Defoliation – Art Or Science?

Rusty Mitchell
Rusty Mitchell

By Rusty Mitchell, Louisville MS

  • Resident of Louisville, Miss.
  • Southern Region Technical Support Lead for FMC Agricultural Solutions.
  • Earned B.S. and M.S. from Miss. State University.
  • Earned Ph.D. from LSU in entomology.
  • Has served in numerous positions at FMC in product development and technical support.

I have been fascinated with cotton my entire life. I recall my father, while working on his graduate degree at the USDA Boll Weevil Research Lab in Starkville, Miss., in the late 1960s, returning from a trip to Guatemala and Mexico. He told me of native cotton plants he had seen that were large enough to support a man. At that young age, it was explained to me that cotton was a woody perennial plant that man had modified to grow as an annual. Thus, it is of no surprise that as we prepare to terminate the crop, it tends to resist.

Think about it. From the time we place the first seed into the ground, we begin the preparation for crop termination. Throughout the entire season, all management practices are directly or indirectly geared towards the production of lint and termination of the crop. We select varieties and a planting date for earliness. We manage fruit set for uniform maturity and boll load. We utilize plant growth regulators to minimize vegetative growth and develop a plant stature that is conducive for its final act, defoliation.

Defoliation is the use of synthetic chemicals to enhance leaf removal and allow the harvesting of a cotton crop in a timely manner. It’s a balancing act. Your objective is to enhance the natural physiological process of creating the abscission zone, the area that separates the living tissue between the plant and leaf petiole.

If you are too aggressive with your chemicals, the leaf may die before the abscission layer develops, resulting in “stuck” leaves. On the other hand, if you use too few chemicals, the abscission process may not be stimulated, resulting in inadequate leaf removal. The process is of significant importance from a profitability standpoint as it eliminates a main source of trash and stain, resulting in better grades.

There are basically two types of chemical defoliants: herbicidal and hormonal. Herbicidal defoliants, such as PPO inhibitors Aim, Display, ETX, Folex and Sharpen, injure the plant, resulting in increased production of ethylene to promote the formation of the abscission layer in the leaf and ultimately leaf drop. Hormonal defoliants, such as thidiazuron and ethephon, increase ethylene synthesis in the plant, resulting in the activation of the abscission layer in the leaf petioles and boll wall.

Boll maturity is the single most important factor to consider in timing a defoliation application. Remember, minimal maturation occurs after the removal of leaves. Premature leaf removal can result in the reduction of cotton yield and lint quality. However, late defoliation can result in increased boll rot and lint quality loss due to weathering.

The defoliation process can typically begin when 60 percent of the bolls are open, and the youngest boll one expects to harvest is mature. To check boll maturity, choose the uppermost bolls you intend to harvest and slice them cross section with a sharp knife. If mature, they should be difficult to slice. Fiber should string out when cut, and the seed coat should be light brown in color with the seed cavity completely filled with no jelly center.

Another method often used to time defoliation applications is counting the nodes above cracked boll (NACB). Research has shown that at least four nodes above the highest first position cracked boll will be mature enough not to be affected by the application of a harvest aid.

But it’s not as simple as estimating open bolls or counting NACB. Nothing ever is! One has to consider the proper harvest-aid product(s) required to achieve your specific objectives: removal of leaves, enhancing boll opening, regrowth suppression, weed desiccation, etc. And there are other factors which can directly impact the effect of your product choice – the overall condition of the plant, weather condition at the time of application, days following application and the application itself.

In closing, remember, no one harvest-aid product, rate or specific timing is the solution for every field. Decisions should be made based on prior experience, crop maturity, environmental conditions, yield potential and crop value. It’s no wonder you hear about the act of defoliation as more of an art than a science. May you be blessed with a successful defoliation season.

Contact Rusty Mitchell in Louisville, Miss., at (662) 773- 6674 or

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