Get to know active ingredients to fight herbicide resistance

multi-pronged attack Weed scientist urges farmers to 
“hit resistant weeds with many little hammers”

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Many herbicides about to go off patent are now for sale under new names, and producers are being urged to read labels more closely.

“You definitely should check the mode of action group of the herbicide before you use it,” said Agriculture Canada weed scientist Hugh Beckie. “But you may also want to use a different active ingredient. There are differences among chemicals in some groups.”

Some are prepared tank mixes, sometimes with a different surfactant, an activator or safener. Beckie said he has found mixtures of herbicides that attack a single weed are more effective at controlling herbicide-resistant biotypes — which can account for as much as 10 per cent of weed populations, especially in the black soil zone.

“We have to hit resistant weeds with many little hammers, avoid doing the same thing at the same time year after year,” said Beckie. “Some of our new cereal varieties are very competitive. That can make wild oat control less of an issue. And winter wheat is a great tool — you may not even need to use wild oat herbicide on it. Of course, the more diverse your crop rotation, the more opportunities you have to control weeds where they’re relatively easy to control — grassy weeds in broadleaf crops and broadleafs in cereals.”

More and more weeds are becoming resistant to herbicides. Of the top 10 yield robbers, seven have herbicide-resistant populations, said Harry Brook, of Alberta Agriculture’s Ag Info Centre. That means farmers need to be familiar with the chemical names of active ingredients in order to manage the whole range of newly named tank mixes, he said.

Alberta Agriculture’s website lists the active ingredients in products currently being sold in the province by their mode of action group, their chemical family, the name of the active ingredient and the trade names that include that chemical. The website address is$department/deptdocs.nsf/all/prm6487.

Keep records

Brook also recommends keeping a record of the product you use so you can rotate your herbicides. Also, some active ingredients are more effective than others in the same group.

Beckie has assessed the risk of target weed species developing resistant populations. Herbicides in Group 1, the fops and dims, and Group 2, the sulfonyl ureas and some other chemistries, are considered high risk, with resistant biotypes developing with 10 or fewer applications. But even within these groups, the Group 1 active ingredient clethodim (found in Arrow 240 EC, Centurion, Select, Shadow RTM) can be a little more effective. Beckie advises saving it for critical situations — fields where you suspect you have resistant grassy weeds and a less competitive crop.

“Save your best products for your least-competitive crop,” he said. “There are no easy answers to weed resistance, we just have to use our tools as best we can — and use the little hammers that give the crop an edge over the weeds.”

Even the herbicides that are low risk, glyphosate (Group 9) and Liberty (Group 10), can lead to herbicide resistance in 20 or 25 applications, said Beckie.

“Many people could well have used glyphosate more than that since it became a low-cost chemical in the early or mid-’90s. Perhaps it’s time to add another active ingredient to glyphosate when we use it for a burnoff. That gives us two ways to fight weeds and reduces the risk of weed escapes.”

It’s important to double-check that one of the active ingredients in a product you plan to use will control your problem weeds, said Brook.

“You should also look at the resistance potential of each chemical — the more you use a single mode of action in a field, the more likely you’ll have resistant weed biotypes,” he said. “And some modes of action, such as the ACCase inhibitors of Group 1, can lead to resistant biotypes with fewer than 10 applications. Others take more applications.”

Brook recommends checking the herbicide use pyramid developed by Beckie to figure what product to use. The pyramid shows how many applications it can take herbicides of each group to develop a resistant biotype.

“Weed populations can have different ways to survive a single herbicide, too,” said Brook. “Some weeds survive glyphosate through hypersensitivity — the herbicide kills above-ground plant parts on contact, so the chemical is not translocated and the plant can recover. Other plants have so many copies of the protein synthesis system that glyphosate attacks that the chemical can’t block them all.”

resistance     Risk of selection by herbicide group

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