WEED MANAGEMENT GOES HOLISTIC
BY JEANINE MOYER • PHOTO COURTESY OF BREANNE TIDEMANN
Farmers are being backed into a corner, said Breanne Tidemann. “They are losing the go-to weed management solutions they’ve relied on for years—herbicides—and need to consider new tools to keep their fields clean.”
With herbicide resistance on the rise, weed specialists such as Tidemann, a research scientist with Agriculture and Agri-Food Canada (AAFC), look for effective new management tools. This is where Integrated Weed Management (IWM) comes in. A holistic, sustainable approach to weed control, it combines multiple strategies.
“Understanding and applying IWM principles is essential for long-term weed control, minimizing resistance and maintaining crop productivity,” said Tidemann. A robust, multi-year approach is needed to address weed growth and the weed seed bank, she added.
IWM diversifies weed control methods that includes chemical, cultural, mechanical and biological practices. These range from crop rotation and cover crops to tillage and tank mixing herbicides with multiple modes of action. IWM isn’t new, but its methods became less common decades ago when herbicides became the easiest, most effective weed control tool. “Life was good with herbicides, until it wasn’t,” said Tidemann. The development of herbicide resistance through the 1990s and 2000s brought renewed use of IWM strategies.
The need to implement IWM is critical due to the increasing prevalence of herbicide-resistant weeds, said Tidemann. She sees new types of resistance and new weed biotypes annually. And yet, farmers still rely on herbicides as their main control method.
Tidemann is one of many Canadian weed researchers in pursuit of effective IWM methods. “There’s never been only one option for weed control, and the most effective strategy requires diversity and layering strategies that work for individual farmers,” she said.
NEW DIRECTIONS IN RESEARCH
According to Lewis Baarda, kochia poses the greatest threat to crop production on the Prairies. This threat intensifies significantly when the weed becomes established in marginal areas. He manages a three-year, on-farm research project to evaluate IWM strategies against kochia in low-lying and saline areas.
“Kochia is a smart weed, but it’s not a competitive one,” said Baarda, field tested manager with Farming Smarter. Once kochia patches are established on marginal land where crops aren’t present to choke it out, it’s hard to control due to its growing herbicide resistance to at least four chemical groups. “If kochia is established in marginal areas, like unproductive land, ditches and train tracks, it’s a problem for everyone.”
Research has identified effective IWM kochia management practices, but this is the first Canadian study conducted on non-productive land. The weed’s ability to thrive in drought conditions has added urgency to the project. “Thanks to recent dry weather patterns kochia is taking over everywhere. We need to find new ways to stop the spread,” said Baarda. “We can’t rely solely on herbicides to manage these weeds in low-lying saline areas, so we’re looking into alternative strategies.”
FIELD-TESTED SOLUTIONS
Four field locations have been set up within a 150-kilometre radius of Lethbridge to test the effectiveness of 10 IWM strategies on kochia patches in unproductive, low-lying and saline areas of commercial fields.
Starting in spring 2025, Baarda and his team began a three-year crop rotation of canola, lentils and spring wheat and will evaluate the impact of control mechanisms that include tillage, double seeding rates, chemfallow, mowing and cutting, direct seeding and competitive crops. The impacts of each strategy on kochia density, biomass and seed presence in the soil will be calculated. While yields will be assessed for controls that impact crops, Baarda noted the overall goal is to determine which approach best manages kochia plants and seed propagation.
“We are working at the farm level to find practical solutions that can be easily incorporated into any crop and weed management strategy,” said Baarda. He noted the multi-year approach will enable his team to evaluate the immediate and compounding effects of each method. For example, tillage will bury kochia to remove the weed while burying its seeds to reduce germination. Baarda will also monitor the long-term effects of each control method on the kochia seed bank.
He is confident his team’s research will deliver results. “This research will contribute to on-farm solutions for managing kochia. We’re looking to give farmers new options and tools outside of herbicides to maintain the effectiveness of the chemical tools we have, while identifying weed management options that fit their farms.
“Controlling established kochia today will reduce the risk next year. Kochia control is everyone’s responsibility, whether it’s on productive cropland or not.”
SMARTER WEED CONTROL
Tidemann has taken a fresh approach to management of herbicide-resistant weeds. She proposes the use of early maturing crops in rotation to muscle out weeds before they shed seeds and spread.
“There are no silver bullet solutions to managing herbicide-resistant wild oats,” she said. “These weeds adapt to just about anything we throw at them, so we challenge ourselves to think outside of the box and find new ways to layer in IWM control methods.”
In Manitoba, a recent survey indicated 100 per cent of wild oat samples were resistant to at least one herbicide mode of action. In Saskatchewan, the number is 77 per cent. What’s more, resistances to multiple herbicide groups, particularly Groups 1 and 2, is increasingly common.
Building on her earlier PhD research of harvest weed seed control (HWSC), Tidemann conducted a three-year research study (2016 to 2018) to evaluate the effectiveness of combining HWSC with earlier maturing crops to reduce wild oat populations.
“Harvest weed seed control isn’t about controlling weeds this season, it’s about doing what’s best for next season and future crops,” she said. By removing or destroying weed seeds at harvest using HWSC methods, and before they can become established in the weed seed bank, farmers can reduce populations in successive years.
