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Cotton Defoliation
Author: Tom Barber, Extension Weed Scientist

 Defoliation has started with some of the earlier planted cotton and dryland fields. The weather forecast continues to look optimum for defoliant applications. There is only a slight chance of rain over the weekend and a fairly clear week ahead. Temperatures remain high; therefore many products and combinations will work well. Most of this dry-land cotton has received little to no rainfall within the last month to month and a half and has taken up less nitrogen fertilizer from the soil, meaning that there is more available for plant uptake. When it rains the drought stressed plants will take up the moisture and remaining nitrogen and try to grow again. In many of these cases this re-growth will be hard to control. The best we can do is to utilize products with thidiazuron (FreeFall, etc) at high enough rates to suppress the re-growth as much as possible. Pay close attention to the weather forecast in regards to application of the thidiazuron (FreeFall, etc.) products. If you receive rainfall within 24 hours of applying these products, the effectiveness may be reduced.

 I have had several questions on the 3-way mixtures of thidiazuron, ethephon (Prepp, FirstPick, Finish etc.), and phosphate (Folex). I think especially if the weather will cool off a bit that the three-way mix will work well, like it has in the past. There is no doubt that a phosphate (Folex) will help the thidiazuron products absorb into the plant. However this mix could get a little hot and stick a few leaves when temperatures reach the high 90’s or 100 degree mark. If you want to try it, I would use a super low rate of the phosphate products, 1:32, or 1:40. Many have asked about using a Folex in the first application instead of a Dropp or FreFall with a boll opener (Prepp, Finish,). I would be careful with the phosphate desiccants with temperatures in the 100’s. I would rather use a FreeFall product at 1.8 to 2 fluid oz/A instead of the Folex because they will be less likely to stick leaves and will also provide some re-growth inhibition if rains do come. I would rather make a second application than stick the leaves the first time around.  Generally in well irrigated cotton a lower rate range of 1:80 FreFall, 1:32 Folex and 1:24 Finish will do a really good job of defoliation with the first application. A follow-up application will be needed approximately a week later with a boll opening rate of Prep or Finish (1:4 or 1:6, respectively) in a tankmix of Folex10-12oz/A or Aim 0..75 oz/A for opening bolls in the top of the plant and removing the lower canopy leaves.

I have had several quiestions regarding the use of Roundup as a harvest aid on Liberty Link cotton cultivars.  First if you are growing the Liberty Link cotton for seed, do not use glyphosate because seed germ can be affected.  Glyphosate or Roundup can be used in combination with products listed above to aid in regrowth suppression and as a harvest aid to kill left over grass. The application rate can vary depending on the situation but in Liberty Link varieties (ST 4145 LLB2, ST 5445 LLB2 etc.) it can be used to control/prevent regrowth at rates ranging from 16-440z/A (Roundup PowerMax).  I would not substitue the Roundup for the FreeFall (thidiazuron) but use it in combination for best results and incresased suppression of regrowth.  It can be used in both applications with a 7 day pre harvest interval.  Formulations such as Roundup PowerMax contain a high surfactant load, so higher rates may cause more burn and leaf stick because of the increased levels of surfactant.  If  large grass is a problem 22 0z/A will be adequate for control and adding the Roundup to the second defoliation application will increase performance due to coverage. 

Timing continues to be a question in everyone’s mind, where some areas of the field received more water than others. In these fields the cotton may range from shoulder high and barely open to knee high and 80% open. These are difficult decisions, but it is important to look at the field as an average. The cotton that stays open longer will most likely decrease in quality if you wait for the ranker cotton to mature. If the majority of the field is ready to defoliate, go ahead and defoliate. With the hot dry weather, we may have higher mic and shorter staple on some of this cotton; timely defoliation will help with the higher micronaire problems. Before you pull the trigger, cut some bolls. Cut the uppermost harvestable boll, most of the time 4 to 5 nodes from the terminal.  If the cotyledons are developed and the seed coat is turning dark brown, the boll can be opened without decreasing yield and quality.

 The two main things that can make or break defoliation are tip selection and amount of water applied (GPA). Make sure to utilize a tip such as a flat fan, twin jet or a cone nozzle that provide good coverage and droplet size. Coverage is essential. If air-mix, or air induction tips are used, expect to make a second shot. The application should be made with no less than 15 GPA (Gallons Per Acre) by ground (20 GPA is better) and 5 GPA by air. If you have the proper combination of tips, pressure and water the product selection is somewhat less important.

For more information on harvest aid products, rates and defoliation issues click on the following link to the defoliation guide:

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