Considerations for Cotton Defoliation (Collins & Edmisten)
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The 2016 cotton crop is quickly winding down and it’s nearly time to be thinking about preparing this crop for harvest. Earlier planted fields, especially those that had little to no rain in late August, now have a noticeable number of bolls open, and the wind and rain from Tropical Storm Hermine may have caused some defoliation of some leaves. In areas that received significant rains from this storm and slow drainage thereafter, boll rot and hardlock could be a concern for bottom bolls.
Defoliation and harvest aid decisions can often be difficult as many factors influence product selection and rates. Even the most experienced growers and crop advisors may make defoliation errors, if weather or crop conditions change after an application is made. It’s generally easier to make these decisions by considering the basic underlying principles behind the defoliation process, and “breaking down” all of the components into their simplest form. The following are some considerations that should help growers with these decisions.
Physiology: It is helpful to understand a few basic principles of defoliation and senescence which can help improve defoliation decisions. It’s important to remember that we do not want to kill the cotton plant with our defoliation practices. This is a common misunderstanding. In reality, we want the cotton plant to remain alive so that we can chemically accelerate the natural senescence process before the plant would do it on it’s own (i.e. before cold weather sets in later in the fall, and well before a killing frost). Killing the plant, on the other hand, would likely cause significant bark issues and discounts in lint quality. The senescence process is largely driven by hormones released as the plant ages. As cotton leaves mature and bolls develop, the leaves become depleted of nutrients due to the demands from the developing boll load. This, along with cooler weather, leads to chlorophyll degradation and reduced chlorophyll activity, and auxins (growth hormones) begin to migrate out of the leaf. As chlorophyll begins to degrade, light-quenching compounds called anthocyanins remain, which gives leaves the bright yellow/orange/red colors that are typical in the fall. The degradation of chlorophyll releases ethylene (ripening hormone) and degrative enzymes which begins to weaken cell walls, eventually causing them to rupture, which releases more degrative enzymes and ethylene to further weaken other adjacent leaf cells. This starts the cascade of cell degradation and ethylene build-up in the leaf. Meanwhile, the auxins that migrated out of the leaf begin to accumulate in the mainstem at the base of the petiole, causing cell elongation at the junction of the petiole and mainstem. As ethylene builds on the leaf side of this junction and weakens those cells, the result is differential cell growth called the abscission zone which is the junction at which the leaf will break off from the mainstem and fall to the ground. The abscission zone does not form in the vascular cells (pipework of cells that move water, nutrients, or photosynthates into or out of the leaf), therefore we want the leaf to remain alive so that the natural water weight will help break the vascular cells with the help of a little wind. If defoliation practices are too harsh (excessive rates during high temperatures for example), then leaf cells will die too rapidly before ethylene can build up, and these leaves will dry out rapidly, resulting in “leaf sticking” or desiccation. Desiccated leaves will likely end up in harvested lint, potentially leading to discounts. Keep in mind that senescence is a natural process that we simply want to accelerate before the onset of unfavorable weather or frost.
Defoliation Timing: As you know, cotton can generally be considered safe for defoliation when 60 percent of the bolls are open and/or when the plant reaches 4 nodes above cracked boll. However, there are deviations to these rules depending on the situation. Focusing on the unopened bolls can often help improve precision of defoliation timing decisions. If a compact fruiting crop or a noticeable fruiting gap occurs for example, simply focusing on the opened bolls can be misleading, therefore relying more on nodes above cracked boll and/or slicing open a few unopened bolls to determine maturity can lead to more accurate decisions. Bolls are generally mature enough for defoliation when they are difficult to slice, when the lint is beginning to dry out and strings out when sliced, and when there is a distinguishable brownish seed coat with little or no jelly inside them.
Product selection: It would be impossible to discuss every possible scenario for product selection, application rates, and tank mixes in a single article. A detailed discussion of harvest aid products and recommended rates can be found in the NC Ag Chemicals Manual (http://content.ces.ncsu.edu/publication/north-carolina-agricultural-chemicals-manual/) and the 2016 Cotton Information Book (https://content.ces.ncsu.edu/cotton-information/cotton-defoliation). Understanding what each of these products do, and determining the need for leaf removal, boll opening, and regrowth prevention on a field by field basis with respect to soil moisture and prevailing temperatures will help make the best decisions possible. Your county agent and consultant are excellent resources for defoliation answers. Regardless, it is worth making a few quick points here.
