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Pyrithiobac sodium and quizalofop-p-ethyl (cotton)

Pyrithiobac sodium and quizalofop-p-ethyl are the two CIBRC-labelled selective post-emergence herbicides for cotton in India. They are used either as standalone sprays (Hitweed 10% EC; Targa Super 5% EC) or as ready-mix MEC formulations (Hitweed Maxx Cotton and Kevat Ultra Cotton) for one-shot broadleaf and grass control.

Composition / Active ingredient

  • Pyrithiobac sodium: 10% EC (Hitweed) — Pyrimidinylthiobenzoate
  • Quizalofop-p-ethyl: 5% EC (Targa Super) — Aryloxyphenoxy-propionate "fop"
  • Ready-mixes: Pyrithiobac 6% + Quizalofop-ethyl 4% MEC (Hitweed Maxx, Kevat Ultra)

Mode of action

The two molecules cover complementary weed spectra through different enzyme targets:

  • Pyrithiobac sodium inhibits acetolactate synthase (ALS), the entry enzyme of branched-chain amino acid biosynthesis. HRAC Group 2. Active mainly on broadleaf weeds with some grass activity.
  • Quizalofop-p-ethyl inhibits acetyl-CoA carboxylase (ACCase), the entry enzyme of fatty acid biosynthesis in grass meristems. HRAC Group 1. Active only on grasses; broadleaves are inherently tolerant.

Cotton is naturally tolerant to both molecules because of differential herbicide metabolism.

Target use and dose

Registered post-emergent timing is at the 2-4 leaf stage of weeds (typically 20-30 days after sowing). Pyrithiobac controls Trianthema, Amaranthus, Digera, Phyllanthus and other broadleaves; quizalofop controls Echinochloa, Digitaria, Dinebra, Cynodon and other grasses. The ready-mix MEC products achieve both in a single pass without the operator preparing a tank-mix.

Safety and regulatory status

Both molecules are selective only on cotton in India; drift to pulses, oilseeds and most vegetables causes severe injury. ACCase and ALS resistance has been documented in several grass and broadleaf weed species globally; rotation with Pendimethalin Cotton Preemergent (HRAC 3) at pre-emergence and Upl Sweep Power Glufosinate (HRAC 10) on bunds is the standard resistance-management practice.

See also: Hitweed Maxx Cotton, Kevat Ultra Cotton, Agil Propaquizafop Cotton, Pendimethalin Cotton Preemergent.

References

  1. Hitweed Maxx. Godrej Agrovet.
  2. Tata Kevat Ultra. Kisan Center.