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Weed management in cotton

Weed competition is one of the largest avoidable yield losses in Indian cotton, with uncontrolled grasses and broadleaf weeds capable of reducing seed-cotton yield by up to 45%. The standard integrated programme combines pre-emergence and post-emergence herbicides with mechanical inter-cultivation and one hand-weeding.

Principle

Cotton is slow to establish a competitive canopy and is most vulnerable to weeds in the first 60 days. A residual pre-emergence herbicide suppresses early flushes; a post-emergence directed spray controls escapes; inter-cultivation maintains a weed-free zone and a soil mulch. Combining modes of action reduces selection pressure for herbicide-resistant weed biotypes.

Implementation

The Indian standard programme on Bt cotton consists of:

  • Pre-emergence: pendimethalin at 1.0 kg a.i./ha applied within 48 hours of sowing.
  • Post-emergence (25-40 DAS): pyrithiobac sodium 75 g a.i./ha plus quizalofop-ethyl 75 g a.i./ha as a directed spray, or the registered 6%+4% MEC ready-mix.
  • Mechanical: two to three inter-cultivations with a blade harrow at 15-day intervals.
  • Hand-weeding: one supplementary pass to clear in-row escapes.

Trials in north-west and north Gujarat cotton report seed-cotton yields of approximately 2,068 kg/ha under this sequential programme, with weed-control efficiency comparable to weed-free check plots. The programme integrates with the broader early-season management sequence in Cotton First 30 Days Management.

Adoption context

The pre-emergence-plus-post-emergence approach is the foundation of Bt cotton (Bt Cotton Bg Ii) weed management because Bt cotton does not carry a glyphosate-tolerance trait — herbicide-tolerant stacks remain unapproved in India (see Roundup Ready Cotton Regulatory). Off-label glyphosate use on Bt cotton frequently produces phytotoxicity that requires recovery sprays (Cotton Herbicide Recovery).

Limitations

Efficacy of pendimethalin depends on adequate soil moisture at sowing. Post-emergence directed spraying requires shielded nozzles and trained operators to avoid drift onto cotton foliage. Repeated use of single modes of action accelerates resistance.

See also Cotton First 30 Days Management, Cotton Herbicide Recovery, Roundup Ready Cotton Regulatory, Bt Cotton Bg Ii.

References

  1. Weed control in Bt cotton with pyrithiobac + quizalofop pre-mix. Crop Protection / ScienceDirect.
  2. Optimizing cotton production through efficient weed management. Plant Science Today.