34 Follow us on Twitter @BetterFarmingON Better Farming | May 2024 crops: the lynch fileS CAUSES OF INCREASE IN RESISTANT WEEDS We can address the resistance problem. By Patrick Lynch Editor’s Note: This is a timely repeat of a popular column Patrick wrote five years ago, looking at issues that Ontario producers continue to encounter. My wife Sandra and I spend an extensive amount of time driving across the province. One day, we travelled in an area with many soybean fields. On one side of the road was a weed-free field. On the other side, the field had a lot of fleabane. “What are those weeds?” she asked. “I’d guess glyphosate-resistant fleabane,” I responded. “Why,” she asked, “does one farm have so many and the other none?” “Well, because the weeds are resistant,” I said. “So, why are they resistant?” she asked. Everywhere you go, people are talking about resistant weeds. In the 1950s, we first had 2,4-D-resistant wild carrot. Next, triazine-resistant lamb’s quarters appeared in the 1970s. The following decade, we discovered Group 2-resistant pigweed, lamb’s quarters and nightshade in soybeans. The latter weed was the worst. In the last 15 years, we have found the following weeds to be resistant to glyphosate: Fleabane, ragweed and Palmer amaranth. Again, all these herbicide-resistant weeds first showed up first in soybeans. Undoubtedly, we have more resistant weed species now and the rate at which they develop is increasing. So, how come we have so many resistant weeds and what can we do? In a nutshell, they developed because of continuous use of a single mode of action herbicide. Weeds began developing glyphosate resistance when we entered the Roundup Ready era of crop production. To offset the cost of this new technology, some producers opted to cut back on herbicide costs and applied only glyphosate, rather than following best-practices and using an herbicide program with multiple modes of action. We will see more fields with glyphosate-resistant weeds this year, I believe. It is just a matter of time, I think, until we get more glyphosate-resistant nightshade. Then we will be in for a challenge. Many factors contribute to the spread of glyphosate-resistant weeds. First, over half of Ontario’s total combined corn, beans and wheat acres are planted into soybeans. We spray most of these soybean fields with glyphosate – whether in-crop, pre-plant or post-harvest. This situation serves as a recipe for the development of glyphosate-resistant weeds. We can address the resistance problem. We must plan to control glyphosate-resistant fleabane on every soybean acre. A good strategy is the use of a dicamba-resistant soybean variety, as this herbicide is really the only product that we can apply post-emerge to effectively control glyphosate-resistant fleabane in Ontario. Other programs provide about 95 per cent control of this weed. In addition to glyphosate, these programs include: herbicides that contain metribuzin (the active ingredient in Sencor) plus Eragon; other metribuzin-containing products, such as Boundary and Canopy Pro, used in conjunction with metribuzin and Eragon; BlackHawk plus Bifecta; Elevore is an alternative to Eragon. We must also scout our fields meticulously. If we see some weeds escaping a glyphosate application, get rid of them. Watch your field borders for encroaching resistant We must make weedy fields socially unacceptable. If we see some weeds escaping a glyphosate application, get rid of them. Patrick Lynch photos
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