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Grass weeds - especially barnyardgrass (Echinochloa crus-galli and E. colona) - are the most damaging weed group in rice. They mimic rice at early stages, compete for the same nutrients and light, and can cut yields by 50% or more when uncontrolled. Direct-seeded rice is particularly vulnerable.

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When rice is most vulnerable

The first 30 to 45 days after emergence or transplanting - the critical weed-free period. Weeds emerging after that window cause less yield loss.

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How grass weeds show in satellite data

  • Uniformly high NDVI on weedy fields at establishment that masks the underlying poor rice stand - a false-positive risk.
  • Uneven, tall canopy roughness later in the season as weeds emerge above the rice canopy.
  • Weed panicles emerging above rice create visible pale flecks in high-resolution imagery at heading.
  • Compare the field’s NDVI trajectory to the ecosystem baseline - a field that reaches high NDVI too early likely has weed competition.
Weed detection is the weakest area for satellite imagery in rice. NDVI does not distinguish weed biomass from rice biomass. Ground scouting in the first 30 days is essential.

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Field symptoms

  • Grass seedlings that look nearly identical to young rice - hardest to spot early
  • Barnyardgrass: rounded, hairless leaf collar; no ligule (rice has a ligule)
  • Weeds emerging in gaps between rice hills
  • Later: weed heads taller than the rice, distinctive seed heads

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Reduce the damage by

  • Use clean, weed-free seed - contaminated seed is a major source of weed infestation.
  • Land preparation - thorough puddling reduces weed emergence in transplanted rice.
  • Maintain 3 to 5 cm standing water for the first 30 days in irrigated systems to suppress grass weed germination.
  • Timely transplanting to give rice a head start on weeds.
  • Hand or mechanical weeding at 15 and 30 days after transplanting on small holdings.
  • Pre-emergence herbicide (pretilachlor or oxadiargyl) within 3 to 5 days of transplanting. Butachlor is banned in Malaysia - see Pesticide registration.
  • Post-emergence herbicide (bispyribac-sodium, penoxsulam, or cyhalofop) at 2 to 4 leaf stage of weeds if pre-emergence was skipped or has broken.
  • Rotate herbicide modes of action to slow resistance development in Echinochloa.

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Set an alert for early warning

Turn the diagnosis into a trigger. Focus alerts on the first 30 days when weed control still recovers yield.
Recommended settings for grass weeds on rice:
  • NDVI trajectory alert at 15 to 30 days after transplanting - trigger if NDVI is well above the expected baseline for that stage (weeds contributing to greenness).
  • New Imagery alert during the critical weed-free period to prompt weekly scouting.