Harvest aid chemicals are applied to cotton to increase the rate of leaf loss and desiccation before harvest. Use of these materials allows timely harvesting operations. The primary goals of applying these materials are:
- Stimulate boll opening and maturation.
- Achieve more efficient mechanical harvesting at a time during good weather conditions and the availability of harvest equipment. It is critical to harvest before rain and fog conditions arrive.
- Maximize the collection of harvestable crop.
- Preserve high fiber quality to provide maximum economic returns.
Determining which harvest aid chemicals to use is a complex management decision. Factors such as late-season crop vigor, nitrogen status, and plant water status exert a significant influence on the success of cotton defoliation and desiccation efforts in preparation for harvest. Decisions on whether one or more chemical materials should be used and proper rates and timing will vary according to crop conditions. Generally the process from application to harvest is a 14- to 21-day period. It can take longer with a delayed crop and cool fall weather. Weather conditions (principally air temperature), patterns of boll set and relative boll maturity, crop vigor, and desired harvest schedule also impact choice of materials and their relative efficacy.
The basic categories of chemicals used as harvest aids include boll openers-conditioners, boll openers-enhancers, true defoliants, desiccants, and regrowth inhibitors. Some harvest aid chemicals impact the cotton plant in more than one of these ways.
- Boll openers-conditioners are often recommended in combination with a range of defoliant materials to increase the percentage of open bolls in preparation for a once-over harvest. They are often used with late-maturing crops when the weather may be too cool to provide enough heat units to open late bolls.
- Boll openers-enhancers have an effect similar to openers-conditioners but have been found in most cases to also reduce vegetative regrowth.
- Defoliants are chemicals that either impact plant hormonal activity related to leaf loss or cause direct injury to leaves, both at a level that promotes leaf drop (abscission). Their activity varies with chemical and conditions but takes days or weeks to remove leaves from the plants.
- Desiccants produce quick injury that is more severe than that seen with defoliants, causing leaf dehydration and death within one to several days. Desiccants are often applied as a follow up after application of defoliants.
- Regrowth inhibitors are applied primarily to inhibit late vegetative growth (regrowth) or to enhance activity of defoliant materials.
For more details on harvest aid materials, how they work and application information, see Harvest Aid Materials and Practices for California Cotton. If used improperly, harvest aid materials can injure cotton or neighboring crops. Always follow labels and consult with County Agricultural Commissioners for local regulations.
| Common name | Amount per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| For tank mixes, observe all directions for use on all labels, and employ the most restrictive limits and precautions. Never exceed the maximum a.i. on any label when tank mixing products that contain the same a.i. | ||||
| BOLL OPENERS–CONDITIONERS | ||||
| A. | ETHEPHON | 1–1.995 lb a.i. | ||
| (Prep, Ethephon 6, Super Boll) | 1.3–2.6 pt | 72 | 7 | |
| COMMENTS: Not labeled as a defoliant but may result in defoliation at higher rates or when crop is well prepared for defoliation. Can reduce micronaire and fiber strength if immature bolls are opened when applied too early (before 4th node above cracked boll [NACB]). Often combined with defoliant materials such as Def, Folex, Ginstar, Harvade, or Dropp. Is not compatible with sodium chlorate because the mix can release chlorine fumes. Do not apply if rain is expected within 6 hours. | ||||
| BOLL OPENERS–ENHANCERS | ||||
| A. | ETHEPHON+AMADS | 1.7–1.995 lb a.i. | ||
| (CottonQuik) | 3–3.5 qt | 72 | 7 | |
| COMMENTS: Ethephon plus the synergist AMADS (aminomethanamide dihydrogen tetraoxysulfate) to improve defoliation. Results are best with cotton that is cutout, with mature leaves. Limited regrowth control unless mixed with other harvest aids. Do not mix with sodium chlorate because chlorine gas will be formed. | ||||
| B. | ETHEPHON+CYCLANILIDE | 1–1.995 lb a.i. | ||
| (Finish-6 Pro) | 1.3–2.6 pt | 72 | 7 | |
| COMMENTS: Ethephon plus a synergist to improve defoliation, particularly of cutout cotton with mature leaves. Also provides some regrowth control but gives best regrowth control in combination with other materials such as tribufos. Can be tank mixed with other materials (except for sodium chlorate because mixture releases chlorine gas). | ||||
| DEFOLIANTS | ||||
| A. | CARFENTRAZONE | 0.0097–0.0148 lb a.i. | ||
| (Shark) | 0.66–1 oz | 12 | 7 | |
| COMMENTS: Has activity as both a defoliant and a desiccant. May be used alone or tank mixed with other harvest aids. Use a crop oil concentrate at 1% volume per volume. Apply when 65% of harvestable bolls are open. Good coverage is essential for defoliation. May require a second application. Do not apply more than 2 oz/acre total as a harvest aid. Usually used as a secondary treatment. Provides burndown of remaining annual morningglory. | ||||
