What weed experts say you should have your eye on in 2020

A province-by-province look at what you need to know about weeds this growing season

Wild oat is showing significant herbicide resistance, says Saskatchewan’s provincial weed control specialist.

Three provincial weed specialists offer their insights on the weeds farmers may find in their fields this growing season and what can be done about them.


Tammy Jones, a Manitoba Agriculture weed specialist, lists redroot pigweed, lamb’s quarters and green and yellow foxtail as the biggest weed threats this year. Winter annuals like stinkweed are likely to be less favoured, she says, as there is an expectation of more spring tillage after the wet fall.

“Cultivation should hopefully get rid of them, but how well they are managed depends if there is a good job done of cultivation.… Certain types of tillage equipment have a higher level of disturbance and better uproot the plants.”

Pre-seed herbicides also work for these weeds.

Jones advises growers to make sure their herbicide application timing is optimal by scouting to determine weed growth stage, applying herbicide before the weeds get too large. “It’s also critical to ensure you are using the right water volume for good coverage and to get the correct dose to the target,” she says. “And never use poor-quality water with bicarbonate or sediment, for example. There are a lot of factors in the water that you use that can affect herbicide efficacy.”


Clark Brenzil, a provincial weed control specialist with Saskatchewan Agriculture, says the top five weeds in surveys have always included wild oat, green foxtail, wild buck­wheat, Canada thistle and volunteer canola.

Wild oat is showing significant herbicide resistance, Brenzil reports, with 60 per cent of samples in the last provincial survey showing resistance to Group 1, 30 per cent to Group 2 and 20 per cent to both. He predicts those percentages will increase this year to 70, 40 and maybe 30 to 35 per cent resistance to Groups 1, 2 and both 1 and 2, respectively. Brenzil also reports some wild buckwheat is showing resistance to Group 2 and green foxtail to primarily Groups 1, 2 and 3.

Because resistance is growing at a steady and fairly fast pace, producers need to be proactive. “Many of them don’t yet realize how fast the situation can blow up in their faces,” Brenzil says. “You can go from noticing the escape of a small patch of a weed to a full-blown resistance across the field in two or three years. Many growers at first dismiss what’s happening, thinking it’s a herbicide application problem or a late flush of weeds. But once there has been time for the weed seeds bank to increase, it’s too late.”

As growers well know, the many tools to manage resistance include increased seeding rates and narrower row spacing where appropriate, which Brenzil says can also provide benefits like reduced days to maturity, increased yield and improved test weight in cereals. “There is also research showing side-banding instead of broadcasting will give you 75 per cent less weed biomass with the same fertilizer rate,” he says, adding that nitrification inhibitors have increased the amount of fertilizer being broadcast.

Brenzil also reminds growers that the more diversified their rotations, the bigger will be the impact on reduction of resistant weeds and on disease and insect control. “A full four-year rotation is optimal, alternating grass with broadleaf crops within that,” he states. “There is also research that shows including a three-year perennial forage in the rotation can drop weed populations by 75 to 95 per cent and can also drop the disease and pest cycles that growers are dealing with as well. There has been a strong hay market in the last couple of years where hay in the rotation could have been lucrative.”


“In many fields in Alberta, volunteer crops (primarily canola, wheat and barley) will by far be the most important weed problem of the coming season,” says Chris Neeser, a weed research scientist with Alberta Agriculture and Forestry. “For many growers, other weed issues will pale in comparison to the problem of volunteer crops emerging from seed shatter in fields.”

Because the province was subjected to early snowfall in September 2019, with continued cold temperatures afterwards, winter annuals like shepherd’s purse, stinkweed, flixweed and cleavers didn’t have much of a chance to get growing last fall. This means there is a lot of seed out there that will germinate this spring and into early summer, says Neeser.

“The normal burndown will not get it all and growers should consider pre-emergence applications with residual control in addition to appropriate in-crop options to control the later-emerging weeds.” Other weeds should not be any worse than usual in Alberta, in Neeser’s view, and hopefully no glyphosate-resistant Canada fleabane will be found in 2020.

About the author


Treena Hein is a freelance writer specializing in science, tech and business trends in agriculture and more.



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