In order for cotton to be ready for stripper harvesting, the crop must be dry enough so that bolls can be easily snapped off of the plant. This drying effect occurs after plants have died due to exposure to freezing conditions or it can be expedited prior to cold weather through the use of desiccant type harvest aid chemicals. Harvest aid chemicals help to drop leaves, open bolls, and desiccate plants, facilitating earlier stripper harvesting, higher yields and improved lint quality (Boman et al. (2009).
Proper harvest-aid product selection, tank-mix partners, and rates vary with environmental and crop conditions. What works best in one year is not necessarily the best for the next season. Effectiveness of harvest-aid chemicals is always a concern. Several factors affect the performance or lack of performance of harvest-aid chemicals.
These factors improve the performance of harvest-aid chemicals:
On the other hand, here are some factors that negatively affect harvest-aid chemical performance:
In general, the yield and condition of the cotton crop should determine the choice of harvest-aid product. If the leaves are beginning to shed and are reddish to purple, they will more easily drop off without too much “sticking” (when leaves do not drop and are frozen on the plant). The natural process that causes leaves to drop can be stopped by stress such as a freeze or desiccant application. Also, some cotton varieties do not defoliate properly. Increased leaf content in the harvested cotton can reduce lint quality. Drought-stressed leaves generally have a much thicker waxy coating, which can reduce harvest-aid performance.
Secondary growth (regrowth) sometimes occurs after the plants have “cut out” or stopped blooming due to drought stress or physiological maturity. If the weather is warm and rainy after an extended period of drought stress and cut out, the growth cycle can start again. You might see regrowth in the terminal and on many of the other nodes on the plant. Plants with unopened bolls or young, developing bolls are less likely to produce secondary growth. Regrowth is difficult to control because young foliage does not shed as older leaves do.
Proper spray volume and coverage are also critical to the success of a harvest-aid program. Be sure to calibrate the sprayer to deliver the correct volume at the proper nozzle pressure to ensure adequate distribution and foliage penetration. Read and follow the label directions for use of the product. The harvest-aid label contains information based on many years of testing and results. Avoid applying on windy days to reduce the hazard of spray drift to nontarget vegetation. Some harvest-aid chemicals are very toxic and should be properly handled and stored, especially around small children and pets. Harvest-aid products are basically classed in three categories: desiccants, defoliants, and boll openers.
Desiccants (paraquat formulations such as Gramoxone Inteon®, Firestorm®, Parazone® and various tank-mixes with other products) dry down the plant by causing the cells to rupture. The old rule of thumb is that desiccants are normally applied when approximately 80% of the productive bolls are open, or at two to three nodes above cracked boll. However, if enough bolls are mature (based on the knife test), then desiccants may be applied to fields with a lower percentage of open bolls.
Do not use paraquat-based desiccants when seedling-stage small grains or other crops are near targeted cotton fields. Paraquat drift can severely damage developing small grains grown for cover or harvest. Gramaxone Inteon®, Firestorm®, and Parazone® are similar products that have paraquat as the active ingredient. The Texas Department of Agriculture (TDA) has granted a 24c special local needs (SLN) label for several paraquat-based products. See labels for specifics. The SLN approves higher use rates for desiccation of stripper-harvested cotton for many Texas counties.
Paraquat applications made in the late afternoon before a bright, sunny day seem to boost the effectiveness of desiccation and tend to increase regrowth control. We suggest the use of nonionic surfactant (NIS) with paraquat. Use the NIS at a minimum rate of 0.125% or 0.25% volume/volume (v/v), depending on the percent concentration of surface-active agent (see individual product labels). You may need to increase the NIS rate to 1% v/v and spray late in the day to effectively desiccate some fields.
In some years, protoporphyrinogen oxidase (PPO) inhibitor defoliant/desiccant products applied at higher rates work well to desiccate juvenile growth and regrowth, which is often difficult to do with paraquat. PPO inhibitor products include Aim®, Blizzard®, ET®, and Resource®. Unlike the problem with paraquat, drift from desiccant rates of PPO inhibitors should not injure small grains.
Defoliants cause plants to begin developing an “abscission layer,” or zone of cells that eventually break down and cause leaves to separate from the stem and drop. Abscission is a natural process, but it is enhanced by the defoliant. Some defoliants are classified as hormonal, some are herbicidal, and some are mixtures of both.