Early in the fall, temperatures can still be relatively high. Naturally, we wouldn’t want to use rates that are too high during these temperatures. However, TDZ (thidiazuron….the active ingredient in Dropp or FreeFall etc) can often be underutilized in tankmixes during the early part of the defoliation season. Let’s be clear….not every situation will need TDZ but in some cases, ineffective defoliation could result if TDZ is not used at appropriate rates. When the cotton plant meets all the demands of its boll load and the majority of those bolls have opened, warm temperatures in late September or early October could cause the plant to “cycle around” and develop regrowth. Keep in mind that cotton is a perennial plant, and as long as there are warm temperatures, the cotton plant will try to develop new vegetative branches (basal regrowth) and renew terminal growth (terminal regrowth). Defoliation with little regard to regrowth potential will remove mature leaves and allow light to reach axillary buds at the base of each branch which tends to trigger regrowth. Since regrowth is generally small tissue, there is little demand for water in this new tissue, therefore even marginal soil moisture can result in noticeable regrowth problems. Naturally, any remaining residual N in the soil would increase the incidence of regrowth. In my observations, several growers have tried to trim or cut rates of TDZ to make room for some other product in a tankmixture. However, if the aforementioned conditions prevail, overall defoliation and regrowth control can be drastically improved by using slightly higher rates of TDZ (up to 3.2 oz/A of TDZ products containing 4 lbs a.i./gal) during the early season (but only if conditions favor the development of regrowth). If regrowth continues into cooler weather, the higher rates of TDZ or products containing TDZ+diuron (Ginstar, Adios, CutOut, several others) can be more effective. Much of the 2016 crop was planted later than normal, meaning that it will likely be defoliated somewhat later than normal and possibly under cooler conditions. Therefore, generally speaking, regrowth may not be significant in many later planted fields. However, growers are encouraged to evaluate each fields needs with consideration to regrowth materials. Fields that were impacted by dry weather in late August with a significant number of bolls open now, could very easily experience regrowth now that we have had some moisture from Tropical Storm Hermine. Additionally, some fields have lost a few leaves as a result of windy conditions over Labor Day weekend, therefore light penetration down to the axillary buds may also trigger regrowth.
Ethephon products (Prep, SuperBoll, Ethephon 6, several others) can be suitable in nearly every tankmixture. The primary role of ethephon is the stimulate boll opening and aid other products in promoting defoliation. This is a hormonal product and its activity can drastically be influenced by temperature. If little boll opening activity is needed, rates should be 1 to 1.33 pt/A, however if cool weather or a significant number of bolls need to be opened, rates should be increased to 2 to 2.667 pt/A. The season-long use rate for ethephon in any form is 2.667 pt/A. As most growers know, other products such as Finish, CottonQuik, FirstPick, contain ethephon + a synergist of various sorts, and can be used to achieve both defoliation and boll opening.
Comments on Additives: Generally speaking, it is wise to default to defoliant manufacturer recommendations (label restrictions….not necessary dealer recommendations) when it comes to the use of additives like non-ionic surfactants, crop oils, methylated seed oils, AMS, etc. These additives can offer benefits in rainfastness, uptake of defoliants, etc. but if used inappropriately, they can lead to desiccation, especially while temperatures and humidity are high. In other cases, they may be simply unnecessary. Be cautious, watch the weather, and read labels.
Nozzles and Application Volume: This may very well be the most important point in this article and it applies to every field in every situation. When we get calls about ineffective defoliation, the number 1 most common factor is…..APPLICATION VOLUME. Although some folks may get away with it from time to time, application volumes of 8 to 10 GPA run a real risk of resulting in inadequate defoliation which necessitates another application. You could potentially save yourself a lot of money and minimize the need for multiple applications by taking the time to apply appropriate application volumes, which are no less than 15 GPA and up to 20 GPA. These rates are for ground rigs….defoliation using airplanes can be effective at much lower application volumes. Nozzle selection is also important. The smaller the droplet size, the better the coverage and thus defoliation in most cases. Defoliants applied using high pressure through hollow cones are very effective at covering the entire canopy , especially when the plants are tall and the canopy is dense. However, hollow cone tips are prone to drift problems, and may be less effective when applied at high ground speeds. The very large spray droplets as observed from air induction or TTI tips do run the risk of only contacting the top leaves and could cause desiccation of the upper canopy and poor defoliation of the lower canopy, especially in taller cotton with dense foliage. A feasible tradeoff to achieving decent coverage while minimizing drift is the use of regular flat fan tips (or nozzles that produce a similar droplet size), but again, ground speed and application volume should be appropriate. As always, there are exceptions. Lower application volumes may be effective in short cotton with a sparse or open canopy, however higher application volumes are generally needed for taller cotton with more dense foliage. The use of appropriate application volumes (15 to 20 GPA) will minimize the need for a second application in many cases.