In a study featured in the most recent edition of Weed Science, a team of researchers tilled four fields every two weeks during the growing season. They then monitored each site to quantify the density and species of seedlings that emerged from the weed seed bank six weeks after each till. They found that total weed density tended to be greatest when soil was tilled early in the growing season. More than 50 percent fewer weeds emerged after late-season tillage. | READ MORE
In 2013, two University of Guelph weed scientists began collaborating on alternatives to herbicides for weed control. The report, by Francois Tardif and Mike Cowbrough, was released in 2016.
Multiple modes of action make a big difference when it comes to slowing down the development of herbicide-resistant weed populations. That’s the message echoed throughout Nufarm Agriculture’s field plot tour, held outside Saskatoon on July 5.

The tour featured cereal, canola, soybean and pulse plots to demonstrate Nufarm’s line of herbicides for pre-emergent control in cereals, canola, soybeans and pulses, and in-crop weed control in cereals. The overriding focus was on strategies to reduce the risk of weeds developing resistance to herbicide groups.

“Using multiple modes of action across multiple application timings helps to manage the selection pressure placed on weed populations by each individual mode of action, and can reduce the proliferation of resistance mechanisms through a weed population,” says Nufarm’s Technical Services Manager Graham Collier in a press release. 

Agriculture and Agri-Food Canada research scientist Dr. Hugh Beckie updated attendees on the state of herbicide resistant weeds in Western Canada. Dr. Beckie, who worked on the very first case of herbicide resistance back in 1988, said the issue is growing in both complexity and severity. “In 2003 we were seeing 10 per cent of fields impacted, and as of last year we were seeing 57 per cent,” he said. Herbicide resistance hits farmers right in the pocket book, Dr. Beckie explained, citing a recent survey of 300 sites in Saskatchewan that found farmers reporting extra costs of $15-20 per acre to address herbicide resistance. 

Weed resistance is a growing challenge for all growers, but how likely resistance is to develop isn’t a mystery. Collier reviewed the key factors that impact herbicide resistance development. “Herbicide resistance can increase exponentially in a field, year by year, depending on the herbicide mode of action, the selection pressure applied to the population, the biology of the weed, and the number of times a specific mode of action is used,” says Collier. “For example, resistance will spread through a kochia population much faster than a wild oat population due to cross pollination, seed production and seed bank longevity. “

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Hard to identify and distinguish from one another, the annual grasses compete with winter wheat and fall rye because their growth habits are similar. Downy brome (Bromus tectorum) densities of 50 to 100 plants per square metre that emerge within three weeks of the crop can reduce winter wheat yields by 30 to 40 per cent. Both downy brome and Japanese brome (Bromus japonicas) are classified as noxious weeds in Alberta.  
Japanese brome (Bromus japonicas) exists as a winter annual or summer annual grass weed in the Canadian Prairies.
New herbicide technology, carefully applied and coupled with managing modes of action, is Ontario’s best hope for winning the war against what is arguably the worst broadleaf weed in Canada today.
Key cropping strategies
There are a couple of key cropping strategies that can have a major impact on weed seed predators.

Avoid indiscriminate use of insecticides: "[It's] discouraged, as applications can also kill beneficial insects, as can some herbicides,” says Chris Willenborg, assistant professor in the department of plant science at the University of Saskatchewan. “Trying to avoid insecticide applications where they are not necessary or critical can help protect carabid populations.

Cover crops: very important for seed predators, which is often lacking in the fall across Western Canada. Cover crops become extremely important not just for building soil, but also for maintaining cover for seed predators because these insects are easily consumed by birds without adequate cover for concealment. Interseeding and relay-cropping practices help maintain that cover, as can perennial crops. In Europe and the U.S., field margins and buffer strips can be planted and maintained, which provide refuge for the carabid beetles to return to after dispersing to forage for weed seeds.”