Hormonal defoliants work two ways: (1) they enhance production of “ethylene,” a hormone that stimulates leaf abscission; or (2) they inhibit a plant’s ability to transport “auxin,” a plant growth hormone. Lower temperatures are more likely to reduce the effectiveness of hormonal defoliants than herbicidal defoliants. Hormonal defoliants include Dropp® (thidiazuron) and related products. Because of fall temperatures, Dropp® is not generally used in the Texas High Plains and Rolling Plains regions.
Herbicidal defoliants include Def® (tribufos) and related products, the PPO inhibitors (Aim®, Blizzard®, ET®, and Resource®), and low rates of paraquat or other desiccants (which injure but do not kill the leaves). Some products may have mixtures of both hormonal and herbicidal defoliants. These products include Ginstar® (thidiazuron plus diuron) and related products.
To maximize leaf drop, defoliants require fairly healthy and active leaves that still function properly and are not severely drought stressed (tough and leathery). Warm air temperatures generally enhance a defoliant’s effectiveness. According to the commonly used rule of thumb, defoliants can be safely applied when 50-60% of the bolls are open and the remaining bolls are mature enough to obtain a good yield. Defoliation generally causes mature bolls to open, but green, unopened bolls can still remain a challenge. Frequently, a killing freeze or a follow-up application of paraquat or other desiccant product is needed to allow stripper harvest of the crop.
Defoliant rates of PPO inhibitors disrupt a plant’s cell membrane, which triggers increased ethylene production in leaves and causes abscission. Texas High Plains research trials indicate that the PPO-inhibitor products can be effective defoliants, as well as effective desiccants in some instances when used at higher rates. These products tend to work equally well, but some may work better under certain crop conditions.
PPO inhibitors can be tank-mixed with other products such as paraquat, Def®, Ginstar®, Prep®, Finish 6 Pro®, and FirstPick®. We suggest the use of crop oil concentrate (COC) or other adjuvant for the Aim EC®, Blizzard®, ET®, and Resource® spray mixtures. See specific product labels for details. Failure to include proper adjuvants with these products will likely result in significantly reduced activity.
Ethephon-based boll-opener products increase the rate of boll opening and defoliation to allow for more rapid harvesting of the crop. Primary ethephon materials include Prep® and other related products such as Boll’d®, Boll Buster®, Setup®, and SuperBoll®. A few years ago, some enhanced boll-opener/defoliant products were marketed: Finish 6 Pro®, which contains ethephon and cyclanilide; and FirstPick®, which contains ethephon and urea sulfate.
These chemicals affect the natural boll-opening process, but they do not cause bolls or fiber to mature faster. Plants convert ethephon to ethylene, an aging-related hormone that speeds up abscission layer formation. Ethephon-based products usually reach a level of maximum effect within 14 days.
Tank mixes of ethephon and defoliants (for example, Def® or Ginstar®) are effective at opening bolls and dropping leaves in higher yielding cotton. Higher rates of ethephon products are often very effective for defoliation, but lower rates are generally effective for boll opening. The maximum labeled rate for ethephon products is 2 pounds of active ingredient per acre. Defoliant chemicals can be tank-mixed with ethephon products to enhance defoliation.
Ethephon must be applied to an active plant to be effective, and temperatures generally drive its effectiveness. Ethephon product labels generally state that plants need “sufficient mature unopened bolls present to produce desired crop.” Mature bolls are defined as “too hard to be dented when squeezed between the thumb and fingers, too hard to be sliced with a sharp knife, and when the seedcoat becomes light brown in color.” If you apply boll-opening products when bolls are not mature enough, you will likely see reduced lint yield and micronaire.
Results from several High Plains studies indicate that lint yield and micronaire reductions occurred when applications were made at 25% open bolls but not at 50% open bolls. Lint yields were reduced at least 10%, and micronaire was decreased by about 5%. When you first use tank mixes of boll opener and defoliant products, you often need a follow-up application of paraquat (or other product with desiccant activity) to sufficiently condition the cotton for stripper harvest in the High Plains region. Although this step adds more expense to the overall harvest-aid program, it is sometimes necessary to complete the season-long earliness investment you have made.