Resistant Waterhemp, Volunteer Corn, and a Four-Hour Rain Window
A soybean field has heavy escapes of waterhemp that survived two postemergence glyphosate applications; the population is confirmed glyphosate- and ALS-resistant. Volunteer corn from last year's crop is scattered throughout. The grower wants to make one trip: glyphosate at 1.1 lb acid equivalent per acre, plus clethodim at 0.1 lb active ingredient per acre, plus fomesafen at 0.25 lb active ingredient per acre, with a nonionic surfactant, mixed in water testing 420 ppm hardness at pH 8.2. Soybeans are at R1, waterhemp is 8–12 inches tall, volunteer corn is about 10 inches, the air temperature is 92°F, and rain is forecast in four hours. The grower plans to sow spinach in that field next spring.
1. Evaluate the tank mix as proposed. What problems will it create, and how would you redesign the program?
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Problems with the one-trip plan:
- Glyphosate plus clethodim is antagonistic. The glyphosate formulation reduces clethodim uptake and activity on grasses, so volunteer corn control will be erratic. Clethodim needs to go out alone or be separated in time; where a tank mix is unavoidable, use the higher clethodim rate and the adjuvant the label specifies for grass control.
- Wrong adjuvant for the grass partner. A nonionic surfactant is the glyphosate partner; clethodim labels generally call for a crop oil concentrate. One surfactant choice cannot serve both.
- Water quality. At 420 ppm hardness, glyphosate needs ammonium sulfate; at pH 8.2, clethodim is in the range where it degrades faster in the tank, so spray the same day and avoid leaving the mix standing.
- Glyphosate contributes nothing against the waterhemp - it is confirmed resistant. Relying on it wastes an active ingredient and selects harder.
- Size and timing. Waterhemp at 8–12 inches at R1 is at the edge of controllable; a PPO (fomesafen) alone on a large, possibly PPO-shifted population is high risk.
- Rain window. Four hours at 92°F with a multi-product mix means at least one partner will be washed off before it is rainfast.
Redesign: apply clethodim alone in the early morning with the labeled oil concentrate and no rain in the forecast window; apply the broadleaf program separately with a different effective site of action plus a residual Group 15 for the next flush; rogue or mow escapes before seed set; and plan a fall/spring program that does not depend on glyphosate or ALS chemistry in this field.
2. The grower intends to plant spinach in this field next spring. What should you tell him about this tank mix, and what would you verify before agreeing?
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Fomesafen is the constraint. It is a long-residual PPO herbicide, and leafy vegetables such as spinach are among the most sensitive rotational crops. Two factors make this field worse than average:
- Soil pH 8.2 is high, and fomesafen carryover injury to rotational crops increases as pH rises; the interval on the label is written for a defined pH range, and this field sits above the typical window.
- The label rotation interval for spinach-type crops is long - commonly a year or more, and longer where pH is elevated.
What to verify before agreeing:
- The current label's rotational crop table for spinach and for any other crop the grower plans in the next 18 months - read the actual label, not a summary.
- Soil pH and organic matter from a current soil test, and whether the field has a history of PPO use.
- Whether the rate can be cut, the product swapped for a residual with a friendlier rotation, or the field split so only the part not going to spinach receives fomesafen.
Recommendation: if spinach is a firm plan for next spring, do not use fomesafen on that field at all. A carryover injury in a high-value vegetable crop costs far more than a different residual choice this season. Put the decision and the rotation interval in writing to the grower.
3. Would you expect crop injury from this application at 92°F, and how should that factor into the decision?
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Yes - expect transient soybean leaf burn, and plan around it. Fomesafen plus a surfactant at 92°F commonly produces speckling, crinkling, or bronzing of soybean leaves within a few days. The judgment question is whether the injury is cosmetic or yield-limiting.
- Cosmetic injury does not justify abandoning a necessary treatment, but it does justify warning the grower in advance so a scout is not called out for a problem the applicator created and did not predict.
- Real risk rises when products stack. Adding a grass herbicide and a high-load surfactant in hot, humid, slow-drying conditions increases the chance of more than leaf speckling.
- Spray timing matters. Early morning or evening applications at 92°F still reduce the chance of surfactant-related burn compared with midday, and the clethodim partner is better off in cooler hours anyway.
- Rainfastness. In a four-hour window, at least one product will not be rainfast; splitting the trip also removes that gamble.
Decision rule: accept predictable, label-acknowledged cosmetic injury when the weed pressure justifies it; do not accept a mix assembled to save a trip when the same trip creates antagonism, off-target risk, and an unpredictable injury complex.