[UPDATED: Mar. 26, 2019] Alberta kochia populations have exploded following back-to-back years of dry growing conditions.
“Kochia was one of the worst weed problems across the southern Prairies last year,” said federal research scientist Charles Geddes. “If we have another dry year this year, there will certainly be issues with kochia management.”
The past two seasons created the “perfect breeding ground” for kochia — which thrives in dry, saline conditions — over much of the south. But the full impact won’t hit until this year.
“Kochia populations tend to show up in very high densities the year following a very dry year,” said Geddes.
Historically, kochia has been particularly problematic in low-lying areas of the field that tend to be more saline. But from there, the weed can spread rapidly into the more productive parts of the field, causing significant yield loss, especially among crops that can’t compete as well.
On top of that — and more worrying still — is that this full blast of kochia infestation will only accelerate herbicide resistance in the weed.
“It’s essentially breeding herbicide resistance,” said Geddes. “The problem starts in the low-lying areas because there’s nothing competing with the kochia there, but it’s still receiving the herbicide applications. That can create selection pressure and herbicide resistance, and then from there, the population spreads out to the rest of the field.”
And as resistance spreads, the modes of action that producers can use to control kochia dwindle.
Today, kochia is considered to be fully resistant to Group 2 herbicides, and more than 50 per cent of kochia populations in southern Alberta are also resistant to glyphosate (a Group 9 herbicide). Group 4 — or dicamba — resistance was also found in 18 per cent of kochia populations in 2017.
And now Alberta farmers are starting to see more triple-resistant kochia — weeds resistant to all three groups. In 2017, 10 per cent of kochia populations were triple resistant, and that number has likely risen in the two years since. (The next kochia survey in Alberta is scheduled for 2020.)
“Resistance is certainly a contributing factor as to why the weed is so abundant as of late,” said Geddes. “We do have very few options for kochia management.”
Managing resistant kochia
But the best chance farmers have to get ahead of kochia is in the spring, before the crop emerges.
“Pre-emergence is basically one of the best windows to utilize for herbicide control of kochia,” said Geddes, adding it is one of the earliest weeds to emerge in the spring, soon after the snow melts.
“So a pre-seed or pre-emergence herbicide application is a must when it comes to kochia management.”
These herbicide options vary by crop type.
*Researchers have done some preliminary research on different chemical control options, some of which are not registered for kochia control yet. These herbicide options vary by crop type. (Check with your input supplier before using a product if you’re unsure whether it’s been registered for kochia control.)
For spring wheat, Geddes has found that a herbicide such as Authority — a Group 14 herbicide — has shown “fairly consistent and high control of kochia.” For field pea, Heat (also a Group 14 herbicide) or Aim mixed with Authority (two Group 14 herbicides) have worked well in the preliminary research. Edge (Group 3) may be a viable option for canola.
“It’s very important not to just rely on glyphosate as a pre-seed herbicide,” said Geddes. “It’s important to include other herbicide modes of action as well.”
And if farmers don’t switch up their modes of action, they could lose the ones that remain.
“I do see many growers shifting toward relying on these Group 14 herbicides,” said Geddes. “That can create selection pressure. We don’t have any resistance to the Group 14s that we know of yet, but if we lose Group 14s, that would be very bad.”
In addition to rotating modes of action, farmers should also consider herbicide layering, “including multiple different modes of action at different times throughout the growing season.”
“A pre-emergence or pre-plant herbicide is a very good tool to help manage kochia,” he said. “But it’s also important to come in with a post-emergence or in-crop herbicide application of a different herbicide mode of action.
“The largest part of the population emerges very early in the spring, but some emerges throughout the growing season, so it’s very difficult to catch the entire kochia population with a single herbicide application. Layering can help.”
As resistance continues to break down, farmers will also need to look at other ways of managing herbicide-resistant weeds, he added.
“We’re at the point where we can’t rely solely on chemical tools because we’re seeing such great selection pressure for herbicide resistance,” he said. “There are some other options that look like they would be effective and that could be implemented before you end up seeding your crop.”
One of those options is spot tillage.
“As much as I hate to say it, if it’s below five to seven centimetres, kochia will have a difficult time emerging. If you bury it for a year or two, you can decrease the viability of the seeds.”
An extended crop rotation is another valuable tool because different herbicide modes of action are introduced into the rotation.
Beyond that, consider ways to make crops more competitive.
One way to do that is by increasing the seeding rate and decreasing row spacing. Giving winter cereals a shot is another.
“Most of our crop rotations consist of summer annuals, but including a winter annual in your crop rotation can be an effective tool for kochia management, as you have something established in the spring as kochia is trying to emerge,” said Geddes.
Going forward, focus on management practices that will improve their stewardship of the remaining modes of action, because once those are gone, kochia will be virtually unstoppable as it spreads across the southern Prairies.
“With kochia, we have limited options for control, and the control options that remain are less effective than the ones that we’ve already lost,” said Geddes.
“If we lose these remaining modes of action, we’ll be in a very bad situation.”
*[Update] Research into these herbicide options is in its early stages, and some of the herbicides suggested are not yet registered for kochia control. Check with your input supplier before using a product if you’re unsure whether it’s been registered for kochia control.