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Effective Defoliation: Temperatures Make a Difference
Author: Tom Barber, Extension Weed Scientist

Harvest aid applications have been in full force since last week. Pay close attention to the weather forecast, especially temperatures and rain chances before making applications.In cooler temperatures application rates of thidiazuron (Dropp, etc.), ethephon (Prep, etc.) and tribufos (Def, etc.) will need to be increased according to the environmental conditions. If temperatures fall below an average of 65 F, effectiveness of thidiazuron products is greatly reduced.

Temperatures today (9/15) through Saturday (9/17) are too low to use thidiazuron (Dropp) products. Boll opener (ethephon) effectiveness is also reduced under cool temperatures and results will be delayed until warm temperatures return. The second application with a combination of Aim or ET with a boll opening rate of ethephon (Prep) can be applied with the understanding that the boll opener will have little effect until temperatures warm up. 

The forecast for next week shows highs reaching higher 80s which will increase effectiveness of harvest aid products. As always, use prior knowledge of field and conditions when selecting harvest aids and rates. One situation does not fit all and success will be determined based on field and environmental conditions during and after defoliation application. The following link will take you to the new defoliation publication and I have listed below some suggestions on products, tankmixes and rates. The following is not the “only” mixtures available or only way to defoliate a crop. They are simply suggestions based on previous years’ data and conversations with producers and consultants on what programs have been effective thus far.

Some of the early crop has already had one application of harvest aid and second applications should be made as soon as weather allows. In this case applications of Def/Folex 8 to 16 oz plus a high rate of ethephon (Prep, 25-32oz or Finsh 6 Pro, 25 oz) should work well. Aim (0.75-1.0 oz/A plus 1% Crop Oil) can be substituted for Def/Folex and may be necessary if re-growth has already occurred, or vines are present.

In situations where cotton is mature >80% open with natural defoliation, there may be an opportunity to defoliate the remaining leaves and open bolls with one application. In this case Def (8-12oz) plus Prep (ethephon) 32oz or Finish 25oz would be an option. Aim (0.5-0.75 oz) or ET (1.5 oz) plus crop oil can be substituted for Def/Folex. If temperatures warm up (average above 65 F) addition of Dropp (thidiazuron) at 2 oz/acre will help defoliation. If Dropp (thidiazuron) is used, rates of Def/Folex should be reduced to 5 oz/A. The total mix would be: Dropp 2 oz + Def 5 oz + Prep 32 oz or Finish 25 oz. This is a risky application and the two shot approach may be a better bet. Be sure to use at least 15-20 GPA by ground if you try this.

In situations where the cotton is mature (20-60% open with dark ring around seedcoat in uppermost harvestable boll) and a two application approach is necessary, Def 8 oz plus Prep (ethephon) 8 oz looks really good for the first application. If temperatures increase, Dropp (thidiazuron) 2oz/A + Def/Folex 5 oz/A + Prep 8-12oz or Finish 6 Pro 6 oz/A will provide good defoliation and re-growth protection. The second application will include the boll opening rates of ethephon (Prep) 32 oz or Finish 6 Pro 25 oz + Def/Folex 8-16 oz.   Aim (0.75 oz) or ET (1.5 oz) can be substituted for Def/Folex in second application.

The late cotton continues to remain green and much of it has yet to open a boll. This cotton is still young to the point that young leaves and squares are common in the top. We knew that this cotton may not accumulate enough heat units to fully mature. The extended forecast does not look very promising in regards to this late cotton. It is very likely that we will not accumulate enough heat units from now to the second week in October. With this in mind, we need to be thinking about making a harvest aid application on this late cotton before temperatures get to the point where the products are no longer effective. The first week in October looks to be the best time to begin defoliation on this late crop. Some consultants that I have spoken with will begin as early as next week on this young cotton. If the bolls in the top of the plant, still contain jelly inside the seedcoats, they are immature and will most likely not open. If the cotyledons have formed inside the seedcoat, the boll will probably open, but fiber quality will be reduced. In these situations starting out with Def/Folex (8-12 oz) + ethephon (Prep 8-10oz, or Finish 6-8 oz) followed by a second application of Def/Folex 8-16 oz + ethephon (Prep 32 – 48 oz or Finish 25 – 32 oz) depending on temperature. Aim (0.75oz), ET (1.5 oz) or Ginstar (2-4) oz can be substituted for Def/Folex in second application and may be needed if temperatures decrease.

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