Soybean School: Mind your fencerows to reduce weed resistance


Are we doing enough to reduce the herbicide-resistance weed seedbank on Ontario farms?

That’s a question AGRIS Co-operative agronomist Dale Cowan wants farmers in the province to consider when they’re planning their weed control programs for this year. Cowan recently attended the Commodity Classic in Anaheim, California, where he talked with farmers from Mississippi who maintain full-time sprayers to spray fencerows and waste areas to help control palmer amaranth.

The conversations started Cowan thinking about how Ontario farmers are managing herbicide-resistant weeds, such as Canada fleabane. In this edition of RealAgriculture’s Soybean School, Cowan notes that farmers are working hard to handle the weeds in their soybean fields, but he feels we really have to be more diligent in managing areas beyond the field.

Cowan notes that a fenceline 1,000 feet long and three feet wide with a fleabane plant every square foot can produce enough seed to populate a 50-acre field at 275 plants per square foot.

“Developing resistant weeds really is a numbers game,” says Cowan who uses the following example to illustrate how a growing weed seed bank can lead to the proliferation of resistant weeds. “If you only allow 30 weed seeds per acre – and every billion seeds is a resistant biotype – you have to look at 10 million acres to find your first resistant biotype. But if you get to 3.2 million seeds per acre you only have to look at 300 acres to find your first resistant biotype, which is the case with Canada fleabane.”

Reducing weed seeds also decreases the selection pressure and protects existing chemistry farmers use to control resistant weeds. “If we want residuals and different modes of action to maintain their effectiveness, we have to reduce the amount of weed seeds they face,” says Cowan.

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