Glyphosate Escapes in Continuous Sorghum
Cody manages 240 acres of continuous grain sorghum in central Kansas. For six of the last seven seasons the only burndown and in-season program has been glyphosate, and an ALS-inhibiting herbicide went out in four of those years. In late August he finds Palmer amaranth escapes clustered along the headlands and in two planter skip rows. About 40% of the plants in those clusters survived a full labeled glyphosate rate applied 10 days earlier; several are already 5 feet tall and beginning to set seed. His budget allows one pre-emergence and one post-emergence pass next season. His retailer's advice is to raise the glyphosate rate by 50% and add a surfactant. Cody asks whether that will solve the problem.
1. What is the most likely explanation for the escapes, and what would you check or test before committing to that explanation?
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Most likely explanation: selection for a glyphosate-resistant biotype in those patches. The pattern fits: multiple years of a single site of action, survivors that received a labeled rate, and clustering where seed moved (headlands, planter skips, harvest equipment).
But do not stop there - rule out the cheap causes first:
- Actual carrier volume and product rate delivered vs. intended (check the sprayer log, not the intention).
- Nozzle condition and spray pattern overlap.
- Adjuvant omitted, wrong water volume, or antagonism from hard/high-pH carrier.
- Weed size at application - 5-foot plants are past the labeled size, so survival on those plants proves nothing about resistance.
- Dust or drought stress reducing uptake.
Confirmation: a discriminating-dose or dose-response screen on seed collected from survivors before they shatter, plus leaving an untreated check strip next season. Resistance is field- and patch-specific; a positive test in one headland does not justify writing off the whole 240 acres.
Now, before seed shed: mow or hand-rogue those clusters. Every plant that drops seed multiplies next year's problem.
2. Design next season's program within the one pre-emergence + one post-emergence budget, and explain the logic of each choice.
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Pre-emergence: a full rate of a residual herbicide from a site of action not used in this field recently (a Group 15 chloroacetamide or a Group 14 PPO inhibitor), applied to a clean, weed-free seedbed. Residuals are the highest-value dollar here because they remove the early-season cohort before it can be selected.
Post-emergence: apply at the labeled weed height (2–4 inches, not 5 feet) with a different effective site of action than glyphosate, and include a residual partner so the pass does more than burn down what is already up.
Non-chemical pieces that cost little:
- Start clean - a burndown or tillage pass so the post product is not fighting established weeds.
- Clean the combine and any tillage equipment leaving the infested fields.
- Rotate to a different crop one year if possible; a sorghum-corn-soybean rotation changes the selection pressure more than any single herbicide swap.
- Keep a non-treated strip to measure whether the program is actually working.
- Rogue survivors before they set seed; two seasons of preventing seed return buys more than any rate increase.
3. The retailer proposes a 50% rate increase plus surfactant. Evaluate that recommendation.
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It is the wrong fix on three counts.
- Legality: the label rate is a maximum. Exceeding it is a misuse regardless of intent, and no state exemption covers "the weeds got tough."
- Biology: resistance is not a dose problem. A rate bump at best kills the weakly resistant individuals at the margin while selecting hard for the strongly resistant ones - the same mechanism that got the field here. On plants past labeled size it will also fail for ordinary uptake reasons and teach Cody the wrong lesson.
- Economics: the extra product buys no additional control against a resistant biotype and adds carryover and crop-injury risk.
What the surfactant question reveals: if the product applied 10 days ago was mixed without a labeled adjuvant, that is worth checking - but it explains a poor job, not a patchy one with 40% survival next to dead plants of the same size. Fix the application question separately from the resistance question.