Isoproturon has stopped working on Phalaris minor
Mahesh Yadav has 2 ha of wheat (HD-2967) sown on 18 November after rice. On 20 December, at 32 days after sowing, the crop is tillering and Phalaris minor is dense across about 60% of the field, with a little Chenopodium on the bunds. He applied isoproturon 75 WP at 1.2 kg/ha on 17 December in 350 L of water with a knapsack sprayer and irrigated on 20 December. He has used isoproturon every wheat season for six years at 1.0–1.5 kg/ha. Two neighbours have shifted to clodinafop because their isoproturon also stopped working. He wants to re-spray isoproturon at 2.5 kg/ha now, or, as another neighbour suggested, add 2,4-D. He plans to grow greengram after the wheat harvest in April.
1. Explain why the isoproturon application failed and how you would confirm the cause without a laboratory.
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Isoproturon is a photosystem-II (urea) inhibitor, taken up mainly through the roots and used pre-emergence to early post-emergence, roughly up to 30–35 days after sowing. It needs soil moisture and actively growing, small weeds. Six consecutive seasons at 1.0–1.5 kg/ha in the same field, with the same experience in two neighbouring fields, points first to selection of resistant Phalaris minor - a well-documented history across the rice–wheat belt.
Other contributors to check before blaming resistance alone:
- Weed stage. Activity falls steeply if Phalaris minor was beyond the two-to-three leaf stage on 17 December.
- Soil moisture at application. Applied on 17 December and irrigated on 20 December: if the surface was dry and crusted, root uptake in the first days was poor.
- Application quality. He used 350 L/ha through a knapsack - check calibration, overlaps and misses. But 60% of the field being infested argues against simple misses.
- Dose and product. 1.2 kg/ha of a 75 WP is within label range, so under-dosing is not the explanation.
Confirming the cause without a lab.
- Strip test: spray a 5 × 5 m strip with a different mode of action (for example sulfosulfuron) and compare survival three weeks later. If the alternative works, resistance to isoproturon is likely - suggestive, not proof.
- Seed bioassay: collect mature Phalaris minor seed from the worst patch and send it to an SAU or ICAR station for a discriminatory-dose test. This is the only real confirmation, and it costs almost nothing next to another failed spray.
Do not re-spray isoproturon at 2.5 kg/ha. Doubling the dose of a failing herbicide accelerates resistance, risks crop injury and increases carryover.
2. Decide the action for this field now, at 32–35 days after sowing, evaluating the isoproturon re-spray and the 2,4-D suggestion.
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Reject both of his proposals.
- Isoproturon at 2.5 kg/ha: doubling the dose of a herbicide that has already failed selects harder biotypes, adds carryover risk and buys nothing.
- 2,4-D alone: it is a broadleaf herbicide. It will not touch Phalaris minor and will only suppress the odd Chenopodium; applied late it also carries drift risk to neighbouring mustard and lentil. It is not an answer to a grass problem.
Choose a different mode of action for the grass weed. Because his history is isoproturon (PS-II) only, the rotation logic favours an ALS inhibitor: sulfosulfuron 75 WG at about 25 g/ha with a surfactant, in 500 L/ha, on moist soil, while Phalaris minor is still at two to four leaves. Alternatives are clodinafop-propargyl 15 WP (ACCase), pinoxaden, or pyroxsulam-based mixtures - but ACCase-resistant Phalaris minor is widespread where clodinafop has been used repeatedly, so ask what the neighbours' clodinafop result actually was before choosing it.
Be honest about timing. If Phalaris minor has already tillered and the wheat is past about 40 days, herbicide efficacy will be poor whatever is chosen. In that case the correct advice is mechanical: hoe or hand-pull the worst patches, accept reduced control this season, and above all stop the seed return - the seeds produced this year are next year's problem.
Broadleaf partner. Add a broadleaf herbicide only if broadleaf weeds are above threshold, and choose one whose label rotation interval suits greengram.
Record species, stage, product, dose, water volume and result - this record is the basis of next season's decision.
3. Design next season's programme and judge the carryover risk to the greengram crop planned for April.
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Rotate modes of action across seasons, not just within one. A workable sequence: a pre-emergence dinitroaniline such as pendimethalin where the seedbed is fine and moisture is available, followed by a post-emergence grass herbicide from a family not used the previous year. Never run the same family two seasons in a row.
Break the monoculture where possible. Rice–wheat every year with the same grass weed is the engine of resistance. A single season of berseem, potato or mustard sharply reduces the Phalaris minor seed bank. Zero-till with residue retention can reduce early Phalaris minor emergence, but watch for a shift towards other weeds.
Carryover to greengram. Wheat herbicides go on in December–January and greengram follows in April, so the interval is roughly three to five months - enough for short-residue products, but not for all sulfonylureas.
- Clodinafop has short soil persistence and is the safer choice for a following pulse provided ACCase resistance is not suspected in his field.
- Sulfosulfuron and especially metsulfuron-methyl have longer soil activity and label rotation restrictions; some can injure greengram. Read the label rotation statement for the exact product rather than assuming, and avoid metsulfuron if greengram follows.
Practical decision rule. If ACCase resistance is not suspected, use clodinafop this season for the shorter residue; if it is suspected, use sulfosulfuron and check the label's approved interval to a pulse crop before sowing greengram.
Records. Keep a simple weed/herbicide log per field. Six years of undocumented isoproturon is exactly why this decision is difficult; the next farmer should not repeat that.