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A new plan of attack for resistant wild oats

With more wild oats showing resistance to Group 8 herbicides, this action committee is going on the offence

RWOAC chair Eric Johnson wants to work with farmers to find out their ideas for managing resistant wild oats.
The new committee has adopted the logo from the original Wild Oat Action Committee from the 1970s. photo: Supplied

In the 1970s, the Wild Oat Action Committee was formed to encourage farmers to control the pesky weed, especially with herbicides. Now a new version is back, but with an ironic twist — the herbicides aren’t working. A recent Prairie-wide study reveals 69 per cent of tested wild oats proved resistant to some modes of action, with a quarter unable to be controlled in wheat and barley.

That’s why a group of weed scientists, agronomists, herbicide company reps and producers has taken a page out of an old book by forming the Resistant Wild Oat Action Committee (RWOAC). Member Charles Geddes says its goal is to carve a path for wild oat management.

“We’re going to be looking at research that’s already been done, priorities for new research in wild oat and working at developing extension and outreach materials so we can get that message about wild oat out to farmers,” says Geddes, a researcher with Agriculture and Agri-Food Canada in Lethbridge, Alta.

RWOAC chair Eric Johnson believes the most effective management ideas may come from farmers. Although the COVID-19 pandemic stifled the committee’s schedule in 2020, it plans to develop a full roster of farmer engagements in 2021, whether in-person or online.

“I think one of the ways we will be successful is if we can work with farmers to try to develop some solutions that they can use,” says Johnson, a plant scientist with the University of Saskatchewan.

“There is more research required but I think we can make stronger gains if we engage the ideas of farmers.”

Triallate resistance developing

The story of wild oats’ herbicide resistance would come as no surprise to anyone who’s followed the concerns of weed scientists and farmers over the past several decades. Wild oats are primarily controlled by selective Group 1 and Group 2 post-emergent herbicides. Their growing lack of efficacy follows a familiar pattern, Geddes says.

“Most often across the Prairies you will see Group 1 resistance show up first. In response to that, farmers shift over to Group 2 herbicides to manage those Group 1-resistant biotypes. Essentially what happens is you end up developing resistance to both of those groups. You basically eliminate all of your post-emergence herbicide options in some of the main crops like wheat, barley, peas and lentils.”

Geddes says a notable exception is herbicide-resistant canola. “One of the only main crops grown on the Prairies with a post-emergent herbicide option would be herbicide-resistant canola varieties such as LibertyLink or Roundup Ready canola. With those you either have glyphosate or glufosinate to at least help manage the wild oats.”

Once Groups 1 and 2 prove ineffective, the next step is usually toward pre-emergent Group 8 herbicides. Triallate — an active ingredient found in herbicides such as Avadex and Fortress — is the only pre-emergent active ingredient registered for wild oat control in cereals in Canada. But there are signs wild oats may be resisting triallate as well.

“In Alberta, for example, 15 per cent of the populations tested were triallate- or Group 8-resistant in 2007,” Geddes says, adding that RWOAC does not have recent Prairie-wide data on wild oat resistance to triallate.

Once these options are exhausted, Geddes says the only chemical options farmers have left are pre-emergent Group 15 herbicides such as Focus and Zidua which suppress — but do not control — the weed.

Additional options for pre-emergent suppression include the Group 3 herbicides ethalfluralin and trifluralin in certain cropping scenarios. Examples include Edge and Rival, respectively.

Don’t assume: test

How do you know if the wild oats in your fields are herbicide-resistant? Don’t assume resistance by the mere presence of wild oats, Geddes says. The only practical way to find out for sure is testing.

“There are different resistance profiles. Within the Group 1 herbicides, for example, we have a few different chemical classes or families that are abbreviated as fops, dims and dens. Every Group 1 herbicide falls into one of those classes.

“When we do resistance testing, we don’t always see Group 1 resistance to all of those categories. For example, sometimes there’s resistance to all of those categories and sometimes there’s resistance to two but one will remain effective. That is where this resistance testing comes into play. You might still have herbicides that remain effective.”

Labs throughout the Prairies can test resistance. The first step is to collect seed samples just before harvest when the seed is mature.

“Collect those seeds, dry them down and send the sample into a lab that tests for resistance,” Geddes says, adding that most labs focus on Groups 1 and 2 resistance but some test for Group 8 resistance as well.

“The results come back over the winter and then farmers can use them to determine not only what kind of resistance they have but also which herbicides remain effective on their wild oat populations.”

Non-chemical approaches

Geddes says that if there’s a bright spot, it’s that resistance patches can be isolated and managed without chemicals. “Wild oat seeds generally don’t spread far from the mother plant because they usually just drop off that plant in a localized area. Resistance can be unique to certain fields or even patches within fields because either it’s selected for within that field or it’s due to some kind of seed contamination issue.”

That’s not a call for complacency. Wild oats can be easily spread by equipment travelling between fields, so sanitation is essential. They can also be dispersed by up to 145 metres in the chaff from the back of a combine.

One option is to isolate resistant patches and then bale, mow or burn them.

But what about longer-term management, especially when herbicide options are no longer viable? In-crop competition is key, says RWOAC chair Johnson.

“Increasing plant density through higher seeding rates is one of the best ways of reducing weed competition with just about any crop. Now growers are looking at variable seed rates across the field … increasing their seed rates where wild oat densities are the worst, is one thing they can do.”

Fall-planted cereals such as fall rye and winter wheat can be extremely competitive with wild oats.

“Barley also tends to be very competitive. Silaging the wild oat patches or cutting them for greenfeed before seed formation are other ways to manage these populations.”

One of the RWOAC’s biggest achievements to date has been the development of a series of infographics offering further information on wild oats. These include details on resistance, ways to stop the spread of wild oats, best practices for shipping samples to labs and locations of some Prairie labs which test for wild oat herbicide resistance.

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