Reducing tillage: This is something that has already been done well in Western Canada. Strip tillage or ridge tillage, another strategy that is becoming more common in the U.S., creates a ridge or strip between crop rows, leaving weed seed predator habitat in that space. Shallow tillage is also less disruptive than deep tillage, which mixes both predators and their larvae deep into the soil.

Mowing weeds: Doing this in the fall is another practice that could help improve habitat conditions for carabids. Mowing weeds or delaying tillage gives seed predators time to consume seeds before they are buried or their habitat is disturbed.
Largely overlooked and previously not studied a lot in Canada, weed seed predation provides the second-largest loss of weed seeds from the seed bank, second only to germination. Although research has been almost exclusively carried out in Europe and the United States, recent research at the University of Saskatchewan proves weed seed predation is occurring in western Canadian cropping systems and can be measured.
In wheat, in canola or in pea, the message is the same: control weeds early for highest yields. Those messages have been repeated in the past and now new research highlights the need to repeat that same message with respect to wheat crops.
Glyphosate-resistant (GR) waterhemp was first found in Ontario in 2014, but it already has a foothold in three counties in the southwest of the province. Fortunately, Peter Sikkema’s research group at the University of Guelph’s Ridgetown Campus has made a good start on finding effective options for controlling this challenging weed.
Originally from Asia, woolly cupgrass has been in the United States since about the 1950s and has caused problems in field crops across the corn belt. This annual grassy weed was first found in Canada in 2000, when it was discovered in Quebec. Since then, government agencies and producers have been working to prevent the weed from getting out of hand, and researchers have been learning about the weed and its management under Canadian conditions.
Another weed control tool bites the dust. A field in southwest Saskatchewan was confirmed to have Group 4-resistant kochia in the fall of 2015. The durum field had been sprayed with OcTTain herbicide (2,4-D and fluroxypyr; both Group 4 active ingredients) and it had little effect on the kochia population.
The key to controlling tufted vetch in soybeans is to try to maximize control in all crops in the rotation and in all kinds of windows. That’s the advice of Mike Cowbrough, weed management specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). He has been investigating options for tufted vetch control for about 14 years so he knows just how difficult this weed is to conquer.
Researchers writing in the latest issue of the journal Weed Science provide important insights on the control of herbicide-resistant giant ragweed - a plant shown to produce significant yield losses in Midwest corn and soybean crops. 

Since giant ragweed is resistant to multiple herbicide sites of action, researchers at the University of Minnesota set out to determine the impact of alternative control strategies on both the emergence of giant ragweed and the number of giant ragweed seeds in the weed seedbank. They evaluated six, three-year crop rotation systems, including continuous corn, soybean-corn-corn, corn-soybean-corn, soybean-wheat-corn, soybean-alfalfa-corn and alfalfa-alfalfa-corn. 

Researchers found that corn and soybean rotations were more conducive to giant ragweed emergence. Thirty-eight percent fewer giant ragweed plants emerged when the crop rotation system included wheat or alfalfa. 

They also found that adopting a zero-weed threshold can be a viable approach to depleting the weed seedbank, regardless of the crop rotation system used. When a zero-weed threshold was maintained, 96 percent of the giant ragweed seedbank was depleted within just two years. 

"Since the ragweed seedbank is short-lived, our research shows it is possible to manage fields infested with giant ragweed by simply eliminating weeds that emerge before they go to seed," says Jared Goplen, a member of the research team. 

Herbicide-resistant giant ragweed is rapidly becoming a major threat to corn and soybean production in the Midwest and elsewhere. This research will help growers utilize crop rotation as a much-needed additional strategy for managing this weed.
Weed control challenges are becoming even more difficult as the number of herbicide-resistant weeds in pulse crops continues to grow. With more than 60 unique cases of herbicide resistance identified in Canada and some weeds developing resistance to key pulse herbicides such as Pursuit (imazethapyr, Group 2) and Solo (imazamox, Group 2), the challenges will become even more daunting in the future.
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