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Mix it up to reduce herbicide tolerance

It’s doing the same thing with the same products year after year that increases the risk of herbicide resistant weeds

The discovery of glyphosate-tolerant kochia in southern Alberta is a wake up call for Western Canadian farmers to pay particular attention to proper herbicide rotation, and also to get back to the basics of good agronomic practices, say weed and herbicide specialists.

While this is only one case of glyphosate-tolerant kochia, more are likely to follow. Glyphosate has been a very cost effective tool for producers, particularly over the past 25 years in the move toward direct seeding and zero-till cropping systems. It is an option farmers do not want to lose.

“The discovery of a glyphosate-resistant kochia is a bit alarming,” says Neil Harker, a weed research scientist with Agriculture and Agri-Food Canada (AAFC) at Lacombe, Alta. “It is the first case of glyphosate resistance on the prairies, and it is found in a major weed species that already has common resistance to Group 2 herbicides, so it is a concern. And I think it behooves us to pay more attention to proper herbicide rotations and to increase crop diversity.”

The news of a glyphosate-resistant weed doesn’t mean farmers should stop using glyphosate pre-seeding or in-crop, but it does mean farmers need to look at all the tools in the proverbial herbicide and management toolbox to reduce the risk or spreading resistance.

“It is a wake-up call for producers,” says Hugh Beckie, a research scientist specializing in the weed resistance with AAFC at Saskatoon. “Glyphosate resistance is fairly common south of the border, so we knew it was coming. It is no great surprise. But it is a wake up call to how growers use glyphosate in their cropping systems so we can preserve the effectiveness of the herbicide as long as we can.”

Mixing it up

Mix things up — that sums up the general management strategy in dealing with herbicide resistance. Use different herbicide groups and different combinations of herbicides, grow a wider diversity of crops, and apply herbicides at different times of the year to avoid repetition and trends that weeds adapt to.

Brian Wintonyk, agronomist with Dow AgroSciences at Calgary says management strategies will differ depending on whether farmers are already dealing with cases of herbicide resistance, or whether they are trying to prevent it.

“If a producer already has some herbicide resistant weeds in their fields the consequence of that is a need for some significant management,” says Wintonyk. “And if you don’t have any resistance problems, then what do you have to do so you don’t get it?”

Double whammy

Using two herbicides is one approach all specialists recommend. At the pre-seeding burn-off stage, producers are urged to tank mix glyphosate with some other herbicide. It could be as basic as tank mixing with 2,4-D, however producers need to know their weed spectrum and use whichever “second” herbicide is most effective.

“Some general figures in the industry suggest of all producers using a pre-seeding herbicide, about 20 per cent use glyphosate tank mixed with a second herbicide and about 80 per cent just use straight glyphosate,” says Bob Blackshaw, a weed research scientist with AAFC at Lethbridge, Alta.

To reduce the risk of weeds becoming resistant to glyphosate, he recommends a tank mix with another herbicide group. In the case of kochia, in particular, using something like 2,4-D, a Group 2 product, may not be the best option since kochia can become resistant to Group 2 herbicides. But the point is to look at your weed spectrum and look at effective options that can be used for weeds that are there and are compatible with the crop to be planted.

With kochia expanding its range across Western Canada, Blackshaw urges farmers to consider using a relatively new group — Group 14 herbicides — such as Heat and Cleanstart in combination with glyphosate to reduce risk of developing herbicide tolerance.

Blackshaw also says post-harvest weed control is another opportunity to apply a glyphosate tank mix to deal with the weed spectrum. It takes the pressure off glyphosate to do all the weed control work.

“As we deal with the risk of herbicide resistance, weed control will become more expensive as different chemistries are used in different combinations to lower the risk,” he says. “It also means producers will need more advanced planning. They will need good records of treatments that have been made in past years, and will need to look ahead three or four years to use the right combination of products, in the proper rotation with crops.”

In-crop treatments

Wintonyk says producers also need to pay attention to in-crop herbicide treatments in cereal crops as well. Research has shown using two compatible herbicides in one application to tackle weeds is more effective than using different herbicides at different times.

One objective of combining two products such as Simplicity and Attain, Simplicity/Frontline/2,4-D, or Simplicity and a new version of Attain being called OcTTain XL, and Tandem/2,4-D ester, is to cover a wider weed spectrum and give producers more options. But, perhaps even more importantly is to combine two products with different chemistry effective on the same weed, to reduce the risk of developing herbicide tolerance.

Wintonyk also urges farmers to learn more about growth and reproduction patterns of weeds on their land. With kochia, for example, one plant can produce as many as 14,000 seeds and the majority will germinate a year after seed production. Also, kochia seeds don’t germinate as well if the seed is one or more inches deep in the soil.

He’s not suggesting farmers revert to tillage for weed control, but he says in some circumstances, in some areas it may have a fit.

Good agronomics

Harker along with others also emphasizes the value of following good agronomic practices which helps control weeds and risk of herbicide tolerance. Greater crop diversity and maintaining optimum plant stands are important practices he says.

He understands farming economics, but says back-to-back cropping or even two-year rotations of crops such as canola can increase the risk of developing herbicide tolerance.

“We are better off than some areas of the United States where their rotations can be Roundup Ready corn, followed by Roundup Ready cotton and then Roundup Ready soybeans,” says Harker. “Using glyphosate after glyphosate after glyhphosate year after year puts a huge selection pressure on weeds for glyphosate resistance.”

A cereal/canola rotation isn’t as bad, but he encourages farmers to use all three herbicide-resistant canola systems which helps reduce the selection pressure of glyphosate. Along with cereals and oilseeds, he also encourages the use of pulse crops in rotation, and where possible the use of winter cereals such as winter wheat, fall rye and fall and winter triticale to extend rotations and change the seasonal timing of herbicide applications.

“Farmers need to have a look at all the practices of an integrated weed management system,” he says. “Maintaining higher seed rates is also important to improve weed control. Higher seeding rates and healthy plant stands help to suppress weeds and also promotes more even crop maturity and reduced green seed.”

While many canola growers are looking to reduce seeding rates to reduce the cost of seed, “I think we need to resist that temptation,” says Harker. In an ideal situation thinner stands can be productive, but if stressed by frost, excessive moisture or drought those thinner stands aren’t as competitive with weeds, and if weeds flourish it may mean farmers are applying two or three treatments of glyphosate to control weeds — more glyphosate increases the risk of herbicide-resistance. †

About the author

Field Editor

Lee Hart

Lee Hart is editor of Cattleman’s Corner based in Calgary.

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