It’s time to expand the tool box when it comes to managing weeds.
That was the message to attendees at this year’s BASF Knowledge Harvest in Brandon Feb. 13.
As of 2015, there were 65 unique weed-resistance cases in Canada, AAFC’s Bob Blackshaw said, a number that rose from near negligible in 1975 and sat under 50 in 2005.
“It’s very difficult because we have resistance in insects and in diseases, but we’ve had some new modes of actions being introduced in recent years by various different companies, so that’s helped a lot,” Blackshaw said. “It sort of helps manage our existing resistance and it doesn’t stop us from future resistance, but it’s another tool for producers to use, but we just haven’t had a new mode of action, a truly new mode of action herbicide, for more than 25 years, so that’s why the resistance story is a little bit more critical than in some of our other pests in crop.”
Resistance has also been on the rise in the province. The 2016 Manitoba herbicide-resistance survey found that 102 of 151 fields surveyed, or 68 per cent, had herbicide-resistant weeds, up from 48 per cent in 2008.
Breaking down those results by weed, producers may be unsurprised to see Group 1-resistant wild oats pulling ahead of the pack, present in 78 per cent of surveyed fields.
Of greater concern, according to Blackshaw, is the number of wild oats now resistant to Group 1 and Group 2 herbicides, a class that now accounts for 42 per cent of surveyed fields, according to the 2016 study.
Wild oats are among the top weeds in the province, although their abundance slipped in 2016 relative to wild buckwheat and barnyard grass, according to the provincial weed survey.
Manitoba’s top weed, however, also makes the list of herbicide-resistance concerns. Green foxtail was found in 24.4 per cent of the 659 fields in the 2016 weed survey (and at an average density of 14.6 plants per square metre). Resistance numbers, meanwhile, showed Group 1 resistance in 44 per cent of the 50 fields it was found in during the resistance survey that same year. Fortunately, Group 2 resistance still remains low.
A similar story has emerged for yellow foxtail, a weed that had provincial specialists concerned with its debut on the province’s top 20 weed list for the first time in 2016. Thirty-two per cent of tested fields showed Group 1 resistance and 17 per cent were resistant to Group 2 in the 2016 resistance survey, although weeds resistant to both were still low.
Glyphosate-resistant kochia is not the challenge it has become in other parts of North America, but Blackshaw warned farmers not to drop their guard. Western Manitoba is at risk of resistant kochia, he said, despite the fact that both confirmed resistant kochia cases have so far been closer to Winnipeg and the Red River Valley.
“I’m not aware of any glyphosate-resistant kochia in this region,” he said after his Brandon talk. “But what I’ve become very aware of in recent years is just how much kochia is present in the Brandon area, the Neepawa area, the Virden area. I grew up on a farm near Virden; we had no kochia on the farm 30 years ago. I was on the farm this past fall and we had an incredible amount of kochia on the farm associated with areas that were too wet to seed in the spring and there was salinity development because of the high water table and all the wet years we’ve had and kochia is tolerant to salinity and it tolerates some of those wet conditions.”
The weed is a high risk for developing glyphosate resistance, he warned.
What’s the solution?
Andrew Reid, BASF technical service specialist, wants producers to think long term in their herbicide plans.
“The big thing from our perspective is proper usage of our products, understanding the risks associated with resistance development and continuing to search for innovation as we go forward into the 2020s and beyond,” he said.
The specialist urged producers to use tank mixes, rotate groups and employ multiple modes of action. More importantly, he told the audience, multiple modes of action should have two active ingredients targeting the same problem.
The chemical side of weed control is not producing new modes of action like it once was, both because of increased cost and regulatory hoops, Reid said.
Both Reid and Blackshaw said there has not been a truly new mode of action for decades, and another is not expected for years to come, although Reid says there are some active ingredients waiting in the wings.
“We screen about 100,000 molecules per year looking for activity on weeds or fungi or insecticides, but the big thing is, even if we could find some activity, (that) doesn’t necessarily guarantee that it will ever make it to market because there’s a lot of very strict regulatory processes that we have to go through,” he said.
At the same time, the AAFC researcher added, tailoring a tank mix may come with its own challenges.
“I think cost is always foremost in producers’ minds, so that is a barrier, there’s no doubt about that,” Blackshaw said. “They have to sort of think, ‘OK, if I’m going to spend an extra $10 or $15 an acre, am I going to get value today or maybe there’s sort of value if you think in terms of multiple years.’ Then we talk about using multiple modes of action on herbicides, whether it’s a tank mix or different kinds, but it’s very hard to, on every weed species, (to make sure) that there are actually multiple modes.
“Even if you’re willing to spend the extra money, it’s hard to come up with another herbicide that will be the perfect tank mix partner, especially in some crops,” he added. “You might be able to do it in wheat, but the chances of doing it in your pea crop or your lentil crop or something else is virtually zero.”
Those challenges have put a new focus on adding non-chemical strategies to the mix. Increased seeding rate may be an economical way to deal with weeds, particularly in crops like wheat, Blackshaw said. Mechanical weed control is, likewise, something that producers have explored and that the organic sector is constantly looking to improve, although Blackshaw remains leery of erosion risk and increased tillage. He did tag the practice, however, for headlands, slough areas and very weedy fields, and noted that shallow tillage might limit soil damage.
“Tillage is a complicated issue and it depends on what kind of tillage and the frequency of tillage and how deep it is and stuff,” he said.
Longer rotations, however, have become a consistent recommendation among experts, Blackshaw included. The researcher argued that two- and three-year rotations have shown yield boosts while cutting down resistance pressure. But while Blackshaw joins the voices urging producers to space out their crops, tight rotations are still among the most popular in the field.
The ideal mix of chemical and alternative strategies will vary from farm to farm, Reid said.
“The biggest viewpoint that we have in understanding and understanding what the problem is and, generally, where I see the best place to start is scouting and testing and understanding what issues or what problems they have to manage in their operation and from there, depending on what their situation is, we can start to look at different perspectives or different solutions that would work for them,” he said.
The BASF employee suggested that farmers flag potential problem areas to watch and submit samples if they suspect a resistance problem.