Use herbicide tank mix against resistance: agrologist

Reducing risk | Glyphosate resistant kochia is spreading, but officials worry about resistant wild oats, green foxtail and cleavers

Weed biologist Eric Johnson was telling farmers as recently as a few years ago to fight problematic weeds with a higher rate of glyphosate.

The Agriculture Canada scientist has since changed his message, armed with a better understanding of how weeds develop resistance to commonly used herbicides and knowledge of glyphosate-resistant kochia on the Prairies.

At the Agri-Trend 2012 Farm Forum Event in Saskatoon, Johnson urged producers to guard against herbicide resistance by tank mixing herbicides with multiple modes of action, especially in chem-fallowed fields where resistant weeds are first appearing.

“So even people that advise (farmers) are a bit of the problem or contributed to the problem,” he said in an interview.

“And so there’s still a bit of that mentality … but really, we should be tank mixing.”

Officials announced earlier this year that glyphosate-resistant kochia had been discovered in three chem-fallowed fields in southern Alberta in 2011, the result of extensive use of the Group 9 herbicide regularly applied as Roundup.

Further survey work uncovered more populations of resistant kochia nearby and all signs point to the presence of the weeds in 2012 at a location further north in Alberta and a few sites in southern Saskatchewan.

Johnson told the conference that only 15 years ago scientists predicted weeds wouldn’t develop resistance.

“I think we’ve learned as weed scientists now to realize we’ll never say that a plant won’t develop resistance to herbicides because there’s so many different ways they can adapt.”

Johnson said officials want to do more extensive surveying for resistant kochia in Saskatchewan. As a tumbleweed, it produces large amounts of seed and travels quickly.

“Kochia responds to the environment. It likes dry, saline conditions,” said Johnson.

“The last few years, it’s been wetter, so it hasn’t been quite as predominant and, I don’t think, quite as on people’s minds. One dry year and it’ll be back and they’ll remember how serious it actually is.”

All kochia populations on the Prairies are already assumed to be resistant to Group 2 herbicides, which spread quickly in a five-year period in the 2000s. Johnson said growers must keep this in mind when tank-mixing herbicides to delay or manage Group 9 resistant populations because the Group 2 herbicide won’t be an effective selection.

He said producers have several options for tank mixes in wheat and barley crops including dicamba, Cleanstart and Blackhawk, although he noted that 2,4-D alone won’t effectively manage glyphosate-resistant kochia populations.

Growers have fewer options in pulse and canola crops, but those available include Cleanstart and Amitrole 240.

Johnson is researching the use of alternative Group 14 and 15 herbicides on some crops.

“It does buy us time. We can’t lose sight of those other things you have to do that are important: good plant density, competitive crops, crop rotations,” he said.

“Those are very critical as well into extending the life of our herbicides.”

Glyphosate-resistant kochia is appearing in chem-fallow fields, where Johnson recommended producers tank mix with Cleanstart, a Group 9 herbicide, or Distinct, a dicamba product.

“I would like to talk to the growers more in the infected area that are chem-fallowing and discuss it with them. I don’t want to come out and say that you shouldn’t be doing this because they are doing it for a reason and it is a drier area and fallow has been and probably will continue to be a part of their system down there,” said Johnson. “We might just have to work on some options to reduce their risk.”

Based on their abundance, officials believe wild oats, green foxtail and cleavers are the weed populations in which glyphosate resistance may appear in next.

About the author


Stories from our other publications