The number of grassy weeds resistant to in-crop herbicides is increasing. Dr. Breanne Tidemann, research scientist with Agriculture and Agri-Food Canada in Lacombe, AB, presented some sobering statistics during the FMC Pre-School webinar, AGRC*104: Grassy Weeds in the Prairies: The Pests, the Problems, the Plans.
In a weed survey conducted from 2014-2017 across Western Canada, of the fields that had wild oats, 62 per cent of those were Group 1 resistant populations. Group 2-resistant wild oats accounted for 34 per cent of the populations. Even worse, wild oats resistant to both Group 1 and Group 2 herbicides accounted for 27 per cent of the populations across the Prairie provinces.
But wait, there’s more. Results from the most recent weed survey conducted in 2019-2020 in Saskatchewan showed that 77 per cent of fields surveyed had Group-1 resistant wild oat populations. Weed researchers are also seeing a rapid increase in Group 1 and Group 2 yellow foxtail resistance in SK, a weed that has shown up relatively recently in the province.
Herbicide layering starts pre-seed
Weed resistance is top of mind for Dr. Tidemann. She said one way to defend against herbicide resistant weeds is to use a pre-seed herbicide, which is the best way to incorporate herbicide layering as part of an integrated herbicide management strategy.
With herbicide layering, you control weeds with multiple different herbicide groups at different timings. Using a pre-seed herbicide that has a different mode of action is one of the best ways to get multiple modes of action on one crop in a given year.
Dr. Tidemann says that resistance management is a numbers game.
“Even weed suppression is better than nothing,” she says. “The more individual weeds you’re spraying with your post-emergence Group 1, Group 2, or glyphosate, the more likely you are to select for resistance.”
A good pre-seed herbicide knocks down the number of weeds you’re spraying in-crop, giving you a better chance of extending the life of that post-emergence herbicide group. Let your pre-emerge herbicide do a lot of the work upfront followed with a pre-seed treatment on emerged weeds which will reduce the number of weeds you face with the in-crop herbicide.
Use soil-applied products simultaneously to control weeds that emerge after the initial application. Once activated by rain or moisture, pre-emergent herbicides create a protective “barrier” within the top layer of soil that controls weeds as they germinate. This herbicide barrier can help cut back resistant weed populations right out of the gate.
Increase seeding rate
Dr. Tidemann also shared research from Saskatchewan that shows that combining a pre-herbicide with increased seeding rates is even more effective at reducing weed biomass.
Using a pre-seed herbicide alone significantly reduced wild oat biomass in the field. Increasing the wheat seeding rate from 150 seeds/m2 to 450 seeds/m2 combined with a pre-seed herbicide was most effective at reducing weed biomass. Using a pre-seed herbicide with increased seeding rates reduced weed biomass up to 94 percent.
Silaging
For those who have a market for it, early cut silaging of a crop like barley can also significantly cut down wild oat pressure because the wild oats are being cut at the panicle formation
Silaging isn’t selective to the weeds, of course. If you can adapt the timing of your silaging operation to one to two weeks after head emergence, it can be effective at reducing weed biomass, particularly of wild oats. You want to be sure to cut the wild oats at panicle formation when the weed is just starting to form its seeds. The silage is taken off the field and wild oat seeds go with it. And doubling the seed rate can be incredibly effective in keeping wild oat populations low.
Patch management
Finally, there is some hopeful work being done on managing patches of resistant weeds in a field. For instance, you might have Group-1 resistant wild oats in a patch in the field but are achieving a high rate of control everywhere else. A study shows that harvest weed seed control might be very effective at keeping resistant weed populations to a limited space in fields.
A final piece of advice from Dr. Tidemann:
“It is important to use a good integrated herbicide management strategy, but it's also, important to incorporate true integrated weed management strategies. Get your canopy closed, give the crop as much competitive advantage as you can and really think about trying to manage the weeds at all times in your cropping system.”
You can watch the complete webinar here. It’s jam-packed with weed-science knowledge. You’ll find other webinars and resources at www.fmcpreschool.com, the education and stewardship extension of FMC Canada.
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