Defoliation Ideas in Early September
A good deal of cotton around the state is ready to defoliate. Producers will not want to spend much money on defoliation with most of the crop having poor to average yield potential. In such a dry year the biggest concern with defoliation is regrowth. A poor crop has little bolls to fill now and residual nitrogen due to the lack of moisture. Because of this the cotton will likely have tremendous regrowth if the rains return. Different years and different crop condition often have us focus our attention on one aspect or another. Some years it may be boll opening and some years it might be defoliating under rank conditions. This year is set up to be a year to focus on regrowth.
The worst treatment for regrowth is probably a Def + Prep application. Finish does prevent some terminal regrowth but does little for basal regrowth. Many growers have changed from basing defoliation mixtures around Def to the PPO products. The PPO products include Aim, Blizzard, ET, and Resource. These product are herbicidal and do not have the hormonal mode of action that products containing thidiazuron (Dropp, Ginstar, Freefall etc.) has. When compared to Def, the PPO’s, appear to have less regrowth. This appears to be because the PPO’s do a better job of taking of small existing regrowth and not due to hormonal inhibition of future regrowth. Therefore, these products will buy us a little more time than a mixture of Def with an Ethephon product. They will not buy us as much time as products containing thidiazuron (active ingredient in Dropp and several generic products) will. The higher the rate of thidiazuron you use the longer the regrowth control you will see. The rates of thidiazuron should be chosen with harvest timing in mind.
Defoliating cotton that already has tremendous regrowth: Unfortunately we are set up to have to deal with this at some point in our defoliation schedule. It is not easy to prevent regrowth, but it is easier to prevent it than to take off severe regrowth. We have had drought years where regrowth was thick up and down the plant. It is almost impossible to take it all off. If you use a herbicidal defoliant you tend to burn the upper regrowth and not cover or do much to the regrowth on the lower part of the plant. Then you are picking through trashy cotton due to desiccation of the upper portion of the plant and green leaves at the bottom. You are probably better off to pick through green cotton entirely and avoid the trash. Any ginner will tell you they would rather you pick through green material than desiccated material as they can’t remove all the trash caused by desiccation. Products with thidiazuron in them tend to do a decent job of removing the upper portion of the regrowth and doing very little to the lower regrowth. I think in both situation the problem is coverage. Once regrowth gets heavy the leaves are all close to the main stem and coverage is very difficult. Defoliating a “normal” I usually see little difference in nozzle types. In this situation I would use hollow cone nozzles and make sure the pressure was high to get the best coverage possible. Be careful as this also increases the potential for drift problems.
Adjuvants: We hear a lot about needing special adjuvants on drought stressed cotton to get the material into the leaf. I think this is primarily a concern with thidiazuron. I would use recommended adjuvants with herbicidal defoliants and avoid the temptation to “soup up” the mixture with some adjuvant not recommended by the company. A lot of our defoliation will likely take place in high temperatures and “souped up” mixtures may cause excessive desiccation. Research in Georgia has shown that a pound of AMS (ammonium sulfate) can help get thidiazuron in the plant. I think this is probably of more benefit on the wettable powder formulations than the liquid formulations.
Mixing PPO’s and Dropp: PPO’s tend to act very quickly and burn all regrowth that is contacted with the defoliant. This tends to reduce the uptake of thidiazuron into the plant and can result in a shorter period of regrowth control than you might expect for a given rate of thidiazuron. Def is a good partner with Dropp.
I think the take home is that nobody has ever invented a defoliant that fits all situations and all possible conditions of cotton. I think a good way to go is to base your defoliation around a PPO if you are going to harvest quickly and around thidiazuron if you are not. Use boll openers where needed.