NOTE: This article first appeared in the February 25, 2021 issue of The Western Producer
A multi-pronged approach before and after seeding brings best results at year-end.
As cases of herbicide resistance proliferate in problem weeds, some growers and researchers are calling for more proactive management strategies.
Using multiple modes of action in a combined burnoff-residual pre-seed application, they say, is particularly effective at reducing populations of yield-robbing weeds such as cleavers, kochia, round-leaved mallow, stinkweed, volunteer canola, and many others.
At the least, the practice can make in-season and post-harvest control more manageable.
Getting in early with a pre-seed application, followed by an in-crop, is supported by the Canola Council of Canada (CCC). As described by the organization, employing this approach before the 4-leaf stage of the crop is generally enough to eliminate yield loss from weed competition.
Keep the Australian experience in mind
Josh Lade, SK
For Josh Lade, the struggles farmers face in his native Australia offer a telling example of what happens when herbicide resistance gets out of control. Now farming in central Saskatchewan, he encourages Canadian growers to take a more critical look at their historic herbicide application record.
His main causes of concern are the wide variety of pre-existing herbicide resistance issues, and the continued employment of practices which increase selection pressure. In his own area of the province, he lists glyphosate resistant kochia, Group 2 resistant wild mustard and wild oats, and Group 9 selection pressure on cleavers as concerning examples.
With this in mind, Lade reminds his fellow growers how bad resistance issues can get.
“Herbicide resistance is always at the forefront of my mind just because of what I’ve seen,” says Lade in reference to Australia’s long struggle with a wide range of acute resistance cases.
“We need to be adding two modes of action at least on specific weeds.”
Layering multiple modes of action
On his own farm, resistance management starts with pre-seed herbicide layering – specifically, combining multiple modes of action with both burnoff and residual activity.
“I’m a big fan of residuals just because of the selection pressure reduction…A lot of Australian growers, all they have is residuals,” Lade says.
The expense of in-crop chemistries – as well as yield losses from crops metabolizing the herbicide – have also contributed to Lade’s move away from regular in-crop applications.
“We can get weed populations down and just spray where we need to. I just think that crop injury is overlooked.”
Cleaver management specifics
Nolan Kowalchuk, Alberta-based technical sales manager for FMC, expresses a similar sentiment, adding in-season applications are generally less effective when trying to manage cleavers.
In-crop chemistries can control cleavers, Kowalchuk says, however, they are less affectatious once the cleavers are past the two and three whorl stage, or can be very slow acting, depending on the herbicide tolerant system you are using. In a crop like canola, too, the canopy itself can actually prevent in-crop herbicides from reaching the weed. “Herbicide layering and extended control of cleavers with an effective mode of action, is key to controlling them.”
“Cleavers germinate in both the spring and fall and regularly produce 300 to 400 seeds per plant, though that number can climb to 3500 seeds per plant given optimal conditions,” Kowalchuk says.
“If enough mature seed makes it into harvested canola, it can result in both yield losses and a drop in grade as well. At the end of the day, profitability can take a major hit.”
The problem posed by cleavers becomes even more significant given the widespread presence of populations resistant to Group 2 herbicides, and concerns about increases in Group 4 resistant biotypes. The utilization of Group 4 modes of action products for broadleaf weed control in cereals is absolutely critical for weed management. With the introduction of more Group 4 mode of action products available for canola as well as cereals and other major crops, Kowalchuk cautions growers against using this mode of action in every crop and timing for there is the potential to foster greater selection pressure to group 4.
“If we continue to use Group 4 in cereals and use it in canola, we’re not giving that mode of action a break…We need to use other useful novel modes of action where we can.”
A multi-pronged approach
Combining burnoff and residual herbicides in a layering approach might be the best chemical means of fighting resistance and supporting yield, but Lade insists it shouldn’t be viewed as a magic bullet. Employing targeted in-season applications when required is still important, as is weed management at harvest.
Like many growers in Australia, Lade operates weed crushers on his combines. While expensive – Lade prices the crushing machines in the neighbourhood of $100,000 – he believes their ability to widdle-down the weed seed bank can be well worth the cost for some growers. Where such an investment doesn’t make sense, other strategies can be used (e.g pushing chaff into rows and collecting it, rather than broadcasting it onto the field).
“The movement is harvest weed seed control. We’ve been playing with the seed terminator since 2018 and got it to a point where it’s working. It’s not perfect but nothing is,” says Lade, later reiterating pre-seed, strategic in-season, and harvest weed management strategies have a much greater impact when used in conjunction.
Evidence of how effective such multi-pronged approaches can be – or, at least, how effective pre-seed and harvest efforts can be – is visible in Australia. Growers down-under are finally, as Lade describes, “turning their systems around and getting control.
Volunteer canola brings a lot of baggage
Volunteer canola increases competition for nutrients and moisture, along with decreases grain quality resulting in dockage. Since volunteer canola sprout from untreated seed, it can also heighten flea beetle pressure and play host to clubroot and blackleg disease.
Used alone, glyphosate is only effective on two herbicide-tolerant varieties – InVigor and Clearfield canola. As in other cases, that means more than one mode of action is critical in gaining control.