Top Crop Manager

Features Agronomy Identity Preserved
Keeping weeds off balance

November 30, 1999  By Bruce Barker

Herbicide-tolerant (HT) weeds have been on the radar screen for many years, but the big elephant in the room for canola growers is glyphosate-resistant weeds. Although only one glyphosate-resistant weed species has been detected in Canada in Ontario in 2010 (giant ragweed) and the selection pressure here is less than in other countries, Agriculture and Agri-Food Canada (AAFC) weed scientist Neil Harker at Lacombe, Alberta, says integrated weed management (IWM) is “absolutely crucial to ensure we do not overuse HT canola technology.”“Sometimes when we depend on HT crops, there can be consequences we haven’t anticipated and the problems can appear rather suddenly,” explains Harker from his office. 

Harker believes that implementing IWM can help preserve HT technology, but he defines IWM as using non-herbicide tools in addition to herbicides. “In canola, there are still opportunities to practise IWM.”

Harker cites research by Lester Ehler (2006) from the Pest Management Science journal as providing a good example of what IWM can be.“Real IWM” involves at least two, and possibly more than two, weed control methods (one of which can be herbicides). Cultural, physical or biological methods are necessary in addition to herbicides to constitute real IWM. Real IWM is also not just biological, or just physical, or just cultural; it is a combination of methods.”


Greater diversity can reduce the likelihood of a weed biotype adapting and developing resistance, which explains why U.S. Midwest growers have a high concentration of glyphosate-resistant weeds in their HT corn-soybean rotations.

Seed right
Even though canola seed costs in the $50 per acre range, and despite farmers’ best efforts, only around 50 percent of planted canola seed actually emerges. Harker explains that poor stands of canola do not compete with weeds well, and that in his trials, canola seeded too deep are much more likely to require a second herbicide application – adding additional selection pressure for weed resistance. 

Harker has looked at canola seeding speed and depth (see page 6 Top Crop Manager, November 2011) to assess canola emergence in conjunction with the Alberta Canola Producers Commission and the Canola Council of Canada. He found a vigorous and early-emerging canola crop closes the canopy early, competes well with weeds and may improve herbicide performance.

Research by John O’Donovan with AAFC also found that high seedrow nitrogen (60-90 kg/ha) caused seedling injury and allowed wild oats to grow more vigorously than when the N was sidebanded, allowing the barley to outcompete the wild oats.  Poor N placement increased wild oat growth by up to five times.

Own the podium
Another component of IWM is planting competitive crop cultivars. At one time, canola was thought to be a relatively poor competitor, but the advent of vigorous growing hybrid canola is changing that perception. Harker has run several crop competition studies and found that hybrid canola can be a very strong competitor with weeds. Under relatively cool and dry conditions, canola hybrids compete with weeds as well as barley. 

“The trials became a contest to see which crop would have the least weed biomass. The common trend we found was the crop with higher biomass outcompeted the rest,” explains Harker.

For canola, there are some environmental factors deciding whether it was on the top step of the podium. At Scott, Saskatchewan (hotter and drier), all cereals outcompeted all canola cultivars. At Lacombe, Alberta (cooler and wetter), the ranking was switched around with canola on top. Canola also competes more successfully against broadleaf weeds than against grassy weeds. 

Early weed removal saves thousands of dollars
Early weed removal has been a mantra for Harker and other weed scientists for years. Still, the temptation is to wait for that extra flush of weeds. Don’t, advises Harker. 

In a recent study on farmer fields, Harker found that canola yield dropped three or seven bushels per acre when herbicide was delayed from 1-2 to the 3-5 or the 6-7 leaf stage. Using $12/bu canola, the cost of delaying herbicide application to the 3-5 leaf stage would be $23,000 on a section of canola, and more than $50,000 if control was delayed to the 6-7 leaf stage. 

“You’re wasting the herbicide if you are putting it on too late,” says Harker, “and adding herbicide selection pressure without all of the benefits.”

Change it up
With the recent tightening of canola rotations on the Prairies from the traditional one-in-four to one-in-two years, selection pressure on canola weeds is becoming more intense. Just as disease pressure may build up, so will the selection for glyphosate-resistant weeds. Research over the years by scientists such as Bob Blackshaw at AAFC Lethbridge and Doug Derksen at AAFC Brandon has shown that diverse crop rotations minimized weed densities by constantly changing the selection pressure on weed communities. 

As proof, Harker cites the fact that green foxtail, wild oat and wild buckwheat remain the top three weeds on the Canadian Prairies over the last 40 years. Despite spending $500 million annually on wild oat herbicides, wild oat continues to thrive because Prairie cropping practices rely primarily on summer-annual crops, which wild oat is well adapted to.

Harvest losses hurt
Harker has just embarked on a new study looking at canola losses during combining, but older research supports what he is finding in his preliminary results.  The older study by Robert Gulden at the University of Manitoba found that on an average of 35 Saskatchewan fields studied, 3000 canola seeds per square metre was left on the ground. Although not all those seeds will volunteer the next spring, consider that the targeted plant population as recommended by the Canola Council of Canada is 40 to 200 plants per square metre – the harvest losses are about 20 times the recommended seed rate for canola and represent two to four bushels per acre loss behind the combine. 

“The losses can be fairly substantial and can result in a huge potential for canola volunteers in following crops,” says Harker. “Effective IWM would include reducing combine losses and volunteer canola seedbanks.” 

Put it all together
Studies have also shown that using “real” IWM is more effective than using just one practice. One study found that planting a competitive canola hybrid, combined with early weed control and a high seeding rate (200 seeds per square metre) yielded 41 percent more than the worst practice of planting a poorly competitive cultivar, combined with late herbicide application and a low seeding rate. These best yield combinations also resulted in better weed control and lower weed biomass. 

In another, agronomic study, Harker also looked at crop rotation, seeding rate, herbicide rate and cultivar selection in barley. The best practices of growing a tall barley variety with 2x seeding rate and a barley-canola-barley-pea-barley rotation reduced wild oat emergence, growth and seed production.

“When we combined all factors, there was a 19x reduction in wild oat biomass at the one-quarter herbicide rate,” says Harker. “The synergistic reaction was much more than the individual effects.”


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