Attack weeds early to avoid yield loss, resistance

If history is a guide, a higher number of herbicide resistant weeds will be discovered this year in Alberta when Agriculture Canada researchers survey the province.

Sixty percent of cropland in Western Canada has a weed resistance problem to some degree compared to 15 to 20 percent 10 to 15 years ago.

Research scientist Bob Blackshaw told those at a recent Farming Smarter field event that good weed control is an important part of avoiding herbicide resistance.

“We do want good weed control because resistance is a bit of a numbers game,” he said. “So if you have a lot of weeds in your field, you have a greater chance of that resistant gene being present.”

The prevailing theory is that herbicide resistance is a natural mutation.

“The herbicide does not cause resistance,” he said. “That resistant gene is in the population and then we are just selecting for that resistant gene.”

That’s where early in-crop weed control enters the picture.

Weeds are easier to kill with herbicides when they are small, potentially preventing any with burgeoning resistance from surviving and setting seed.

Most yield damage is done by the first flush of weeds, said Blackshaw.

“If you wait, especially in crops that aren’t very competitive, like lentil and field pea and whatever, you can have a 10, 15 percent yield loss just by waiting 10 days to spray a herbicide,” he said.

“So don’t wait for every last weed to come up in your post-emerge sprays. Try and get a really good control of that first flush and then if you have a good crop stand, it’ll cover over and close in the canopy and shade out the weeds.”

Most resistant problems in southern Alberta involve Group 1 and 2 herbicides. Only kochia is resistant to glyphosate, a Group 9 herbicide.

“We’re lucky that it’s only kochia, and I dread the day that we have wild oat that is resistant to glyphosate because we have such resistance to Group 1 and Group 2 already.”

Blackshaw said wild oat is the most economically important weed in Western Canada, and its resistance first to Group 1 and then to Group 2 chemicals severely limited options for control.

“What some people are doing … is trying to fit in a Treflan or an Edge or an Avadex back into your cropping systems because in many cases they still work on those weeds,” he said. “They aren’t going to give you 100 percent control but they can be a good start in giving you 70 to 80 percent control.”

Farmers were asked to pay attention to weed problems and report suspected herbicide resistance as early as possible. Blackshaw was the person at Agriculture Canada who first got the call about herbicide-resistant kochia, which he later investigated and confirmed in work with researcher Hugh Beckie.

Blackshaw learned then that farmers had noticed kochia surviving after spraying but did not think to report it at first.

“We’re always going to be two, three, four years behind the curve in terms of identification, so it’s really, really important if you have an inkling … that you have glypho-sate resistance in whatever (to report it),” he said.“We’re worried about cleavers. We’re definitely worried about wild oat and green foxtail.”

Weeds also adapt to cropping systems and climate. Blackshaw said he was surprised to find hemp nettle in southern Alberta research plots this spring because it usually prefers wetter, cooler conditions.

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