Cotton Defoliation in Georgia

Cotton defoliation is a complex production decision with many chemical options to consider. Harvest aids are utilized to prepare the crop for machine harvest, and timely defoliation and harvest of cotton can reduce weathering losses (yield and quality) and decrease trash in the lint. A basic knowledge of crop development and maturity as well as an understanding of the physiological effects of harvest aids on cotton plants is necessary in making decisions concerning defoliation.

Harvest Aid Functions

Harvest aids have four functions. Based on the time of year defoliation is occurring, all processes may not be necessary for cotton harvest. It's important to understand these processes to determine appropriate products and rates. These four functions are (a) removal of mature foliage, (b) removal of juvenile foliage, (c) boll opening, and (d) regrowth suppression.

The first two processes are considered defoliation. Defoliation is a natural plant process—but in a cotton crop, leaf drop does not occur simultaneously throughout the canopy. To facilitate a timely harvest, producers must manipulate the plant to drop its leaves over a short period of time.

Auxin and ethylene are the two plant hormones involved in defoliation. Auxin promotes growth and prevents abscission, whereas ethylene is a ripening hormone that promotes abscission. Leaves fall from the plant once ethylene moves from the leaf to the base of the petiole to activate cell-wall-degrading enzymes that form the abscission layer. The ratio of auxin and ethylene concentration in the leaf is based on leaf age. Younger leaves have higher concentrations of auxin while older leaves have higher concentrations of ethylene. This makes older leaves more conditioned for defoliation, while young leaves might be more difficult to remove. Although higher rates of defoliants might be necessary to remove juvenile growth, this could also lead to leaf sticking—where the leaf desiccates and remains on the plant. Increased ethylene from defoliants can hasten boll opening, and regrowth suppression is necessary to prevent follow-up applications to control regrowth. Manipulating these hormones with harvest aids will facilitate the leaf abscission process and allow for suppression of boll opening and regrowth.

Types of Defoliants

There are two main types of defoliants for cotton: herbicidal and hormonal. Herbicidal defoliants injure the leaf, stimulating the production of ethylene. Hormonal defoliants increase the ethylene concentration in the leaves without causing injury. Specific examples of each type can be found below.

Herbicidal defoliants

Tribufos (Folex)—Injures leaf below the cuticle, causing stress and stimulating ethylene production.

PPO-inhibiting herbicides [Aim (carfentrazone-ethyl), ET (pyraflufen-ethyl), Resource (flumiclorac), Blizzard (fluthiacet-methyl), Sharpen (saflufenacil), and others]—Degrades cell membranes, causing ethylene production.

Hormonal defoliants

Ethephon (Prep, others)—Increases production of ethylene, leading to leaf drop and accelerated boll opening. Other ethephon-containing products include Finish 6 Pro (ethephon plus cyclanilide) and FirstPick (ethephon plus urea sulfate).

Thidiazuron (Dropp, Freefall, others)—Enhances the production of ethylene and inhibits auxin transport. Primarily used for juvenile growth removal and regrowth suppression. Ginstar is a premix of thidiazuron plus diuron.

Defoliation Timing

Determining when to defoliate your cotton crop is of the utmost importance because defoliation timing can impact both yield and fiber quality. Poor defoliation reduces fiber quality; early defoliation reduces yield and micronaire; and late defoliation increases the likelihood of boll rot and reductions in yield and quality from weathering. Additionally, lower temperatures later in the season can cause suboptimal defoliant performance.

There are three primary ways to determine crop maturity and defoliation timing:

1. Percent open boll—60% to 75% open boll (60% in a uniform crop ONLY)
2. Sharp knife test—Using a sharp knife, cut into the uppermost boll that has a chance of contributing to yield. The cotton should string out when the boll is cut; the seeds should be fully developed, with a brown seed coat, and visibly developed cotyledons inside the seed.
3. Nodes above cracked boll (NACB)—observing four NACB or less. There is a relationship between the percent open boll and the number of nodes between the uppermost first position cracked boll and the uppermost first position harvestable boll. That relationship is illustrated in Figure 1.

Defoliant Applications

Most defoliants do not translocate through the plant, making spray coverage extremely important. To ensure adequate spray coverage, growers must use the proper spray pressure, ground speed, and nozzle size for the appropriate application volume (according to label instructions). Much attention has been given to defoliation using auxin nozzles (i.e., nozzles labeled for 2,4-D and dicamba applications), and there have been concerns about adequate spray coverage. A study conducted across the cotton belt found that sprayer output (GPA) was far more important in defoliation than nozzle type. Higher GPA results in greater defoliation, with a good output target being 15 GPA.

Many growers across the state of Georgia have participated in the Using Pesticides Wisely training over the past few years. Although this training primarily has to do with the use of 2,4-D and dicamba, these lessons should not be dismissed when applying other pesticides. Care should be taken to prevent off-target movement of defoliants, particularly around sensitive crops and urban areas, as the effects of defoliant drift are readily apparent.

When defoliating, only treat the acres you anticipate harvesting in the next 10 to 14 days. Rain occurring after application can affect defoliant activity, so weather forecasts should be consulted before application. In Table 1, some of the more common defoliants are listed with their rates, rain-free periods, PHIs, and expected activity. This table also can be found in the 2021 Mid-South Cotton Defoliation Guide (https://cottoncultivated.cottoninc.com/2021-mid-south-cotton-defoliation-guide/)

Specific Recommendations

For particular defoliant tank-mixture recommendations, refer to the cotton section in the latest edition of the Georgia Pest Management Handbook (https://extension.uga.edu/publications/detail.html?number=SB28-16). In the defoliant section of the handbook, recommendations are broken up by “season”—the relative range of temperatures at and around the time of application—and expected defoliant activity.

Although there are many options when choosing defoliants, one of the most common tank mixtures is a three-way mix of ethephon (Prep, others), thidiazuron (Dropp, others), and tribufos (Folex/Def). Rate recommendations based on temperature at and in the time surrounding defoliation can be found in Table 2. For more information on these and any recommendations and how to incorporate them into your operation, please contact your local UGA County Extension Agent.

Weed Management at Defoliation

In cases where weeds are present at harvest, note that some defoliants also have herbicidal activity on certain weeds. Table 3 lists some of those options. These treatments should be followed by desiccants to further prepare the cotton crop for harvest, including products containing paraquat or sodium chlorate.

Status and Revision History
In Review on Feb 07, 2024
Published on Apr 23, 2024