Defoliation Management

by Jeffrey C. Silvertooth,
Extension Agronomist - Cotton

Crop conditions and the best defoliation program often vary considerably from year to year. Accordingly, recommendations for defoliation in a given season are often not developed until the end of the season. The purpose of this article is to outline and review several important factors associated with crop defoliation that can be useful when developing a defoliation plan.

Cotton is a perennial plant that is being managed as an annual in a production system. Defoliation is basically an effort on the part of the manager to speed up the natural physiological process involving plant senescence (aging) and leaf drop. Among several factors influencing senescence and leaf drop is plant-water relations at the time of defoliant application. A certain degree of water stress on the plant is desirable because it favors plant senescence and the formation of the abscission layer at the base of the leaf petiole. However, if plant water stress is too severe, then defoliant materials are less effective because they require a physiologically active plant. A plant that is too severely water stressed at the time of defoliant application may exhibit leaf burn, leaf dessication, and an increase in plant trash (pin trash), which can diminish grades.

Termination of irrigations starts the process of plant senescence, leading ultimately to defoliation. Therefore, it is important to identify the last set of blooms intended for harvest (realistically). These blooms will require 600 additional heat units (HU, 86/55 oF thresholds) or about three weeks to progress from a bloom to a full sized, hard, green boll (fiber length development complete). Water stress should be avoided during this period to facilitate fiber length development. Approximately an additional 400 HUs will then be needed to develop a hard, green boll into an open boll. Defoliants can be applied when the last set of bolls intended for harvest have reached sufficient maturity.

An old, standard rule of thumb is to apply defoliants when 60-65% (or more) of the bolls on the plant are open. Under irrigated conditions in the desert Southwest another rule of thumb is to apply defoliants after the final irrigation at an interval approximately 2 times that of normal in-season irrigations. For example, if normal in-season irrigation intervals are 14 days, one should wait approximately 28 days after the last irrigation to make a defoliant application. This indicator of defoliation can depend on a number of factors including: boll load, crop vigor, amount of water applied in the last irrigation, soil water holding capacity, weather, etc. Thus, it is important to watch the crop and adjust defoliant applications accordingly.

Another plant measurement used in some areas to estimate the proper timing of defoliant applications involves the use of the number of nodes above the uppermost, first position cracked boll (NACB). Common recommendations indicate proper defoliant timing when NACB is about 4. With the use of this measurement it is important to identify the top boll intended for harvest and use it as the point of reference for the NACB measurement and not to include nodes above this point in determining NACB. This is particularly critical in Arizona and low desert regions where top-growth/re-growth at the top of the plant can produce additional squares and bolls that may confound this measure. Thus, by use of the standard NACB measurement, it would be appropriate to defoliate when four nodes separate the top cracked boll and the uppermost boll intended for harvest. In Arizona, some recent studies have indicated that a NACB of about 4 may coincide with approximately 80% open bolls, which might be late to some degree for proper defoliation. Hence, this measure should be used with caution in managing defoliation.

A good method of estimating boll maturity is to cut into bolls with a knife and inspect seed development. Mature seeds should reveal seed embryos with tiny folded leaves instead of a light green "jelly-like" material (indicates immature seed).

The nitrogen (N) fertility status can also impact defoliation efforts. If a plant is high in N fertility at the end of the season, the typical effect will be a delay in maturity, a more vigorous plant, and a difficult case for defoliation with a high potential for top-growth and/or re-growth problems. From defoliation research across the state in recent years, we have found that petiole nitrate-N concentrations greater than 3,000 ppm can lead to defoliation problems. This is where applications of fertilizer N past peak bloom may complicate late season management, which is one reason we recommend that fertilizer N applications be completed by peak bloom.

As we progress through the defoliation season, rates of many defoliant materials can be adjusted in relation to weather conditions. Basically, lower rates of many defoliants can be very effective under hot conditions. As temperatures drop in the fall, increasing rates of defoliant materials will commonly be needed to accomplish a satisfactory defoliation (>75% defoliation from a single application). In Arizona, we can apply general guidelines based on expected HU accumulations in the 14 day period following defoliant application (Table 1). Exercising patience during this 14 day period is also important.

Table 1. General guidelines for adjusting defoliant rates on irrigated cotton in Arizona (rates refer to those specified by product labels)

* Heat units, 86/55 degrees F thresholds, accumulated 14 days following defoliant application.
** General day time maximum temperatures.
*** General night time minimum temperatures.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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