ONTARIO GRAIN FARMER AGRONOMY Viatude™ fungicide with Onmira™ active delivers best-in-class defense against white mould. With multiple modes of action, you can enjoy premium protection that keeps your soybeans healthy all season long for higher yield potential. Engineered excellence in every jug. Innovation unleashed. Visit us at Viatude.corteva.ca ™® Trademarks of Corteva Agriscience and its affiliated companies. © 2025 Corteva won’t be enough for activation.” It’s therefore important not only to look at the local weather forecast but also to get a reading on your soil moisture levels (and in all areas of your field if there is in-field variation in soil type). At the same time, excessive rain after application of soil-applied herbicides can rapidly leach a portion of your product beneath the weed seed germination zone, Sikkema notes, also reducing effectiveness. With post-emergence products, weather that’s dry and also too hot will also cause problems. Sikkema points to research showing that glyphosate absorption by common milkweed plants is 15 per cent lower when temperatures are up. With other products, effectiveness in these conditions can be lower by 35 per cent or more. Soil characteristics also matter. “Most soilapplied herbicides are adsorbed to clean organic matter,” Sikkema explains. “Always remember that if a herbicide is bound to the soil, it provides zero weed control, and there are more binding sites on a heavy clay, textured soil high in organic matter. So, the herbicide rate should be adjusted depending on your soil type and organic matter level.” WEEDS—AGE, TYPE, AND DENSITY The waxy cuticle on weed leaves varies in composition and thickness among weed species, but of course, younger plants usually have thinner and less-developed cuticles more permeable to herbicide. “Always remember young or small weeds are easier to kill,” Sikkema says. “If you're going to make a mistake as a grower, always make the mistake of spraying too early rather than too late. If you apply too late, you're going to get poor weed control with almost every post-emergence herbicide.” Herbicide effectiveness is also dependent on species. Sikkema’s research with colleagues has shown that five times as much Roundup is required to kill a common ragweed plant than a green foxtail plant (but note that these study plants were large). “So, you need to know what weed species you have in each field,” he says, “and adjust your rate depending on species composition.” Looking at weed plant numbers, Sikkema points to a study where a herbicide applied at the same rate had a control decrease of about 20 per cent in higher densities. “It is a fact with every single herbicide, whether it's a soil-applied herbicide or a post-emergence herbicide, herbicides always work better in low-density environments,” he says. “You should do everything you can to reduce weed seed return to the soil.” Read more practical tips for spray application at www.ontariograinfarmer.ca.
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