| B. | PYRAFLUFEN ETHYL | 0.00446875 lb a.i. | ||
| (ET) | 2.75 fl oz | 12 | 7 | |
| COMMENTS: Has activity as both a defoliant and a desiccant. May be used alone or tank mixed with other harvest aids. Use a crop oil concentrate at 1% volume per volume. Apply when 65% of harvestable bolls are open. Good coverage is essential for defoliation. May require a second application. Do not exceed 2 applications or 5.5 fl oz/acre. Usually used as a secondary treatment. Provides burndown of remaining annual morningglory. | ||||
| C. | SODIUM CHLORATE | 3–4.5 lb a.i. | ||
| (Defol 750) | 1.6–2.4 qt | 12 | 7 | |
| COMMENTS: Can be used as both defoliant and desiccant, depending on timing and rate of application. When used at lower rate for defoliation, is less effective than thidiazuron plus diuron (Ginstar) or organophosphate defoliants on Acala and Pima cotton. Higher rated used for desiccation may stick leaves to plants. No major effect on limiting regrowth and is usually ineffective in preparing young leaves for senescence. Usually used as a secondary treatment in combination with paraquat, carfentrazone (Shark), or pyraflufen (ET). Provides burndown of remaining annual morningglory. | ||||
| D. | THIDIAZURON | 0.1–0.2 lb a.i. | ||
| (Dropp) | 3.2–6.4 oz | 24 | 5 | |
| COMMENTS: Works well in controlling regrowth and in removing younger leaves. | ||||
| E. | THIDIAZURON+DIURON (preconditioning) | 0.046875–0.07 lb a.i. | ||
| (Ginstar) | 4–6 oz | 24 | 5 | |
| . . . or . . . | ||||
| THIDIAZURON+DIURON (defoliation) | 0.075–0.1875 lb a.i. | |||
| (Ginstar) | 6.4–16 oz | 24 | 5 | |
| COMMENTS: Has better activity than thidiazuron alone in California. Low to medium rates work well under warm-to-hot conditions and have good activity on young leaves. Use highest allowed rate only under cool conditions and when applying to Pima or very vigorous Upland varieties; high rate during warm conditions can desiccate and stick leaves. Label allows mixing with ethephon products but not with tribufos or other phosphate defoliants. Do not apply more than 1 pt/acre/season. Under cool conditions or when cotton is vigorous works best combined with ethephon treatments. Apply at least 5 days before harvest. | ||||
| F. | TRIBUFOS* | 0.9975–1.875 lb a.i. | ||
| (Folex 6) | 1.33–2.5 pt | 7 days | 7 | |
| COMMENTS: Organophosphate –based defoliant. Effective for defoliation in both Pima and Acala cotton under a wide range of crop and environmental conditions. Very good and fairly quick at removing mature leaves. Works best if tank mixed with ethephon. Not effective in regrowth control or in removing younger leaves. Do not apply more than 2.5 pt/acre/crop season. Effective in reducing whiteflies, especially in combination with a pyrethroid. | ||||
| DESICCANTS | ||||
| A. | PARAQUAT* | 0.0578–0.325 a.i. | ||
| (Gramoxone Inteon) | 3.7–20.8 oz | 24 | 3 | |
| COMMENTS: Considered a desiccant because at label rates it rapidly desiccates leaves and can cause them to stick to the plants rather than to abscise. Used to help open mature bolls by causing direct injury but is not generally applied as a desiccant until after 80% or more of bolls are open because it can prevent further boll development and opening if applied too early. Other crops may be sensitive to paraquat; follow label precautions and control drift carefully. See local restrictions for use. | ||||
| B. | SODIUM CHLORATE | 3–4.5 lb a.i. | ||
| (Defol 750) | 1.6–2.4 qt | 12 | 7 | |
| COMMENTS: Can be used both as a desiccant or defoliant, depending on timing and rate of application. Is effective in combination with paraquat, Shark, or ET to desiccate young leaves and control regrowth before harvest. Is low cost and has relatively low mammalian toxicity for applications near dwellings and public buildings; less damaging to some other crops than paraquat. Check label for plantback restrictions. With 2 applications, toxicity can occur to following crops. | ||||
| REGROWTH INHIBITORS & OTHER CHEMICAL ACTIVITIES | ||||
| A. | ENDOTHALL | 0.0325–0.097 lb a.i. | ||
| (Accelerate) | 0.5–1.5 pt | 48 | 0 | |
| COMMENTS: Usually considered an additive rather than a true desiccant or defoliant. Can be added to sodium chlorate or to organophosphate defoliants (Def, Folex) to increase the rate of early leaf drop, but used alone is not effective. | ||||
| B. | GLYPHOSATE | 0.6875–1.89 lb a.i. | ||
| (Roundup) | 16–44 oz | See label | See label | |
| COMMENTS: Do not apply to cotton grown for seed because seed quality and germination percentage will be affected. Works as a pretreatment applied 7–14 days before defoliation to improve regrowth control, enhance defoliation, and control some late-season weeds. Can provide regrowth control with little impact on yields or fiber quality if applied when 40-50% of bolls are open. | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest. |
| * | Permit required from county agricultural commissioner for purchase or use. |
| 1 | Group numbers are assigned by the Weed Science Society of America (WSSA) according to different modes of action. Although weeds may exhibit multiple resistance across many groups, mode-of-action numbers are useful in planning mixtures or rotations of herbicides with different modes of action. |
| NA | Not applicable. |