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Weed pressure ramps up in late spring

Weeds that emerge a week before the crop can cause up to 50 per cent yield loss

In a late spring like this one, you might be tempted to skip spraying for weeds to make up for lost time — but that decision could cost you.

“The last few years, we’ve had early springs, so people weren’t feeling like they were under the gun,” said Harry Brook, crop specialist with Alberta Agriculture and Forestry.

“This year, people are basically saying, ‘Damn the torpedoes — full speed ahead.’ But it’s important to do early weed control.”

In a typical year, producers would do a pre-seed burn-down to get the early weeds or, at the very least, spray shortly after seeding before the crop emerges. But in a year like this one, where winter stretched into mid-April in many parts of the province and the ground didn’t start to warm up until the start of May, producers are likely to skip that step to get seed in the ground.

It makes sense, but it’s the wrong move, said Brook.

“You want to get the weeds while they’re young. They’re easy to kill when they’re very small,” he said.

“As long as you can get your crop ahead of the weeds, that’s the biggest factor for your yield.”

Weeds emerging at the same time as the crop aren’t ideal, but they don’t affect yield potential compared to ones that emerge before the crop does, he added. In canola, weeds that emerge one week before the crop can cause up to 50 per cent yield loss, while a pre-seed burn-down can increase yields by as much as 15 per cent. Wheat is even less competitive than canola, and when weeds like wild oats (a particular problem in Alberta) are at high density in a wheat crop, yield losses can exceed 50 per cent.

Yield is established in cereals up to the six-leaf stage, and at bolting in canola. If the crop is sprayed between the two- and four-leaf stage, weeds will actually have little effect on yield.

“But if you have problems getting out into that field because it rains and the crop gets to that four-leaf stage, then your weeds are affecting yield potential,” said Brook.

“At that point, you run into the problem of not being able to spray it or having greatly reduced spraying options.

“If you’re past the six-leaf stage in the crop, it’s going to be very tough to find something to control the weeds.”

If the weeds do get ahead of the crop, the plants will be competing for water — the last thing you want in a relatively dry spring.

“Our soils are pretty dry right now, and there’s not a lot of soil moisture available,” said Brook. “We need that rain for the crops, not the weeds.”

The same goes for nutrients. Most producers typically band fertilizer, but in a tight spring, they might decide to broadcast instead. That’s probably a bad call, said Brook.

“If you broadcast fertilizer, weeds will take advantage of it too and become more vigorous,” he said. “Fertilizer is expensive. You want to make sure you get the biggest bang for your buck possible using these inputs.”

Producers should also remember to rotate the chemicals they’re using wherever possible if they’re doing a post-seeding spray, he added.

“We’re starting to run out of a few tools, especially in cereals, so herbicide resistance should always be top of mind,” said Brook. “Whatever product you’re using for weed control, you should always be changing up the game.”

While herbicide resistance is a growing problem, herbicide use has been pretty effective in reducing the weed seed bank in the soil. Even so, it’s hard to predict from year to year how high weed pressure will be, so producers should err on the side of caution and use an integrated approach to managing weeds.

“We’re not really getting any new herbicide products out there. It’s a valuable tool, so we’ve got to be stewards of it so it can last as long as possible,” said Brook. “Otherwise, we’re going to lose it.”

That means seeding early when possible, increasing the seeding rate, planting high-vigour seed, and taking care of weeds quickly. But if weed populations aren’t causing problems in your crop, maybe skip the second in-crop spray later this summer.

“We’ve got this mentality where we don’t want to see weeds growing,” he said. “Maybe we should be looking at it from the point of view that as long as it’s not economically damaging yields, maybe weeds are OK.”

But it’s not do-or-die time yet, he added. There’s still a lot of growing season ahead.

“It really depends on the weather and how many rainstorms we get this spring,” said Brook. “This late spring may be a non-issue.”

About the author

Reporter

Jennifer Blair is a Red Deer-based reporter with a post-secondary education in professional writing and nearly 10 years of experience in corporate communications, policy development, and journalism. She's spent half of her career telling stories about an industry she loves for an audience she admires--the farmers who work every day to build a better agriculture industry in Alberta.

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