CCN-98-8C - August 1998

DEFOLIATION CONSIDERATION FOR 98

Keith Edmisten, Extension Cotton Specialist

Like most years the crop is a mixed bag. Now we throw the hurricane in and it really mixes things up. I will post something on the hurricane damage when I have a better feel for what damage was done. You may want to refer to the two Carolina Cotton Notes from 1996 dealing with hurricane damage to cotton. No two hurricanes are alike. Our cotton in 1996 responded very differently than cotton in other States responded to Hugo and Opal.

One of the things that I think is different about this years crops is that in many fields we have not set as compact a crop as we normally do. Having the fruit spread out over a larger zone of the plant means that the fruit are not as close together in age as normal. While we can often defoliate a compact crop considerably before 60 percent open, this spread out crop will require special attention in terms of defoliation timing. Defoliation will need to be made at 60 percent open or more. In fact, percent open will not be a good tool to use on these fields. I would use nodes above cracked boll where possible. In some cases this will be difficult as the cracked boll will "disappear" once it reaches the fruiting gaps. Cutting bolls to determine maturity will be important this year.

There are several reasons for the "spread out" crop this year:

1. Many fields on Roundup Ready DPL5690 have a lot of cavitation. In some fields I have seen 6 or 7 first positions  with cavitation in a row. With a few fruit set on some early first positions and a few fruit on outer position the cotton had very little boll load to hold down growth. This has made the top crop much more important and delayed defoliation. Some of the fields I have seen cavitation in since the last Carolina Cotton Note have been fields with very little drought stress. Some varieties are more likely to have cavitation than others but I have never seen as much cavitation as we have seen in some Roundup Ready DPL 5690. Cavitation occurs when the fruit bloom causing an increased surface area and demand for moisture to start filling the boll. Cavitation occurs when the secondary wall of the xylem cannot support this increased demand.

2. Many fields suffered through severe drought, set a bottom crop, shed the middle and are now trying to make a top crop. If you feel that the top crop is worth waiting on this causes a "spread out" crop.

3. Some Roundup Ready fields have had anywhere from terrible to fair fruit retention as mentioned in an earlier Carolina Cotton Note. This lack of fruit retention makes for a situation similar to cavitation in item #1.

4. Higher than normal insect pressure in some areas has spread the crop out.

The drought stressed cotton that has since received rains makes defoliation a NIGHTMARE. In 1995 we did a defoliation test with 16 treatments on drought stressed cotton with a great amount of juvenile regrowth. Nothing worked well. Most of the treatments did a fair to good job on the top half of the plant but none of the treatments did a good job on the bottom half of the plant. Even high rates of Starfire left quite a few leaves at the bottom of the plant. Figure 1 shows the results of this study. Note that told defoliation equals 200 percent as I rated both the bottom and top halves of the plant as 0 through 100 percent. Two treatments I wish we had included in this study are 1) Ginstar at 8 oz/acre and 2) Dropp at 0.2 lb product per acre + 4 oz/acre of Def or Folex + 1 lb/acre of ammonium sulfate. One reason I lean towards these treatments is that Dropp is the best defoliant we have for juvenile regrowth

I am worried about this because South Carolina has started defoliating drought stressed cotton with tremendous regrowth and the defoliation they have experienced is similar to the miserable results we got in this test. I think part of the reason for poor activity on the bottom of the plant is that the regrowth is thick up the stalk and coverage of the bottom of the plant is difficult. Growers should use high pressures to get as good coverage as possible.

Figure 1. Defoliation of severe regrowth in 1993.