How weeds outsmart you

Herbicide resistant weeds proliferate in unexpected ways, making scouting more important than ever

For two scorching-hot days in July, farmers and crop consultants attending the University of Guelph’s Diagnostic Days event at Ridgetown College heard about the birds and bees – of herbicide resistant weeds.

A session hosted by Dave Bilyea, research technician at the Ridgetown campus, reviewed the state of herbicide resistance in Ontario and across North America, as well as the various ways problem plants proliferate.

Why it matters: The natural biological processes of herbicide-resistance weeds give them an incredible ability to weather hardship and spread over wide areas. Awareness of those advantages is critical to understanding and responding to the risks posed by problem plant pests.

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With 21 herbicide resistant weeds present in Ontario – 20 of which are considered common – and more clawing northwards from the United States, Bilyea described plant reproduction methods that are both efficient and unexpected. This, he says, makes understanding the risk of herbicide tolerance, and the need for proactive cooperation, more important than ever.

Giant-ragweed’s relationship with worms

One of the best-known herbicide-resistant weeds is in Ontario, says Bilyea, is giant ragweed. Despite producing a fraction of the seeds produced by other weeds, earthworms help ensure it remains a prolific pest.

“Earthworms actively seek the seeds out,” says Bilyea. “They don’t eat them, they move them between two and 22 cm down. Earthworms bury two-thirds of giant ragweed seeds dispersed naturally.”

Bilyea goes on to say both during the sessions and in a fact-sheet distributed to attendees that most seeds dispersed by earthworms remain in the top 10 cm of soil where they can actively emerge. Seeds further down, though, can stay dormant until exposed to open air – say during a construction project or plowing.

Tests at the Ridgetown campus showed earthworms burrows contained an average of 127 seeds each, with an estimated 5000 seeds per square meter.

Palmar amaranth’s waterfowl alliance

With a very competitive nature, resistance to several different modes of action, and an ability to combine male-female traits for improve resistance genetics, Palmer amaranth – widely considered the “worst weed in the United States” – is poised to invade Ontario from the air.

The pest made its way across the United States through a combination of product transportation (specifically cotton), as well as animal feed – and subsequently animal manure. Ducks and other migratory waterfowl also use Palmer amaranth as a food source, with seeds able to exist in their digestive systems for up to 40 hours before being excreted. This puts Ontario well within dispersal range of migrating birds.

Dave Bilyea holds a Palmer amaranth plant. The weed is not yet in Ontario, but will arrive at some time likely hitching a ride with waterfowl.
photo: Matt McIntosh

Palmer amaranth has already established populations in every state in the Great Lakes region. And while not yet present in Ontario, Bilyea says “It’s going to happen, we just don’t know when.”

“Populations will likely begin in and near non-scouted areas like fencerows and ponds.”

By sea and air

Some weed seeds, including Palmer amaranth, can also be effectively dispersed through water systems.

The minute size of Canada Fleabane seeds makes them particularly buoyant, meaning dispersal though water a significant problem.

Unfortunately, Canada Fleabane is also easily dispersed by wind. Indeed, Bilyea says wind-blown seed has been found as high as the planetary-boundary layer of our atmosphere (that’s about one km up). Seed blowing at this level can move 500 km or more.

Wild Carrot also uses the wind to disperse itself. Indeed, seeds make use of a combination of hardened snow and winter storms to glide to new areas – a strategy which has already helped Wild Carrot resistant to group four herbicides proliferate in at least one Ontario county.

Many other methods

Bilyea says many other methods of seed dispersal exist among herbicide-resistant weeds. Giant Foxtail, for example, drops seeds well before crops are ready to harvest. This reproduction method, he says, makes the use of shredders on combines – equipment designed to macerate weed seeds – useless.

Plants like Lambsquarters and groundsel take an assembly line approach to dispersal, with the former creating two generations of seed in a year and the latter continuing to produce seed at a constant rate through the growing season.

“Common groundsel doesn’t have a dormancy period. It starts growing as soon as it hits the ground,” says Bilyea.

Scout in and around the field

Ontario Ministry of Agriculture, Food and Rural Affairs details several methods of controlling and preventing the proliferation of herbicide resistant weeds. This includes only applying herbicides as necessary, using recommended rates, combining two or more herbicides, and diversifying herbicide groups. Crop rotation, using clean seed and equipment, and employing other agronomic practices like cover crops to reduce viable weed seeds in soil also helps.

However, Bilyea reiterates the importance of scouting. More specifically, he encourages producers to not overlooking perimeter weeds, or ignore those that have not responded to herbicide applications.

“Rotational control methods are preferred whenever possible,” he says. “Don’t ignore them if they don’t look like the typical weeds you see. Check it out, send it to someone who can identify it if you can.”

About the author


Matt is a freelance writer based between Essex County and Chatham-Kent. He is interested in all things scientific, as well as rock n' roll, hunting and history. He also works with his parents on their sixth-generation family farm.



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