Cotton Defoliation Timing

When making the decision to defoliate cotton, careful consideration must be made with relation to crop maturity, field conditions, and the environment. Cotton fiber quality and yield can be maintained by paying attention to maturity and understanding growth characteristics of a variety.

Benefits of proper cotton defoliation can include: reduction in the sources of stain and trash (leaves), increased harvest efficiency, quicker drying of dew, straightening of lodged cotton plants, increased potential for boll opening, and a reduction of boll rots.

Factors Affecting Defoliation—When applying a defoliant, desiccant, or boll opener, many factors must be taken into consideration for successful application. The use of cotton plant growth regulators (PGRs) during the growing season can influence the efficacy of defoliants. PGRs work to reduce vegetative growth and slow the development of new nodes, which can help plants mature early and prepare for defoliation. Boll openers may also be used in combination with defoliants to enhance activity. Boll openers increase ethylene production to hasten opening and dry out. If applied to cotton prior to maturity, micronaire and fiber length can be reduced.

Defoliation Application—Successful defoliation depends on leaf coverage by the chemical. Higher sprayer volumes may be recommended to achieve adequate coverage to all plant leaves. Volumes between 10-15 GPA are recommended for most situations.1,3 Recommendations for most defoliant applications include the use of flat-fan or hollow cone nozzles. These types of nozzles provide excellent spray coverage.4 Drift-reduction nozzles, while excellent at controlling spray placement, have been shown to decrease coverage within the crop canopy.

Timing Defoliant Application—There is as much art as science involved in making the decision to defoliate. There are several methods producers can use to determine the best time to apply defoliants:

  • Percent Open Bolls – This is the most widely used method, based on determining the total percentage of open bolls in a field. It is commonly recommended to defoliate when 60% of bolls in a field are open.3 Research has shown that maximum yields can be obtained with defoliation applications ranging from 42% up to 81% open bolls.

  • Nodes Above Cracked Boll (NACB) – This method focuses on the un-opened portion of the crop. NACB is determined by locating the uppermost first-position boll that is cracked open with visible lint, and counting the number of main-stem nodes to the uppermost harvestable boll. Most recommendations call for a defoliation application at 4 NACB. With low plant population and skip-row cotton, recommendations call for a defoliation application at 3 NACB.3

  • Accumulated Heat Units After Cutout – This method is based on the unopened portion of the crop. A defoliation application is recommended after an accumulation of 850 heat units (DD60s)3, which is typically after cutout or 5 Nodes Above White Flower (NAWF=5). This method does not always work as each cotton variety varies in the amount of heat units needed to reach maturity.

  • Visual Inspection – Producers may choose to determine maturity by visual inspection. Bolls are generally considered mature when they are difficult to cut in a cross-section with a knife, fibers will string out when cut, and seeds have begun to form a brown or black seed coat.

No one defoliant or harvest aid, rate, or specific timing is the solution for every field. Selections should be based on prior experience, price, environmental and crop conditions, yield potential, and the value of the crop. Knowledge of the lint and growth characteristics of each individual variety is critical in finding the best harvest aid program, with respect to product and timing.

Defoliants do not contribute to cotton yield potential or maturity, but are chemicals used to remove leaves from the plant. It is also important to recognize that problems can occur from the use of defoliants. Once a cotton plant is defoliated, fiber and seed development can slow or stop. If cotton is defoliated too early, bolls may not mature, which reduces fiber quality and yield potential. Defoliants should be applied only when a cotton crop has reached the desired maturity for harvest.