Herbicide layering was on the agenda at Saskatchewan Agriculture’s Crop Diagnostic School near Indian Head this past summer. Cory Jacob, regional crop specialist in Saskatchewan Agriculture’s Watrous office, walked participants through the process.
1. What is herbicide layering? “We’re basically talking about layering on multiple herbicide modes of action and groups in sequential application. So, mainly your pre-seed and your in-crop herbicide applications,” Jacob said.
This means spraying herbicides from different chemical groups at different times during the growing season — not mixing modes of action in a tank mix.
2. How many layers do I need?
“The Ministry recommends at least three to four herbicide groups,” Jacob said. “The more the better.”
Getting to four may be a big step for some farmers. Two is better than one, Jacobs says, “but above three would be a good target.”
3. When should I layer?
“The main thing is to get a good pre-seed herbicide product in there,” Jacob said.
“Later, the weeds that were missed will be killed by the post-emergent application.” The residual is key for getting the weed population down for your in-crop herbicide.”
For example, Jacob said, if you start with a weed population, after the pre-seed herbicide, you’ll have “five, or 10 or maybe even 20 per cent of that population there.” The weed population will be reduced before you apply in-crop herbicide.
“You’re basically reducing your chances of spraying over a resistant biotype and having some more resistance issues show up on your farm.”
4. What if I don’t have resistant weeds?
These days, you can’t be too careful. “For some weed species, assume you have it,” Jacob said. “And for some, don’t let it get there.”
Herbicide layering combats cleavers
As explained by Saskatchewan Agricultural specialist Cory Jacob at the annual Crop Diagnostic School near Indian Head, two recent studies led by University of Saskatchewan weed scientist Chris Willenborg have found that herbicide layering can help control those hard-to-kill Group 2 resistant cleavers.
In 2014-15, researchers found that following a pre-emergent herbicide application with a post-emergent herbicide from a different chemical group (with a different mode of action) was more effective than using chemicals from only one group for controlling Group 2 resistant cleavers in field peas.
In 2013-14, researchers turned to canola, and found that cleaver control in canola was also better when multiple modes of action were applied during the year. This was the case with all canola systems (Liberty Link, Roundup Ready and Clearfield).