Cotton defoliation has started for some and will be starting very soon for others. Proper timing of cotton harvest-aid applications are very important. Defoliation timing decisions should be based on the crop and the crop environment.
Timing of defoliation is critical to insure optimum yield and fiber quality. Several factors can be used to determine the proper time for harvest aid application. The first is the traditional method of counting open and unopen bolls. Defoliation should proceed when at least 60 to 75 percent of bolls are open. This method focuses primarily on the “open” portion of the bolls while ignoring the “unopen” portion, which is also important.
A second indicator involves slicing bolls with a sharp knife. Bolls are considered mature- and ready for harvest aids- when they cannot be sliced without stringing the lint. In addition, bolls are mature when the seed embryo contains only tiny folded leaves (no “jelly” within the developing seed) and the seedcoat begins to turn yellow or tan. (See below picture)
A final method utilized to determine crop maturity is counting nodes above cracked boll (NACB). NACB is determined by counting the number of nodes separating the uppermost first position cracked boll and the uppermost first position boll that is expected to be harvested. Once NACB has reached 4 it is generally safe to apply harvest aids. In some cases, when plant populations are low, a NACB of 3 maybe more appropriate. Growers should understand that each method of determining defoliation timing considers different plant characteristics, therefore the use of a combination of these methods would more accurately depict maturity of plants and provide a better indication for optimal defoliation timing.
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.
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. The below tables 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. 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. 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.
*NOTE: The use of trade names is solely for the purpose of providing specific information. University of Georgia Cooperative 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. https://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. https://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. https://www.alabamacrops.com
Whitaker, J. 2013. Cotton defoliation / harvest aid options. pp. 113-120 in 2013 Georgia Cotton Production Guide. The University of Georgia College of Agricultural and Environmental Sciences Cooperative Extension. https://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. https://edis.ifas.ufl.edu/
Mike Donahoe, County Extension Director, Santa Rosa County, Florida