CCN-98-9A - September 1998

More Thoughts on Defoliation and Roundup Ready Cotton

Keith Edmisten, Extension Cotton Specialist

Defoliation of Drought stressed cotton:

In 1993 we ended up with tremendous regrowth prior to defoliation of much drought stressed cotton similar to the situation I described in the last Carolina Cotton Note. As I mentioned before this tremendous regrowth was very difficult to deal with. One way I think we can deal with this problem in fields that have not developed severe regrowth is to defoliate the cotton before the regrowth gets too bad with Dropp, Dropp Ultra or Ginstar. I would add 4 ounces of Def or Folex to 0.2 lb/acre of Dropp (not Dropp Ultra or Ginstar) and possibly also add 1 pound of ammonium sulfate. Some people have observed better Dropp activity on drought stressed cotton with the addition of ammonium sulfate. Dropp is slower than some of the other defoliants so I would not use it on cotton I want to pick very quickly. Basically what we are trying to do here is defoliate fields that will not be picked for a while in order to get some dropp in them and control regrowth as long as possible.

On fields that the grower would like to harvest quickly I would lean towards our standard defoliants like Def, Folex or Harvade and add one of the boll opening materials if needed to allow harvest prior to regrowth.

Roundup Ready Cotton Fruit Retention:

Although it is too early to know the full impact of cavitation, hawk-billed bolls and poor fruit retention in some Roundup Ready fields many growers are telling me they will need to make variety selection decisions before we will really have a better grasp of this situation. I will discuss below possible ways either prevent or reduce the likelihood of having problems next year.

1. One option is to go back to conventional varieties. Some growers have already indicated that this is what they plan to do at this time. I realize that everyone will not be able to or want to do that. If a grower can at least not grow all Roundup Ready cotton he doesn't have all his eggs in one basket.

2. We have seen problems or heard of problems in almost every Roundup Ready variety. So far I have not heard of any severe problems in stacked varieties. Growers may want to consider increasing the portion of their crop devoted to stacked varieties.

3. So far the problems I have seen that are severe enough that the plant will not likely be able to compensate for have been in fields with 2 or more applications of Roundup. Sometimes both were over-the-top and some times one was post-directed. One way to reduce the likelihood of having problems is to use Roundup only one time over-the-top as early as possible followed by post-direct application of traditional herbicides.

4. Severe cavitation has been mostly in DPL5690RR and to a lesser extent DPL5415RR. I don't think limiting Roundup applications as mentioned above will help prevent cavitation. I would limit the amount of acres I put into these varieties.

5. Where the weed spectrum fits the BXN system growers should consider that system to "spread their risk". In fields with sicklepod a half rate of MSMA will help slow down sicklepod so that post-direct sprays can control it. I would rather take the potential delayed maturity from a half rate of MSMA than to have cotton that looks like some of the Roundup Ready cotton this year.

6. There is another problem with some varieties that is not related to the Roundup system but to genetic predisposition. In some other cotton producing states including LA, MS, GA and SC growers have seen what is called bronze wilt, copper top or red wilt. It is believed that varieties with TAMCOT-SP37 are susceptible to this malady. The disease has been seen in the 1200 Paymaster series and in Stoneville 132 and 373. Some fields have been disced and abandoned due to this problem. Dr. William McCarty, Extension Cotton Specialist in Mississippi has some pictures and description of the disease at:

http://ext.msstate.edu/anr/plantsoil/cotton/csbw.html

He also has some pictures and explanations of potential fruit retention and cavitation problems with Roundup Ready Cotton at:

http://ext.msstate.edu/anr/plantsoil/cotton/csrr.html

7.. I know that growers don't want to hear the next point but I think it is important. There are only two ways I can think of to know if you are paying a penalty for growing Roundup Ready cotton. One is to grow some traditional varieties. The other way is to grow some Roundup Ready cotton in a traditional herbicide program with no Roundup.