Herbicide resistance isn’t something you want to discover at harvest. Catching it early—before it spreads—is key to staying ahead of problem weeds and preserving herbicide options for seasons to come. Here are some practical tips to help you scout effectively and confirm resistance before it becomes more significant.
Scout Early and Often
One of the best times to spot potential resistance is shortly after your pre-seed or early post-emergent herbicide applications. Identify any weeds that have survived, especially if one species stands out while others are controlled. Kochia, wild oats and cleavers are some common culprits in western Canada that often develop resistance. Pay special attention to areas where you’ve had weed escapes before—field edges, fence lines and patches where weed pressure tends to be highest. Make notes of what you’re seeing, as well as specific locations and severity.
Double-Check Before You Conclude Resistance
Before deciding that resistance is the cause of a weed escape, there are several other factors to rule out:
Sprayer calibration:
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Was your equipment properly calibrated? A nozzle blockage or spray pattern issue could leave untreated areas.
Application rate:
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Did you apply at full-label rates?
Weather conditions:
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Was there rainfall before the herbicide became rainfast? Were wind or temperature extremes a factor?
Weed staging and size:
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Were the weeds too large or too mature at the time of spraying for effective control?
Coverage:
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Did dense crop residue or poor water volume affect coverage?
Water quality:
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Hard water or high pH can reduce herbicide efficacy.
Tank-mixes:
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Were tank-mixed products compatible? Did you use multiple modes of action?
Rule out these possibilities first. Missteps in any of these areas can mimic resistance symptoms.
What To Do If Resistance Is Suspected
You must quickly respond if you’ve ruled out application errors and conditions and still see survival. Do everything possible to prevent those surviving weeds from setting seed—that’s how resistance spreads and escalates. Consider mechanical control options, targeted spot sprays, or mowing to manage problem patches.
Additionally, rapid testing for herbicide resistance is becoming more accessible. New tests, like those developed for kochia, can help growers identify resistant populations in season. Early confirmation allows you to change your weed control plan, adjust herbicide strategies, and start planning longer-term management solutions like herbicide layering or crop rotation.
Herbicide resistance is manageable, but only if caught early and handled proactively. Sharp scouting skills and quick action can make the difference between isolated patches and a field-wide problem.