Colquitt County Ag Report

News, events, and happenings in agriculture.

Cotton Defoliation Considerations for 2018 (Freeman and Whitaker)


Most of our early planted cotton is quickly approaching (and has approached in some areas) time for defoliation. Cotton defoliation tends to be one of the most important aspects of cotton production each year. Timing and product selection are two of the more critical components regarding defoliation and confusion occurs not only by the thousands of tank mix concoctions but also the differences in personalities of growers with some wanting to pull the trigger too early and some wanting to wait on the very last boll to crack.

Timing of cotton defoliant application can be determined in many ways. However, the methods which we feel are most beneficial involve monitoring boll opening. The cotton plant has bolls of different ages and those bolls will typically open in the order in which they were set on the plant (such that bolls on the bottom of the plant will open before those on the top of the plant). Research in Georgia has clearly shown defoliation application should be made when 60 to 75% of the bolls on the plant are open. Whether one should lean towards 60 or 75% depends upon how uniform the crop is, as a plant that grown under optimum conditions can often be defoliated earlier than one that has gone through periods of stress. In any situation, it is almost always most profitable to defoliate the crop when the crop reaches 75% open boll.

The process of actually determining open boll percentages can be quite time consuming considering that multiple plants in each field should be assessed (and averaged) and this process should be conducted for each field separately. Therefore, there is another method to determine proper defoliation timing which is much quicker and easier. This process involves counting the number of main-stem nodes between the uppermost first position cracked boll and the uppermost first position boll we want to harvest (in some cases there are late set bolls which will not have time to mature that are set in the top of the canopy). With this method in Georgia we consider it is most profitable to defoliate the crop when it reaches 3 to 4 nodes above cracked boll (NACB).

Another method, which can be used in conjunction with determining open boll % and counting NACB is the “sharp knife technique.” This method involves using a knife to cut the bolls in half so that you can see the cross section. A harvestable boll should have lint that strings out as you cut it and the cotton seed should have a darkened seed coat with developed cotelydons. When we say “harvestable” we mean that the boll should be able to be opened with the use of an ethephon product.  In some cases, we use this method to determine the uppermost harvestable boll when figuring NACB.    

In general, the best way to determine proper defoliation timing is to employ a combination of these methods.  A proper timing of defoliation is the first step to a successful harvest.  We should point out that most producers could defoliate their crop earlier than they typically do.  In Georgia, we have always had to balance peanut digging in with cotton harvest and cotton is typically much more forgiving than peanut when it comes to harvest timing.   However, we should make an effort to defoliate the crop when it is ready, as delays in defoliation applications can turn into lost profit.  Specifically, delays in defoliation relate into delays in harvest and those delays often turn into lost yield and fiber quality.    

Defoliation of a cotton plant can impact it in four ways.  We consider those four functions to be (1) removal of mature leaves, (2) removal of juvenile leaves and plant tissue, (3) preventing regrowth of new leaves after defoliant application and prior to harvest, and (4) opening bolls.  A particular cotton crop may need a defoliant application to impact one or more, typically all four, of these functions.  Since one product is not effective on all of these functions, tank mixtures of products are applied to enhance the overall effect of the application.    Product selection and application rates are determined by the condition of the crop (which functions need to happen) and condition of the environment (temperature, rainfall forecast, soil fertility, etc.).  The charts below show defoliant performance by plant function and rates for two of the most widely used and effective defoliant mixtures.  

Additional Comments:

Application water volume and pressure greatly impact overall performance.  Often, these two factors are as if not more important than choosing which products and what rates to utilize.  One should apply at least 15 GPA when using ground rigs (the more the better, 20 GPA is better than 15 and 25 is better than 20, etc.) and as much water volume as possible with aerial applications.  Aerial applications can be extremely effective when products are applied such that spray is blown into the canopy.  
Follow product labels concerning the addition of additives. Additives may aid in uptake of some hormonal defoliants but may also increase the chances of leaf desiccation when combined with others especially with high temperatures.  In general, when tribufos (Def) is used in the mixture, additives are not needed to improve efficacy.
Rainfastness is always a concern with all products however thidiazuron requires a 24 hour rainfree period. However, when thidiazuron is used with other products (especially when mixed with tribufos) the actual rain-free period is reduced.  If rainfall occurs before the rain-free period is reached, we suggest that you wait about 7 days and evaluate effectiveness and then make decisions on follow-up applications.



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