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Defoliation Thoughts Heading Into A Cold Front
DR. TYSON RAPER
JACKSON, TENN.
Defoliation became a little more complicated this week. While we were lucky enough to miss the mid-week rain, the next few nights are going to be quite cold (upper 40s to low 50s) and it looks like it may be next weekend before lows reach the 60s again. Changing conditions justify a change in rate and often a change in product. In this blog, I highlight a few thoughts on building concoctions as we move through a pretty substantial shift in temperatures.
Comments on defoliant concoctions
Thidiazuron (active in Daze, Dropp, Freefall, Takedown) plus a boll opener has excellent activity on juvenile and mature leaves, rarely sticks leaves, and provides excellent regrowth control BUT requires >65F to achieve the best activity. By targeting the hormonal balance at the base of the petiole, thidiazuron often allows leaves to fall while still turgid. In addition to generally producing very low rates of leaf desiccation and stick, thidiazuron can also provide very good regrowth control if applied at reasonable rates (10-14 days of regrowth control will typically require 0.1 lb active per acre- if using a 4 lb formulation, that is 3.2 oz). The one major downside of thidiazuron is the requirement of ‘heat’. Night temperatures of less than 65F greatly reduce activity. While performance of this mixture has been excellent up to this point, it will be necessary to phase another product into this mix as temperatures drop slightly; I would not run a straight thidiazuron plus boll opener mixture again this year unless night temperatures hold >65F.
Thidiazuron plus tribufos (Def, Folex) plus a boll opener has received less traction up to this point, but this will likely become a go-to mixture as we move into next weekend. The addition of tribufos ‘heats up’ the mixture and creates more injury to the leaf. If the rate of tribufos is too high, the abscission zone may not form quickly enough and one of two things may happen; 1) the petiole may not separate from the stalk (first image below) or 2) the petiole may separate from the stalk but the severely injured leaf may fall into an open boll or across a lower branch (second image below). In order to generate leaf stick with this mixture you have to miss rate by quite a bit; in my opinion, we generally have ‾3 ounces of forgiveness with tribufos. If running this concoction, you will still likely generate regrowth control. A thidiazuron plus tribufos plus boll opener mixture would be my preferred concoction moving into next week if temperatures hold >60F.
Thidiazuron + diuron (Ginstar, Adios, Cutout) plus tribufos plus boll opener is another common mixture as temperatures fall. The use of a thidiazuron + diuron product provides even more ‘heat’ to the mixture and creates even more injury to the leaf. This is usually a very inexpensive mix but will generate little to no regrowth control at the rates typically applied; in warm conditions, rates rarely exceed 2 ounces. A 2 ounce rate of most thidiazuron + diuron formulations contains less than two hundredths of a pound (0.015) of the thidiazuron contained in a 3.2 oz application of a straight thidiazuron product- and thereby very little regrowth control. One other comment- this mixture is fast; leaf injury generated by diuron will develop quickly. Because of this, we typically observe less than 1-2 ounces of forgiveness with thidiazuron + diuron products when temperatures are above 55F; overshoot and you will stick leaves. Fall short and you will not generate enough leaf injury to develop the abscission zone, especially with juvenile growth.
Tribufos plus boll opener is a good option, even in cool conditions, if defoliating cotton which has little to no juvenile growth. Unfortunately, tribufos will not provide much activity on juvenile growth and this mixture will not provide any regrowth control. Still, this is not a bad option for acres that have cutout hard, are ready to defoliate now, and you want to start defoliating first of next week when night temperatures are really too low to be running any thidiazuron product.
Comments on boll openers
Ethephon + cyclanilide (Finish, Terminate) is a premium boll opening mixture that works faster than ethephon alone. This product can provide some defoliation and a little regrowth control. Unfortunately, ethephon + cyclanilide mixtures are expensive. We’ve been looking at running only a portion of the boll opener in a given application as ethephon + cyclanilide and the remainder as a straight ethephon product (Prep, Boll Buster, etc.) and I’ve been very pleased with the results; while it might not be quite as fast as an application which is only Finish, a Finish/Prep blend works quicker than Prep alone.
One or two applications?
Two applications gives you flexibility, is less risky, and results in the cleanest crop. Planning for two applications will allow you to apply a reasonable, fairly safe rate in the first application to generate defoliation and start boll opening. The second application can then be formulated based on what leaves remains on the plant. Furthermore, from a boll opening standpoint, I’ve found it to be much better to have two separate, smaller applications of ethephon than one big application. Unfortunately, most of the acres I’ve walked this year will likely need two applications. Keep in mind we only have 42 oz max season use per acre on 6 lb ethephon products.
If you have questions on products or rates, please reach out to your local agent. We will continue to apply defoliant trials in the coming weeks and I’ll update recommendations as we get more results and conditions change. ∆
DR. TYSON RAPER: Cotton & Small Grains Specialist, University of Tennessee
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