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Rallying against resistance

By Madeleine Baerg

Canada is now home to 65 unique cases of herbicide resistant (HR) weeds. (Update 2020: 76 unique resistant weeds) Though 2017 weed survey results are not fully tabulated, experts warn the total number of prairie acres infested with resistant weeds probably jumped to over 38 million, an increase of more than 50% since the 2007-2009 survey. Integrated weed management (IWM) is the only answer. For it to be achievable on farm, however, it must take agriculture’s economic and practical realities into consideration.  

“Most of the (resistance issue) is an economically derived problem. Margins are so limited for growers in western Canada: that’s the unfortunate fact of the situation. Growers can’t possibly invest money in every area of their rotation because they’d lose all of their profit,” says Dr. Charles Geddes, an AAFC weed researcher in Lethbridge.   

The solution, Geddes says, is to pick and choose resistance-fighting techniques based effectiveness and the farm’s unique risk factors, history, location, management practices and priorities.  

“There are going to be certain areas in your rotation where certain practices are more beneficial. Instead of implementing every possible IWM tool, producers need to choose certain combinations of tools that are most cost effective, most achievable, and offer the most result. That is where knowledge needs to be increased,” he says. Also, IWM tools used in combination can increase the efficacy of weed management.  

“We’ve developed the problem of resistance by relying too heavily on herbicides. I don’t think the solution to that problem is more herbicides,” says Geddes. “Herbicide rotation, so long as it’s rotating modes of action, is definitely beneficial over using the same herbicide over and over. And tank mixing has proven beneficial over just using single modes of action. But herbicide rotation and tank mixing are just delaying the problem.” 

Kochia is the glyphosate resistant weed that particularly worries Dr. Geddes PHOTO: Farming Smarter

Crop rotation should be a given across all prairie fields, says Geddes. And, producers should do all they can to maximize a crop’s competitive edge, from increasing seeding rates, to decreasing row spacing when possible, to optimizing crop in-puts (the latter, ideally, on a site-specific rather than field-wide basis).

To make increasing seeding rates economically viable, opt to do so where higher rates will have the most impact at the least cost. For example, while increasing seeding rate is very effective at controlling weeds in multiple crops, it is far more cost effective in cereals than in canola.

Effective resistant weed management also depends on thoughtful consideration of all the tools in the arsenal, including much more intensive scouting, sanitation between fields and, yes, even tillage.

“In western Canada, tillage is almost a bad word. Tillage has its place if used sparingly where it is needed,” says Geddes. “There are many weed species that are adapted to zero-tillage. If you have a patch of, say, glyphosate-resistant kochia, going in and disturbing the ground could be a very effective option.”

While cover crops are highly effective, they draw vital moisture from dry ground. Their highest benefit is in chem-fallow fields that would otherwise be ideal breeding grounds for resistance. Opt for cover crop varieties with low transpiration to retain as much moisture as possible.

Remember: not all HR weed management techniques need to be complicated or expensive. One of the least expensive tools available to reduce the spread of resistance costs just $0.25: an HB pencil.

“Producers need to be doing more record keeping. If a grower has a suspected resistance case, they should get it tested. Then, they need to make a note of it and keep that in their own records. Even more beneficial, they should let someone else know so it can be recorded in an industry database. The whole industry needs to work together,” says Geddes.

It’s not just growers who need to do a better job of communicating, however, he adds.

Charles Geddes
Charles Geddes

“One of the big things in industry is we need to get better at labelling herbicides with group numbers in ads. As an industry, we could be better at explaining what’s in each herbicide so that producers use different modes of action instead of just different names for the same modes. Something needs to change in the industry as a whole: my hope is that it could be voluntary.”

All resistant weeds concern Geddes, but the spread of HR wild oats and glyphosate resistant weeds, especially kochia worry him most. Different populations of wild oats now show resistance to six different herbicide modes of action: Groups 1, 2, 8, 14, 15 and 25. While no single population appears to have resistance to all six groups yet, growers need to watch for resistance to multiple modes of action. If a weed proves resistant to a single group, it is more likely to develop resistance to another.

Currently, kochia and volunteer canola are the only known weeds with biotypes resistant to glyphosate in western Canada. However, worse is almost certainly ahead. In Ontario, farmers’ increasingly heavy use of GR crops is the likely cause of four weed species now boasting resistance to glyphosate.

Almost all kochia in western Canada is resistant to Group 2 herbicides. Now, certain Group 2 resistant populations in Alberta also show resistance to Group 9 herbicides, while some Group 2 resistant populations in Saskatchewan show resistance to Group 4s. Geddes’ concern is that triple-resistant kochia populations may soon appear.

Data from grower surveys shows that, in fields with identified resistance, farmers change management practices. While Geddes applauds farmers for making strides to limit the spread of resistance, he says the shift needs to be proactive rather than reactive.

“My hope is that growers start including practices on fields that don’t have resistance problems. “Studies show that in the long run, it is more profitable to include non-chemical options that limit your chances of developing resistance than to deal with resistance once it appears.” to “Studies show that in the long run, it is more profitable to include proactive weed management that can limit your chances of developing resistance than to deal with resistance once it appears.”