Wild oats typically mature ahead of harvest. This enables the weed to spread seeds and populations to multiply. To challenge this pattern, she proposed to leverage early maturing crops to beat wild oats before they go to seed. Using HWSC principles, Tidemann set out to remove the weed seeds during harvest.
Previous research has shown, due to greater crop competitiveness, early maturing crops such as winter wheat reduce wild oat populations. Tidemann saw potential to further enhance control with rotational diversity and incorporation of HWSC as complementary strategies.
To test her theory, Tidemann conducted field trials in four locations across the Prairies. Three-year crop rotations with early, normal or late-maturing crops, including canola, faba beans, flax and peas as well as spring and winter wheat were implemented. Barley was then seeded across all rotations in the final year. Weed seeds were gathered, with chaff collection used as an HWSC method. While she evaluated the impact of crop rotation, she also studied the effects of swathing and straight cutting on weed seed numbers, crop biomass and the seed bank.
FRESH THINKING, NEW TACTICS
The study clearly proved a combination of IWM control methods is necessary to manage wild oats. Early maturing crops worked well in combination with HWSC. Wild oat seed bank numbers dropped significantly with early maturing crops. And, when HWSC was introduced,
the two methods delivered results. Tidemann recommends the two-punch combination to farmers battling herbicide-resistant wild oats. She also noted little difference between straight cutting and swathing, although the latter tended to mildly increase efficacy.
“Every farm requires a unique weed management strategy,” said Tidemann. Employing early maturing crops such as spring barley or winter wheat may not work for everyone, but her research may inform new approaches or inspire on-farm, out-of-the-box thinking. “Leveraging HWSC, like weed seed collection or impact mills is another perspective for combating problem weeds that works, too. It isn’t a static control measure—it can be adopted to fit on individual farms.”
PIGWEED RISK
Shaun Sharpe is determined to limit the march of two scary weeds into Western Canada. “The greatest potential threat to Prairie farmers is Palmer amaranth and waterhemp,” said Sharpe, an AAFC research scientist. “Both weeds are wreaking havoc across the U.S., and Eastern Canada has dealt with waterhemp for years. What’s most alarming is they are very prolific and resistant to multiple herbicide groups.”
Amaranths, also known as pigweed species, are common in Canada. Of 14 established species, nine are herbicide resistant. Waterhemp has invaded Manitoba and is expected to creep across the Prairies. Most waterhemp in Canada is resistant to at least one herbicide mode of action, with multiple resistances common. Palmer amaranth remains a looming threat, with a few plants identified in Ontario and Manitoba. One of the most competitive amaranth species, it is resistant to multiple herbicide modes of action.
Sharpe worries about the potential economic impact of these weeds and limited control options. In response, he now leads a four-year research project that aims to reduce the risk posed by the two weeds and help farmers manage them if they do take root.
Both species are known to dramatically increase yield loss and input cost as well as reduce harvest efficiency and grain quality. In Ontario alone, herbicide-resistant waterhemp has caused an average yield loss of 19 per cent in corn and 43 per cent in soybeans. On farms with extremely high waterhemp pressure, yield loss can be up to 99 per cent in corn and 93 per cent in soybeans. A 2001 study of a Kansas corn field reported a yield loss of 91 per cent due to Palmer amaranth.
BIOVIGILANCE IN ACTION
Sharpe predicts Palmer amaranth will reach Saskatchewan between 2030 and 2035 if it hasn’t yet done so unnoticed. Waterhemp will likely spread at a similar pace. Both weed species can produce up to one million seeds per plant and spread easily via contaminated feed mixes, irrigation water and dirty equipment. They are also transported naturally by wind and wildlife.
Sharpe and his team employ biovigilance. This multi-pronged, holistic approach includes monitoring the spread of pigweed species in Western Canada, raising awareness about their impact and development of new tools to help the industry identify and manage these weeds. The work is supported by the analysis of existing research and the experience of other geographic regions, tracking herbicide resistance trends and identifying effective management strategies.
The project kicked off in April 2025 with the development of a weed identification process. Weed images collected and studied by the project team will support the creation of a plant ID app. Farmers and agronomists will use it to identify the weeds, but Sharpe wants to take this one step further by making genetic tissue sample testing available. He now oversees the development of a system to handle samples.
These activities will support development of a predictive niche mapping tool expected to identify areas at risk for weed invasion. “If we know where the weeds are headed, we can intervene by supporting agronomists, extension specialists and farmers in those areas,” said Sharpe. “We want to be ready to help them identify the weeds, manage their risk and deal with the weeds if necessary.”
Field trials were also carried out this year in Saskatoon, SK. Using grain amaranth as a pseudo weed, Sharpe evaluated the effectiveness of cultural control strategies across multiple crops.
Industry awareness is the overarching component of the project. An associated extension campaign began with tours of the grain amaranth demo plot at Ag in Motion, this July in Langham, SK.
“These weeds are a serious threat, and, if established, it would be like battling kochia all over again,” said Sharpe. Every acre is at risk of amaranth invasion, he added. “Everyone needs to be on the lookout, because they can spread so easily.”
Farmers should be on the lookout for these weeds and prepare to engage all necessary weed control measures.
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