard
Persian Darnel	Common Lambsquarters	Common Hempnettle	Horseweed
Narrowleaf Hawksbeard	Redroot Pigweed		Virginia Pepperweed
Shepherd’s-purse			Powell Amaranth
Cowcockle			Common Lambsquarters
			Green Foxtail
			Giant Foxtail
			Smooth Pigweed
			Wild Carrot
			Lateflowering Goosefoot
			Common Groundsel
			Barnyardgrass

HERBICIDE RESISTANT WEEDS IN THE U.S.

Source

For those of you that are based in the U.S., the link below allows you to search by state which unique weed varieties are known to have a herbicide resistance. Click on the link and just hover over your state, that’s all it takes! http://weedscience.org/vmap/statemap.aspx

There is also a feature on the same website that allows you to search any herbicide and it will tell you what weeds are resistant based on your location. It’s a phenomenal tool for you to play around with, especially if you’re still trying to make a decision on which herbicide application to use this season, you can find it by clicking the link below. http://weedscience.org/summary/resistbyactive.aspx

 

Written and Published by Jessica Kohls, BSc, PgCE – Dutch Biologist

SOURCES

http://www.extension.umn.edu/agriculture/crops/weed-management/herbicide-resistant-weeds/

Heap, I. The International Survey of Herbicide Resistant Weeds. Online. Internet. Wednesday, March 18, 2015 . Available www.weedscience.org

http://www.agriculture.gov.sk.ca/Default.aspx?DN=ae24aa39-579c-4617-a630-92f7a9f4ad57