There are a number of reasons why herbicides sometimes fail to control weeds.

Perhaps surprisingly, one of the easier things to rule out is whether the weed is herbicide resistant. Ontario Ministry of Agriculture, Food and Rural Affairs weed specialist Mike Cowbrough says many universities across Canada conduct testing for herbicide resistance, but growers have to ensure they collect a proper sample.

Over the past 20 years, Cowbrough notes that about 20 percent of the weed seed samples submitted to the University of Guelph could not be processed for testing. That’s mainly because the sample contained immature seed, there was no seed in the sample, or there were too few plants.

In this episode of RealAgriculture’s Resistance Management School, Cowbrough provides tips on how to best collect samples of common weeds and grasses found in Ontario, including common ragweed, common lamb’s-quarters, pigweed, and foxtail. He notes that the best time to sample these species for testing is mid to late September

When gathering weed samples, he advises growers to get as much seed as possible of the suspected resistant weed and be sure to grab a representative sample from across the field.

Click here for more Resistance Management School episodes.

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