Direct-Seeded Rice: Herbicide Injury and Escaped Weeds
Prakash sowed 1.2 ha of direct-seeded rice (DSR) on 20 June using a seed drill at 25 kg/ha. At 10 days after sowing (DAS), when the rice had two leaves, he sprayed bispyribac-sodium at 30 g/ha in 250 L water/ha in the afternoon at 34°C, without a surfactant. At 18 DAS, rice in spray-overlap strips is pale yellow, stunted and tillering poorly. The field also has patches of Echinochloa colona, Cyperus iria and Ludwigia parviflora; the sedges and Ludwigia were not controlled. The field is intermittently flooded because the canal runs every 10 days. He plans to re-spray bispyribac-sodium at 40 g/ha immediately and add 2,4-D to control the broadleaf weeds. The previous DSR crop also received bispyribac-sodium. He has no mechanical weeder and says labor is scarce.
1. Diagnose the yellowing and poor tillering. What evidence supports herbicide injury rather than a nutrient deficiency or pest? Explain why the planned re-spray is risky.
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Herbicide injury is most likely. The yellowing is in spray-overlap strips, appears after bispyribac-sodium, and the spray was made at 10 DAS when rice had only two leaves. Bispyribac is normally applied when rice is at the 3-4 leaf stage and weeds are 2-4 leaf; 30 g/ha is above the usual 20-25 g/ha, and 34°C with no surfactant increases stress. A nutrient deficiency would be more uniform; a pest would not follow the sprayer pattern. The weeds that survived-Echinochloa, Cyperus and Ludwigia-suggest the weeds were already too large or the dose/timing was wrong, not that the field needs a double dose.
Why re-spraying is risky. 40 g/ha will increase crop injury, and 2,4-D at this early stage can cause leaf curling, stunting and later sterility; it also does not control sedges. The weeds are now larger and harder to kill, so the answer is not a higher rate.
2. Prepare an integrated weed management plan for the rest of this season and the next DSR crop. Include immediate actions, herbicide choice principles, and non-chemical measures.
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Immediate. Stop further bispyribac. Drain any deep water and keep the field moist but not flooded for a few days; do not add fertilizer to force growth. Hand-weed or use a mechanical weeder at 25-30 DAS, removing Echinochloa and Cyperus before they tiller and Ludwigia before flowering. If a post-emergence herbicide is still justified, use only a registered product after confirming rice stage and weed spectrum; spot-treat rather than broadcast, and keep a non-treated strip.
Next DSR crop. Start with a stale seedbed and one light irrigation to flush weeds before sowing. Use a pre-emergence herbicide such as pretilachlor or pendimethalin at the label rate and timing, then rotate to a different site of action for the post-emergence spray. Keep narrow rows and a competitive crop stand, level the field, and use shallow water early. Hand-weed at 20-25 DAS if escapes remain. Do not use bispyribac year after year.
3. How would you monitor and prevent herbicide resistance, and what would you tell the farmer about 2,4-D?
Read the worked answer
Keep a field map of herbicides, dates and doses. Suspect resistance if the same herbicide at the full label rate fails on the same weed species in the same field for two seasons. Then rotate sites of action-ALS inhibitors (bispyribac), ACCase inhibitors (fenoxaprop), chloroacetamides (pretilachlor) and synthetic auxins-and use mechanical/cultural control. Never mix 2,4-D with the early post-emergence spray; use it only after rice tillering and before panicle initiation, with a drift guard, because it can injure rice and nearby broadleaf crops. Do not use a higher rate to compensate for a missed stage.