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Cotton defoliants and harvest aids for mechanical picking Photo: EqualStock IN · Pexels License · source ↗

Cotton defoliants and harvest aids for mechanical picking

A cotton defoliant is a chemical applied 7-14 days before harvest that triggers controlled abscission of mature leaves, leaving open bolls on a leaf-stripped canopy that a mechanical picker can clear efficiently in a single pass. Defoliation is standard practice in the United States, Australia, China and Brazil, but is still nascent in India because most cotton is hand-picked over multiple selective rounds (Cotton Picking Labour). With picking labour squeezing economics and mechanical-picker prototypes being tested by ICAR-CICR and CIRCOT, harvest-aid use is expected to grow.

Principle

Mechanical pickers — spindle or stripper types — operate cleanly only when (i) ≥ 60% of bolls are open, (ii) the canopy carries minimal green leaf and (iii) regrowth has been suppressed. A harvest-aid chemical achieves these conditions by either inducing leaf abscission (true defoliant) or by desiccating leaves (desiccant) or by promoting late-boll opening (boll opener). Most commercial programmes combine two of these modes.

Active ingredients

  • Tribufos (DEF): a true defoliant; phosphorothioate that triggers ethylene-mediated abscission. Used in the US but not yet registered for cotton in India.
  • Thidiazuron: a substituted urea cytokinin-like compound; widely used internationally. Indian CIB&RC label coverage on cotton is limited.
  • Ethephon: an ethylene-releasing compound; functions as a boll opener. Registered in India for several crops; not labelled specifically for cotton harvest aid but used off-label.
  • Sodium chlorate / magnesium chlorate: desiccants used in seed-cotton seed-production blocks.

Indian context

ICAR-CICR demonstration trials in Maharashtra (2018-22) and CIRCOT pilot blocks have evaluated thidiazuron + ethephon tank-mix at 90-100 days after sowing (when ≥ 60% of bolls are open) on HDPS short-duration varieties (High Density Planting System Cotton). Reported outcomes include 75-85% leaf drop within 10-12 days, improved single-pass picker performance, and a 5-7% reduction in stick-and-trash count in bale samples. None of these chemicals is yet labelled in India for cotton; CIB&RC registration is the current barrier.

When and where to apply

The agronomic precondition is a uniformly-maturing crop. Defoliation suits compact short-duration HDPS plantings far better than tall, indeterminate spreading hybrids (Ajeet 155 Cotton) where mature and immature bolls coexist for weeks. Topping (Cotton Topping Pinching) and timely terminal-irrigation withdrawal improve crop uniformity ahead of defoliant spray.

Limitations

  • No CIB&RC label currently for tribufos or thidiazuron in Indian cotton — regulatory barrier.
  • Mechanical-picker infrastructure (machines, ginning compatibility, contractor service) is still scattered.
  • Defoliation requires correct timing within a narrow window; mis-timed spray can lock in green leaf or trigger boll shedding.
  • Cost of imported pickers and harvest-aid chemistry can erode the labour-replacement savings on smallholdings.

See also Cotton Picking Labour, High Density Planting System Cotton, Cotton Topping Pinching, Ajeet 155 Cotton.

Sources

  1. Mechanical cotton picking and defoliation. ICAR-CICR.
  2. Defoliants for cotton. CIRCOT technology brief.
  3. CIB&RC label data. Central Insecticides Board & Registration Committee.