Tribufos (Def, Folex): Def and Folex are phosphate-based compounds that provide good to excellent defoliation over a range of environmental conditions, but minimal regrowth inhibition.

Thidiazuron (Dropp, Freefall, Ginstar, Leafless): Dropp and Freefall provide good to excellent defoliation, regrowth inhibition and removal of juvenile growth under warm, humid conditions (higher than 70 degrees F). Activity is reduced or slowed as temperatures drop below 60 degrees F before or after application. Addition of 1 pint per acre crop oil concentrate adjuvant or the insecticide methyl parathion at 0.25 pound active ingredient per acre can increase efficacy under such conditions. Ginstar, a combination of thidiazuron and diuron (active ingredient in the herbicide Direx), provides all the positive attributes of thidiazuron alone and is more active under cooler conditions. It is, however, more likely to cause unwanted desiccation of cotton leaves than thidiazuron alone. Higher rates and tank mixtures increase this potential.

Dimethipin (Harvade 5F, Leafless): Harvade 5F provides effective defoliation of mature plants but is a weak inhibitor of regrowth. It can desiccate several weed species including morningglory and prickly sida but exhibits little activity on emerged juvenile growth. Leafless is a combination of dimethipin and thidiazuron. It combines benefits of dimpethipin listed above and good to excellent regrowth inhibition and removal of juvenile growth provided by thidiazuron. Activity of the thidiazuron component is reduced and slowed under cooler conditions (see above). Crop oil concentrate should be added to dimethipin-containing compounds.

Ethephon (Ethophon 6, Prep, Super Boll, Cottonquick, Finish): With adequate spray coverage, Prep and Super Boll accelerate the opening of mature bolls, but are not labeled as defoliants. Therefore, they should be tank-mixed with other defoliants. At low rates, ethephon can enhance the activity of other defoliants. Adequate time should be allowed before harvest for optimum boll opening effect. Cottonquick and Finish are combinations of ethephon and a synergist to increase defoliation and speed boll opening faster than ethephon alone. Neither provides adequate suppression of regrowth or removal of juvenile growth.

Carfentrazone (Aim): Aim is a newly registered compound that has shown fair to good defoliation activity, but it can increase the likelihood of desiccation. Multiple applications or tank-mixtures with other harvest aids are usually needed to achieve complete defoliation. Aim exhibits excellent activity on emerged juvenile growth but provides minimal regrowth inhibition. Limited research with Aim has shown excellent desiccation activity on weed species including morningglory.

Glyphosate (many formulations): Glyphosate provides excellent regrowth inhibition of conventional (non-Roundup Ready) cotton when applied in conjunction with defoliants or ethephon and results in excellent johnsongrass control.

Sodium chlorate: Sodium chlorate is most effective in defoliating mature leaves and provides no regrowth inhibition or removal of juvenile growth. At higher rates, it acts as a desiccant, tending to stick to cotton leaves. Sodium chlorate may be the best defoliant choice when temperatures are below 55 degrees F. Application should not be made before cotton has 85 percent or more open bolls. Sodium chlorate should not be tank-mixed with other defoliants for safety reasons.

Paraquat (Gramoxone Max): Gramoxone Max at lower rates in addition to conventional defoliants may increase defoliation of juvenile growth and stimulate boll opening. It does not, however, inhibit regrowth. Higher rates may result in cotton leaf desiccation and “freezing” of closed bolls. It may be used as a spot treatment for weed desiccation and should not be applied at weed desiccation rates before cotton is at least 85 percent open.
 
(This article was published in the summer 2002 issue of Louisiana Agriculture.)