ONTARIO GRAIN FARMER 18 AGRONOMY Jeanine Moyer Global lessons in herbicide resistance Canadian farmers can learn from international successes and setbacks to protect the tools that still work If herbicide resistance offers one clear lesson, it is that weeds do not respect borders. Around the world, growers are facing the same challenge: herbicides that once provided reliable control are losing their effectiveness as resistant weed populations spread. For Canadian farmers looking ahead, Australia may offer the clearest glimpse of what is to come on the herbicide resistance front. After decades of battling herbicide-resistant annual ryegrass, Australian growers have been forced to adapt and develop new weed management strategies as herbicide tools have become less effective. According to Michael Flessner, professor and weed science specialist at Virginia Tech, the rest of the world is paying close attention. “Australia is being watched globally as other countries look to learn from its weed control strategies and solutions, and determine whether they can be adopted elsewhere to manage and mitigate herbicide resistance,” he says. The experiences of growers around the globe offer both a warning and Experiences in Australia, the U.S., and South America show that growers need to act early by using integrated weed management strategies — including chemical, cultural, mechanical, and emerging technologies — to preserve effective tools and prevent resistance from becoming entrenched. a roadmap. As herbicide resistance continues to evolve, Canadian farmers have an opportunity to learn from the successes, setbacks, and innovations emerging in other countries, and apply those lessons to their own fields. WEED MANAGEMENT CHALLENGES “Most farmers know herbicide resistance is a threat, but because it can be regional and even field-specific, it is easy to believe it is someone else’s problem,” says Flessner. “The challenge is that relying on herbicides alone often works — until it doesn’t. By the time resistance becomes obvious, the opportunity to prevent it has already passed.” The top five herbicide-resistant weeds Flessner is watching in the U.S. are also familiar to Canadian growers: horseweed, Italian ryegrass, which is similar to the problematic annual ryegrass in Australia, kochia, Palmer amaranth, and waterhemp. While most growers recognize herbicide resistance as a growing concern, their responses vary widely. At Virginia Tech, Flessner and his team are focused on helping farmers tackle weed management challenges through integrated strategies that maximize herbicide performance today while preserving their effectiveness for the future. Palmer amaranth offers a clear example of what is possible. Faced with a highly prolific weed that has evolved resistance to multiple herbicide groups, many growers are finding success by diversifying their approach — mixing and rotating chemical modes of action, combining them with crop rotations, and adding cultural and mechanical control measures. It is a strategy aimed not only at controlling existing weeds, but also at preserving herbicide effectiveness for the future. “We’ve made significant progress in understanding resistant weeds over the past decade, but there’s still a perception among growers that we will always have herbicide options to solve the problem,” says Flessner. “Meanwhile, resistance continues to spread. More weed species are becoming resistant, and new cases are being identified every year. Resistance isn’t going away, so our focus has to shift from reacting to the problem to managing it for the long term.” Flessner points to the increased use of pre-emergence herbicides as a positive example of how growers are adapting
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