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Cotton defoliation is the application of harvest aids to help prepare a cotton crop for harvest. Benefits of proper cotton defoliation include: reduction of the main sources of stain and trash (leaves), increased harvest efficiency, quicker drying of dew, straightening of lodged cotton plants, potential for increased boll opening, and a reduction of boll rots.
As a cotton plant matures, a physiological process takes place which separates the living tissue near the leaf petiole, called the abscission zone. Plant hormones regulate enzyme activity causing the cell walls in the abscission zone to dissolve and the leaf to drop.
There are two types of defoliants. A herbicidal defoliant can be used to cause injury to the leaf, upsetting the hormone balance and beginning the abscission process. The application of a hormonal defoliant increases ethylene synthesis in a plant, causing the leaves to fall off. Correct application rates are important, especially with herbicidal defoliants, as overapplication can cause the leaf to die before the abscission process, resulting in “stuck” leaves. Conversely, when too little defoliant is applied, the abscission process may not begin, resulting in no leaf defoliation.
When applying a defoliant, desiccant, or boll opener, many factors must be taken into consideration for successful application. Best results from an application occur when:
Poor results from a defoliant application can occur when:
The use of plant growth regulators (PGRs) during the growing season can also influence the efficacy of defoliants. For more information refer to the Agronomic Spotlight - Cotton Growth Management.
In high-yielding cotton, a desiccant may be applied after a defoliant to help speed up the plant drying process. Boll openers may also be used in combination with defoliants to enhance activity. Boll openers increase ethylene production within a boll to hasten opening and accelerate drying. If applied to cotton prior to maturity, micronaire and fiber length can be reduced.
Successful defoliation depends on sufficient leaf coverage during application. Higher sprayer volumes may be necessary to achieve adequate coverage. Volumes between 10-15 GPA are recommended for most situations.1,3 It is also recommended to use flat-fan or hollow cone nozzles. These types of nozzles provide excellent spray coverage.4 Drift-reduction nozzles, while excellent at controlling drift and spray placement, have been shown to decrease coverage within the crop canopy.
There is as much art as science involved in making the decision to defoliate cotton. There are several methods producers can use to help determine the best time to apply a defoliant to their crop. One or more of the following methods may be used to help make this decision.
Percent open bolls. This is the most widely used method and is based on determining the total percentage of open bolls in a field. The most common recommendation for defoliant application is when 60% of bolls in a given 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 is determined by locating the uppermost first-position cracked boll with visible lint and counting the number of main-stem nodes to the uppermost harvestable boll. Defoliation is generally recommended at four NACB. For skip-row or cotton with a low plant population, defoliation is typically recommended at three NACB.3
Accumulated heat units after cutout. Defoliation is recommended after 850 heat units (DD60s) have accumulated, which is typically after cutout or 5 nodes above white flower (NAWF).3 The main drawback to this method is that the amount of heat units required by each variety can vary. In addition, this method requires a determination of cutout, which can be different for every field.
Visual inspection. Growers may also 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 string out when the boll is cut, and the seeds have begun to form a brown or black seed coat. Once a dark seed coat has formed, defoliation should not adversely affect those bolls.
No one 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.
Harvest aids do not contribute any to cotton yield potential or maturity, but are used to defoliate the plant. It is also important to recognize that 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 can limit fiber quality and yield potential. Defoliants should be applied only when a cotton crop has reached the desired maturity for harvest.