Kochia Survivors After Six Years of Glyphosate
A dryland wheat grower in southern Idaho has used glyphosate at 32 fl oz/acre as a fallow burndown for six consecutive years. This season, kochia survives in irregular patches covering about 25 acres of a 320-acre field. The plants yellow slightly at 10 days, then resume growth and set seed. The grower applied at 4 mph with 15 GPA using a stock pond with visible silt as the water source. A neighboring field sprayed by a custom applicator at the same rate shows almost complete kochia control. The grower wants to double the glyphosate rate next spring and add a second application.
1. Before calling this herbicide resistance, what would you check? Explain how you would confirm or rule out resistance.
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Rule out application and environmental causes first
- Water quality: silt and turbidity can tie up glyphosate; the stock pond is a red flag. A clean water source should be compared.
- Weed size and timing: kochia treated larger than the label's recommended stage often survives and regrows.
- Coverage: dust on leaves, low volume, worn nozzles, or a boom height problem can produce sublethal doses.
- Adjuvant and rate: confirm the labeled surfactant and that the rate was actually delivered (calibration).
- Sprayer contamination or mixing errors: check the previous tank and the order of addition.
The neighbor's clean field with the same rate points toward the grower's water, coverage, or timing rather than a field-wide resistance problem.
Confirmation steps
- Leave an untreated strip and a 1x strip in the patch to compare regrowth.
- Collect seed from survivors and from the controlled area; a greenhouse dose-response (1x, 2x, 4x) is the standard check.
- Map patch boundaries over two seasons. Resistance spreads from seed and pollen movement, so expanding patches are suspicious.
- Verify the sprayer and water source by spraying a small clean-water test strip.
2. Evaluate the plan to double the rate and add a second application.
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Doubling the rate is a weak first response.
- Label limits: the maximum single rate and seasonal total are set by the label; doubling may be illegal.
- Selection pressure: higher rates on a partially resistant population accelerate selection for stronger resistance.
- It may not fix the real problem: if silt, timing, or coverage caused the escapes, a higher rate is wasted money and adds carryover risk.
- A second application may be allowed only under specific crop-stage and interval restrictions; check the label before planning it.
- Resistance is a population issue: the goal is to prevent seed set and diversify selection, not to overpower survivors.
A better approach is to correct the application variables and then test whether the survivors are actually resistant before changing rates.
3. Outline a two-year management plan for the 25-acre patch.
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This season
- Prevent seed set. Hand-pull, mow, or spot-treat small patches before kochia seeds mature. One plant can produce thousands of seeds, and plants tumble and spread seed.
- Do not let the patch produce seed and then harvest through it.
Next season
- Rotate to a different site of action. Use a preemergence product with residual activity plus a tank-mix partner that is effective on kochia.
- Use multiple effective sites of action, not just glyphosate, and rotate them across years.
- Consider crop rotation to a competitive crop or a grass/alfalfa stand to break the cycle.
- Use tillage selectively where it fits erosion control and the conservation plan.
- Clean equipment before leaving the patch, and manage field borders and ditches where kochia spreads.
Monitoring
- Scout before and after each application, map survivors, and keep a record of what worked. If survivors persist under correct application, treat them as resistant and adjust the program.