What are the benefits from defoliation?
. Leaf removal
. Faster opening and drying of bolls, allowing harvest to begin earlier in the season
. Elimination of the main source of stain and trash
. Quicker drying of dew, allowing picking to begin earlier in the day
. Better grades
. Straightening of lodged plants for more efficient picking
. Prevention of boll rot
. Reduction in moisture
. Faster and more efficient picker operation
. Improve storage in modules

Desiccation is drying of plant tissues due to disruption of cell membranes and rapid loss of moisture. Often results in “stuck leaves”.

Application Timing and Conditions
Weather - Weather conditions at the time of application and for 3 to 5 days following application have a great effect on the response to harvest aids. Harvest aids are most active when temperature, sunlight intensity and relative humidity are high. A night temperature above 60 degrees F is especially important. For temperatures above 60 degrees F, the percentage of leaves that drop and the speed of leaf drop roughly doubles for each 10-degree rise in temperature. At least a full day of clear weather following application is needed for best results.

Plant Conditions - Defoliation is best in mature, well-fruited, uniform plants which have cut out but are not completely inactive. The nutrient supply, particularly nitrogen, should be almost exhausted, and the crop should not be under drought stress. Results may be poor when defoliants are applied to drought stressed crops with tough foliage.

Application timing - The single most important factor to consider in timing defoliant application is boll maturity. Little or no boll maturation occurs after the leaves are removed. Premature leaf removal can reduce yield and quality. It is generally safe to defoliate when 50-60 percent of the bolls are opened and the youngest bolls are mature. Under high levels of management, this may occur when 45 percent of the bolls are opened. To check boll maturity, cut the uppermost harvestable boll in cross section with a sharp knife. When the boll is mature, it should be difficult to cut with a sharp knife, the seed will be completely filled out and there is no jelly in the center. The presence of a thin brown line around the seed indicates the seed coat is reaching maturity; the boll is mature enough not to be adversely affected by application of a harvest-aid chemical. When possible, apply defoliants in late afternoon or early morning when humidity is high and winds are calm. The crop is ready to treat when the top-most, first position harvestable boll is 4 nodes or less above the uppermost, first position cracked boll (NACB).

Some defoliation materials do not move within the plant, so coverage is important. Cone-type nozzles are superior to flat fan or flood jet nozzles in a minimum of 10 gallons/acre for good foliar coverage. Two equally spaced hollow cone nozzles per row should give adequate coverage. Spray pressure, ground speed and nozzles should be matched to apply a finished spray rate in accordance with label instructions. A second application of a defoliant may be required for defoliation of rank cotton. Consult the product label for rates and other information.

Other factors to consider when defoliating: Do not defoliate all cotton at one time. Defoliants should be applied 10 to14 days prior to anticipated harvest date. Leaf drop should start in about 4 days and be complete in about 10 days. Rain in 3 to 4 hours after application of a defoliant does not destroy effectiveness of these chemicals except Dropp which requires 24 hours before rain.

RECOMMENDED HARVEST AID MATERIALS

DEFOLIANTS

Def 6 and Folex 6: These phosphate-type materials have been the standard defoliants in Tennessee for several years. Their performance is essentially equal and they are effective over a broad range of environmental conditions. Minimum temperature for optimum performance is 55-60 degrees F. These materials do not inhibit regrowth or appreciably improve boll opening and activity improves with increased maturity of the crop. Leaf removal with each of these products is usually rapid and addition of surfactants offers benefit only under adverse conditions. Leaf stick may be increased when applied under hot conditions and full rates of these products may not be needed.

Harvade: Harvade has generally provided defoliation of mature cotton equal to Def 6 and Folex 6 but is usually slower. Harvade does not inhibit regrowth or improve boll opening. Addition crop oil concentrate (1 pt./A) is necessary for acceptable defoliation. Addition of Harvade will increase desiccation of morningglories and other viney weeds.

Dropp: (Dropp 50WP, FreeFall 50WP): These thidiazuron products are formulated as a wettable powder. They require a 24-hour rainfree period and are also sensitive to cool weather. Dropp should not be applied when the average 24-hour temperature is predicted to be below 60 degrees F for two to three days after application. Although Dropp is slower in leaf removal than Def or Folex and may leave some bottom leaves, Dropp effectively removes younger leaves at the shoot tips. Dropp does not improve boll opening. However, Dropp will strongly inhibit regrowth when applied under favorable weather conditions. If Dropp is used under less than favorable conditions, the addition of crop oil concentrate (1 pt./A) may enhance the activity of this material. It is important to follow suggested cleanout procedures with Dropp or FreeFall.

Ginstar: Ginstar is an emulsifiable concentrate formulation of thidiazuron (active ingredient in Dropp and FreeFall) and diuron (Karmex, Direx). It is applied to mature cotton at least 5 days before harvest, but defoliation may take longer under cool conditions. Addition of adjuvants or excess rates can cause desiccation or “stuck leaves” under warm conditions. Tank mixtures with ethephon and Finish have shown enhanced boll-opening activity and good defoliation results.

Ethephon: (Ethephon 6, Prep, SuperBoll): The active ingredient is ethephon. Ethephon is used primarily for boll opening, but can defoliate at higher rates. Increasing the rate of boll opening may permit harvesting operations to start several days earlier and increase the percentage of the crop harvested during the first picking. Ethephon is labeled to accelerate boll opening and to enhance defoliation. See respective labels for rates. Cotton should be mature and at least 50 percent of the bolls should be open when the application is made. Early application on immature bolls can result in lower micronaire values but care should be taken to avoid premature application which may reduce overall yield.

Paraquat (Gramoxone Max): Paraquat can aid in opening of mature bolls when 2.1 to 3.3 oz./A are mixed with Def, Folex, Dropp, Harvade, or Ethephon.

Leafless: Leafless is a combination of thidiazuron (Dropp) and dimethipin (Harvade). It combines the benefits of dimethipin listed above with good to excellent regrowth inhibition, and removal of juvenile growth provided by thidiazuron. Limited research has also shown that split applications of Leafless are effective in defoliating rank cotton. The recommended rate of 10-12 ounces per acre delivers the equivalent of 0.125-0.15 lbs. Dropp and 6.7-7.7 ounces per acre Harvade. If morningglory desiccation is desired, additional Harvade and Prep will be required. Crop oil concentrate at 0.5 to 1.0 pints per acre should be added to Leafless. The addition of ethephon will enhance defoliation and boll opening.

Aim: Aim is a herbicidal type defoliant that should be applied to mature cotton with at least 60-70% open bolls. Aim activity is extremely rapid and growers may need to be careful in fields where conditions favor desiccation, such as rank growth or high temperatures. Full rates may not be needed when temperatures exceed 90 degrees F. The label states that Aim should be applied with a 1 percent by volume crop oil (1 gallon per 100 gallons of spray solution) and activity is enhanced with the addition of an ethephon product. However, research has shown that COC is not needed when tankmixed with ethephon in hot conditions. Aim has excellent activity in desiccation of juvenile growth, but does not inhibit regrowth. In situations in which two applications are necessary, Aim has performed very well as the second application. Aim has shown excellent activity in desiccating morningglories.