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Practical ways to fight herbicide resistance

Do you have herbicide resistant weeds growing in your field? 

kochia in a canola field

Brian Wintonyk wants farmers to be able to identify herbicide resistant weeds in their fields and know what to do about them.

Wintonyk, western crops agronomist with Dow AgroSciences Canada Inc., was on the agenda at the third annual Grain Expo, a crop production conference held in Regina in conjunction with Agribition in November. He described several conditions that can increase the risk of herbicide resistance developing:

  • Weed characteristics: Weeds with high seed production are more likely to spread resistance. For example, “a single kochia plant can produce 14,000 weeds,” Wintonyk said. Kochia is also quick to reproduce, especially as compared with weeds like wild oats that can stay dormant in the ground for years. Wintonyk said, “the kochia that was grown in 2014 and produced seed, 92 per cent of that population, roughly, is going to emerge next spring.” These physical characteristics increase kochias’ ability to develop resistance to herbicides.
  • Use patterns: “Using the same product year over year,” can speed the development of herbicide resistance. Most importantly, “reliance on a single mode of herbicide activity,” gives weeds a chance to develop resistance to that particular product.
  • Farming practices: Practices like tight crop rotations also contribute to herbicide resistance.

Detecting herbicide resistance

If you believe you have herbicide resistance weeds in one of your fields, ask these questions.

Was the weed control product applied correctly? That is, Wintonyk said, “Were the tank mixes compatible? What was the crop stage?” Was the product applied at the right time? “Some herbicides perform less effectively under cool conditions or cloudy conditions.” If the product wasn’t applied according to the label directions and under good conditions, you may not have a case of herbicide resistance.

Do the weeds show any symptoms of control? Has the product been effective in the field at all?

Did the weeds come up after the spray treatment? That could indicate that this is not a resistance problem but just timing.

Do the weeds occur in patterns? A pattern of natural weed growth can indicate resistance. Weeds in a straight line are more likely to indicate a sprayer miss.

Has more than one weed escaped? If all but one of the weeds that the chemical should have controlled were controlled, that would indicate an effective chemical application. There may be resistance in that one type of weed that survived.

Has that same herbicide or group been used in the field for several years? If so, that can increase the odds that weeds in the field have developed resistance. “Sometimes we think about rotating herbicide brands, but we don’t think about rotating herbicide groups,” Wintonyk said. Rotating brands with the same mode of action does not lower the ability of the weeds to develop resistance.

And finally, what’s going on in the neighbour’s field? Are there herbicide resistant weeds growing there? “Talk to your neighbours,” Wintonyk advised.

If, after asking these question, you believe you do have a chemical failure, call the chemical manufacturer or an agronomist right away. “If it’s not investigated properly, the same use pattern continues, and you have failure next year.”

If you do have resistance in your field, Wintonyk said it’s time to consider developing a long-term strategy. “That doesn’t mean shifting to a different herbicide family and using that until resistance development occurs.”

herbicide resistance risk pyramid

Tips to slow or deal with resistance

Whether you already have herbicide resistant weeds or you just want to slow the development of resistance in your fields, Wintonyk said, the most important step is to “move to a herbicide rotation that consists of multiple modes of action.”

This means using two herbicides together that are both effectively controlling the same weed (either in a tank mix or a pre-mix).

“When you have two modes of action,” Wintonyk said, “both of those ingredients are acting on that same weed.” With two active ingredients, “you can go through multiple generations before that risk increases.”

It’s also important to know that some herbicide Groups have a higher risk of resistance development than other. As shown in the graphic above, there is a much higher risk of weeds developing resistance to herbicides in Group 1 or 2 than to herbicides in Groups 4, 6, 9 or 10.

Changes to cultural practices can also help. Rotate crops that require the use of different herbicides, as well as different life cycles (for example, Wintonyk said, “What about the introduction of winter wheat?”)

Try other methods to change the weeds’ growing conditions. For example, Wintonyk said, the driest field on the farm is usually seeded first. “Weeds are very adaptable,” he said, and they’ll adapt to that practice in the field. “If you can switch the practice in that field by even as much as a week you can completely throw the weeds off.”

Sometimes, “tillage may be a necessary thing,” Wintonyk said. It can reduce the need for chemical application to control persistent weeds in a particular spot.

Using clean seed, growing competitive crops, scouting regularly, and keeping good records of weed occurrences are all practices that can help to slow herbicide resistance development.

Also, Wintonyk said, “be judicious about proper equipment hygiene. Prevent the spread of that seed.” This means not allowing weeds to move from fences, railways, oil and gas leases or utilities.

Tolerance versus resistance

Brian Wintonyk, western crops agronomist at Dow AgroScience explained the difference between “herbicide resistant” weeds and “herbicide tolerant” weeds.

Herbicide resistant weeds are “weeds that were once effectively controlled by a herbicide. Now when that herbicide is applied at the same rate, it doesn’t appear to be controlling those weeds.” An herbicide tolerant weed is “a plant that can naturally survive that herbicide application.”

Generally speaking, herbicide resistance has a chance to take hold when weeds are controlled by the same herbicide in the same conditions multiple years in a row.

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