Donnie K. Miller, Alexander M. "Sandy" Stewart, Ralph D. Bagwell and John W. Barnett

Preparing for cotton harvest involves some of the most important management decisions producers face. Applying chemical harvest aids before harvest can increase harvester efficiency, reduce leaf and trash content in harvested lint, facilitate dew drying, straighten lodged plants, retard boll rot, maintain or improve fiber quality and stimulate boll opening. In addition, using harvest aids with desiccating activity on weed species (causing leaves to dry out) can increase harvest efficiency.

Application Timing

A frequent question about harvest preparation is “When do I defoliate?” Harvest aid application decisions are based primarily on crop maturity as well as crop and weather conditions and harvesting schedule. Premature defoliation can reduce efficacy and yield. Delayed defoliation can cause loss of lint quality from weathering, high micronaire (measure of fiber thickness) values and, in some cases, reduced efficacy because of lower temperatures. Several methods for timing defoliation have been proposed. Each has merits and limitations. Unfortunately, there is no one-size-fits-all recommendation for cotton defoliation.

Open Boll Percentage

The most widely used method is based on percentage of opened bolls in a field, with 60 percent a common recommendation. In many situations, unopened bolls are mature enough to withstand negative effects and will open before harvest. This method, however, has limitations. Research in Louisiana has shown that, depending on fruit distribution on the plant, maximum yield can be obtained when defoliation occurs before 60 percent are open. Additionally, in cases where a large fruiting “gap” (no bolls present at fruiting sites) occurs and a large percentage of bolls are less mature and set in the uppermost region of the plant, optimum defoliation timing may occur later than 70 percent open. Research evaluating optimum defoliation in Louisiana and other states has shown maximum yield can be achieved with application ranging from 42 percent to 81 percent open, depending on crop maturity and fruit distribution.

NACB Method

The node above cracked boll (NACB) method, in contrast to the percent open boll method, focuses on the unopened portion of the crop. (See illustration of cotton plant on page 13.) 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. By focusing on the unopened portion, NACB takes into account potential fruiting gaps. Most recommendations call for defoliation at four NACB. Low plant population and skip-row cotton, however, are often safer defoliated at three NACB. Lower plant population usually means a later-maturing crop with a significant portion of yield coming from outer position bolls and bolls set on vegetative branches.

DD60

Another method recently developed in Arkansas recommends defoliation after accumulation of 850 heat units, or DD60s, after cutout (stage when vegetative growth ceases). A DD60 (degree days above 60) is a measure of accumulated heat needed for growth and development using a 60 degree F minimum threshold. The value is determined by adding the daily high and low temperatures, dividing by two and subtracting 60. LSU AgCenter research indicates that under conditions experienced in Louisiana, the appropriate defoliation timing may be higher than 850 heat units (1050 heat units) beyond a cutout of node above white flower four (four main-stem nodes above uppermost first-position white flower). Although this method does focus on the unopened portion of the crop and is supposed to allow enough time and DD60s for full development of all bolls, it requires that a determination of cutout be made. The definition of cutout is a moving target and can be different for every field.

Boll Maturity

Whatever method is used, growers also should inspect unopened bolls for maturity. A boll is considered mature if it is difficult to slice in cross-section with a knife and seeds have begun to form a brown or black seed coat. Once a dark seed coat has formed, defoliation will not affect yield of those bolls adversely. Cotton bolls need about 40 to 60 days to mature, depending on temperature. Bolls set later in the season will take longer to mature and may never be harvestable. Growers should walk their fields before defoliation and examine only those bolls that can reasonably be expected to mature.

Harvest Aid Considerations

In addition to defoliation, boll opening and weed desiccation, desired effects of harvest aids include inhibition of regrowth of cotton plants and removal of any juvenile growth. Because no one product can accomplish all of these objectives, tank-mix combinations are usually required. Harvest aids exhibit either herbicidal or hormonal activity. Compounds classified as having herbicidal activity injure the plant. This injury changes the hormone balance in the leaf by reducing growth hormone levels and stimulating ethylene production, which leads to leaf abscission (leaf drop). Hormonal harvest aids either deliver ethylene to the plant or stimulate its synthesis. Regardless of the mode of action, thorough coverage of foliage is required for maximum effect. Research has shown that hollow cone nozzles provide superior coverage for harvest aid applications. Research also has shown that harvest aids are affected by environmental conditions. Thus, label precautions must be followed.

Use of defoliants and boll openers for preparing cotton to be harvested is a critical management decision. LSU AgCenter researchers continue to evaluate products and timing strategies to determine the most profitable level of defoliation. A good cotton crop in the field can only be sold once it is harvested and ginned. Proper defoliation is a profitable part of a total cotton management system.

Acknowledgment
Donna R. Lee and A. Lawrence Perritt, research associates at the Northeast Research Station; Derek Scroggs, research associate at the Dean Lee Research Station; and Barrett McKnight, extension associate at the Scott Research and Extension Center for their work in the cotton defoliation research and extension programs and the Cotton Foundation for providing funding.

(This article appeared in the summer 2002 issue of Louisiana Agriculture.)