Cotton defoliation will soon be underway in many older planted fields throughout the state. Proper timing of cotton harvest-aid applications is important for optimizing both yield and quality of the crop. Defoliation decisions should be based on the crop and the crop environment. Plant maturity is usually the most important consideration, but other factors such as picking capacity, custom harvesting, and weather are also important. The goal for the producer is to determine the boll population that contributes significantly to yield and to harvest that crop of bolls at the optimum time.

Percent Open Bolls

Percent open bolls is a useful tool to determine when to defoliate cotton, but it’s only one of several methods to use to make a decision. We need to look at a combination of factors. An old rule of thumb is to defoliate when 60% of the bolls are open. However this method has limitations and depends on fruit distribution and gaps (no bolls present at fruiting sites). Research 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.

Nodes Above Cracked Boll (NACB)

Another method is nodes above cracked boll (NACB). NACB is determined by locating the uppermost first-position boll that is cracked open and counting the number of main-stem nodes to the uppermost harvestable boll. Research has shown that once NACB reaches four, the crop can be safely defoliated without significant weight or quality loss.

Heat Unit Accumulation

Measuring accumulated heat units (DD 60s) past cutout is another method to help schedule defoliation. Generally, cutout is defined as the time when five main-stem nodes are present above the uppermost first position white flower (NAWF=5). DD60 heat units are calculated using the formula: maximum daily temperature + minimum daily temperature divided by 2, minus a base temperature of 60° F equals total daily heat units [(TMax +TMin /2) – T 60 =DD60s]. For example, a daily high and low of 88 and 76° F results in (88 + 76 /2) – 60 = 22 DD60s for the day. In general, when 850-950 DD60s are accumulated from NAWF = 5, the field should be mature enough to defoliate. However, this may vary from year to year and from field to field due to rainfall patterns, soil types, or daily high temperatures. Therefore, the field should also be physically examined before a harvest aid is applied.

Cutting Bolls

Whatever method is used, growers should check bolls for maturity. Mature bolls are difficult to cut in cross section with a sharp knife without stringing the fiber. The seed will be completely filled out with no “jelly” in the center. The seed coats of mature seeds are tan to brown as opposed to the white and pale color of immature seeds. The presence of a thin, brown line around the seed indicates the boll is mature enough not to be adversely affected by application of a harvest-aid chemical. Bolls require 40 to 60 days from bloom to mature, depending on temperature. Bolls set late in the season take longer to mature and may never be harvestable. In most years, blooms after the first week of September will not have enough time to develop into open bolls in north Florida.

The most important thing to remember is that there is no one-size-fits-all approach to defoliation timing. You have to make decisions on a field-by-field basis and not rely on any one method. With any method, check fields regularly to track the development of the crop and sample enough plants in different areas of the field to ensure that the sample is representative of the overall field status. It’s often best to use a combination of these methods to make a final harvest aid treatment decision.

Harvest Scheduling

In addition to crop maturity, another other major consideration for harvest-aid application is picker availability. Applications should be timed so that harvesting can keep up with defoliation. Harvest aids should be applied approximately 12 to 14 days ahead of picking. Under optimum conditions, the crop could be ready to harvest within 7 days after application. The interval between application and harvest may increase as temperatures drop later in the season.

Harvest aid performance is affected by temperature, plant condition, spray coverage, and product rate. Temperature is the main factor in determining harvest-aid rate and it can have a significant impact on the activity of various defoliants. Defoliants work best on mature cotton under warm, humid conditions. Cool temperatures at the time of application, and for 3 to 5 days afterwards, can retard defoliant activity and cause less than desirable results. If possible, materials should not be applied during cool snaps. When nighttime temperatures drop into the low 60s, activity of thidiazuron products (i.e. Dropp, Freefall, Klean-Pik, Thidiazuron, etc.) is reduced. Table 1. lists expected activity of various defoliants.

Most harvest-aids do not translocate throughout the plant. Therefore, thorough spray coverage is essential for acceptable results with all harvest aids. Most labels call for ground applications in 10 to 25 gallons of water per acre (GPA) and aerial in at least 5 GPA. Lower carrier volumes increase the likelihood of needing a second application.

Mode of Action of Harvest-Aids

Harvest-aids work in one of two ways; by herbicidal or hormonal activity. Herbicidal harvest-aids injure the leaf, stimulating the production of ethylene. The ethylene promotes abscission, or leaf drop. If these are applied at rates two high for the temperature, they kill the leaf too quickly before ethylene can be produced. This results in desiccation or “leaf stick” instead of leaf drop. Aim, Blizzard, Def, Folex, ET, Harvade, and Resource are herbicidal-type defoliants.

Hormonal harvest-aids increase the ethylene concentration in the leaves and plant without causing any injury. Dropp, Freefall, Klean-Pik, and ethephon (Prep, Finish, FirstPick, etc.) are hormonal harvest-aids. Because these hormonal-type defoliants do not cause the leaf injury like the herbicidal types, they are not as likely to cause desiccation or “leaf stick”.

There is no best harvest aid material that will defoliate, stimulate boll opening, prevent regrowth, and perform equally well under various conditions. Combinations of products can result in good performance under a broad range of conditions that normally occur in north Florida. Boll-opening materials, listed below, are often used in combination with defoliation materials to increase the percentage of the crop harvested during the first picking or possibly to eliminate the need for a second picking.

Regrowth suppression is important if you cannot harvest the crop within 10 days following application. According to Dr. Mike Patterson, Extension Weed Scientist with Auburn University, on Roundup Ready or Roundup Ready Flex cotton, the only materials that provide significant re-growth suppression are those that contain thidiazuron as an active ingredient. These products include DroppSC (and generic versions) and Ginstar and will usually suppress re-growth for up to three weeks if used at the appropriate rate. On conventional (non-transgenic) varieties, glyphosate (Roundup, etc.) can be used to suppress re-growth.

Updated listings of harvest aid materials and combination choices can be found on the university web sites referenced below.

Table 1. Expected activity of various defoliants as complied by Drs. Donnie Miller, Daniel Stephenson, and Kohn Kruse in the 2012 Cotton Harvest Aid Guidelines for Louisiana.

The use of trade names is solely for the purpose of providing specific information. University of Florida IFAS Extension does not guarantee or warranty the products named, and references to them do not signify approval to the exclusion of other products of suitable composition.

References:

Edmisten, K. 2012. Cotton defoliation. pp. 147-165. In 2012 Cotton Information. North Carolina Cooperative Extension Service. http://www.cotton.ncsu.edu/

Miller, D., D. Stephenson, and J. Kruse. 2012. 2012 Cotton Harvest Aid Guidelines for Louisiana. LouisianaStateUniversityAgricultural Center. Pub. 3194: 10 pp. http://www.lsuagcenter.com/

Patterson, M. 2009. Cotton defoliation 2009. pp. 4-5. In Alabama Cotton Picksack Newsletter. Late Aug/Sep 2009. Alabama Cooperative Extension System. PSK-8-09. http://www.alabamacrops.com

Whitaker, J. 2012. Cotton defoliation / harvest aid options. pp. 113-120. In 2012 Georgia Cotton Production Guide. The University of Georgia College of Agricultural and Environmental Sciences Cooperative Extension. http://ugacotton.com

Wright, D.L., and B.J. Brecke. 2009. 2009 Cotton Defoliation and Harvest Aid Guide. University of Florida/IFAS Extension. SS-AGR-181. http://edis.ifas.ufl.edu/AG188

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