The past two weeks have been excellent for defoliation. Clear skies, nighttime temperatures in excess of 65F degrees, and very few scattered showers have allowed many to shed green leaves and open bolls quickly. Unfortunately, the end of this week is forecast to bring much lower nighttime temperatures; several nights are expected to hit the low 50s. The good news- chances for rain over the next week are slim, daytime temperatures are still relatively warm, and only a few days are forecast to be partly cloudy. Still, as temperatures begin to drop, products and rates will need to be altered.
For those applying the first application to your crop in the next few days, remember Thidiazuron (Daze, Dropp, FreeFall, Takedown) has very little activity after nighttime temperatures drop below 65F. I would not include that in my mix until nighttime temps move back out of the 50s. Additionally, products containing Thidiazuron + Diuron (Ginstar, Adios) and Tribufos (Folex) become much less effective as temperatures drop below 60F. Applications containing Ginstar/Adios or Folex will subsequently require slightly higher rates to achieve the same response noted a week ago.
For those cleaning up leftover leaves and scattered open bolls, I have good news. If you are addressing juvenile growth or regrowth (leaves generally the size of a dime to the size of a half-dollar) products like Aim, ET, Display or Sharpen still work well in cooler temperatures. If you are only trying to shake large leaves at the base of the plant which were not shed during the first application, I would run Tribufos (Folex).
Boll opening in either of these situations (first shot or second shot) will definitely be slower as temperatures drop. Ethephon + Cyclanilide (Finish, Terminate) and straight Ethephon (Superboll, Boll’d, Prep, etc) work best at temperatures above 60F, so expect slower activity and a need for slightly higher rates.
Don’t forget that shedding leaves and opening bolls are physiological processes governed by a handful of hormones. The formation of the boll sutures (splits in the boll where they open) and the abscission zones at the base of the leaf petioles are very environmentally sensitive- particularly to rain, sunlight and temperature. Hopefully we will creep back into the 60s at the end of next week and it again becomes easy to achieve acceptable levels of defoliation and boll opening without ‘hot’ applications.
I’ll post another article next week concerning applying to acres which haven’t yet received an application (the time to pull the trigger on those acres is quickly approaching).