ONTARIO GRAIN FARMER 25 AGRONOMY Herbicide resistance is inevitable. It’s only a matter of time before the genetic makeup of weeds adjusts to repeated chemical use and production practices. “Some production systems are more at risk than others, like those that grow continuous soybeans and use the same mode of action each year,” says Mike Cowbrough, weed specialist for field crops with the Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA). “No matter what best management practice you choose, resistance will only be delayed. And you can do everything right to prevent resistance, but all it takes is resistant weed seeds moving in from equipment or wildlife to cause a management challenge.” The reality is challenging, but Cowbrough says it’s not all doom and gloom. He points out the industry has better tools to identify weed species than it did 20 years ago, and more growers are adopting practices to manage and delay resistance. There are also multiple benefits to managing resistance. “The practices that make good sense for weed control also make good economic sense for the farm business,” he says, pointing to strategies like adequate soil fertility, which speeds crop canopy closure and helps restrict weed growth while contributing to yield. He cites residual weed control as another example, noting that while additional herbicides and applications can add costs, the reduction in weeds during the current and subsequent growing seasons generally results in good economic returns. Cowbrough explains that the key to managing weeds, resistant or not, is to establish the most competitive crop possible and create conditions where weeds are less likely to germinate. He points to row spacing as another example where the right approach not only makes agronomic and financial sense for growers, but also contributes directly to weed management. He references key management strategies for reducing waterhemp seed in the soil from research conducted at the University of Guelph Ridgetown Campus. The study examined integrated weed management strategies targeting herbicideresistant waterhemp in the soil seedbank. “Takeaways from the study show that row spacing, crop rotation, and effective residual herbicides are key management strategies for reducing waterhemp seeds in the soil seedbank, and that translates to more control in-season and for future crops,” Cowbrough says. WESTERN PRESSURES BUILD Widespread kochia infestations across the Prairies are also proving Cowbrough’s point. No matter what growers do to prevent this prolific weed, herbicide-resistant kochia is appearing in fields, ditches, and marginal growing areas everywhere. “Chemicals are still the backbone of how we manage weeds in crop production systems. What we need to do is preserve their effectiveness by introducing cultural practices and strategies that complement chemicals to ease the pressure and reliance. - Shaun Sharpe, AAFC weed scientist “Kochia is quickly eclipsing wild oats as the number one problem weed in Western Canada,” says Shaun Sharpe, a research scientist with Agriculture and Agri-Food Canada in Saskatoon. “It develops resistance quickly, spreads rapidly and in abundance, and seems to thrive in the dry, drought-like conditions we’ve had over the past few years.” Sharpe shares Cowbrough’s view on the inevitability of weed resistance. He’s especially concerned about the eventual arrival of Palmer amaranth, a member of the pigweed species that has shown resistance to multiple herbicide modes of action in the United States, including Groups 2, 3, 4, 5, 9, 10, 14, 15 and 27. “Unfortunately, it’s just a matter of time before Palmer amaranth becomes a problem in Canada, after all, it moves effectively through wildlife, and it’s hard to control waterfowl who are known to spread the weed,” says Sharpe. While kochia tops the list of herbicide-resistant weeds in Western Canada, wild oats and cleavers aren’t far behind. “Wild oats are actually the number one problem weed in all three Prairie provinces,” notes Sharpe, explaining that the weed seeds can last in the soil for up to 10 years and have shown resistance to Groups 1 and 2. There is some good news for Prairie growers: kochia seeds only persist in the soil seedbank for two to three years, making intensive weed management in problem areas an effective option for longer-term control. continued on page 26 Waterhemp
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