A variety of harvest aid chemicals exist to prepare cotton for harvest and can be classified into four main categories: defoliants, desiccants, boll openers/conditioners, and boll openers/defoliants.
These chemicals are applied to enhance the natural process of defoliation and boll opening. It is critical that the crop is stressed but still physiologically active for effective defoliation to occur.
Defoliantsare chemicals that impact plant hormonal balances to cause the leaves to fall off. Defoliant activity is highly temperature dependent. But in general, most defoliants need about two weeks to remove leaves from cotton plants.
Desiccantsnormally dehydrate and kill the leaves within several days. These are often applied after defoliants to remove the remaining leaves and/or to kill juvenile growth or young tissue at the growing points of the main stem and lateral branches.
Note that desiccants can injure un-open bolls and that sodium chlorate (a desiccant and defoliant) cannot be mixed with ethephon-containing products. Chemicals that inhibit regrowth can reduce young, green tissue which may occur at the axillary positions along the main stem.
Boll openers/conditionersare applied with defoliants to enhance the boll opening process. Ethephon-based products (boll-openers) can also be used alone as a conditioning treatment before or after a defoliation event.
Boll openers/defoliantscan increase the percentage of open bolls and reduce vegetative regrowth at the same time.
It is important to note that the application rate for harvest aid chemicals should be lower when the temperature is warmer and higher when cooler.
A rule of thumb is to use low rates when accumulated heat units (86/55 F) in the next 14 days will be over 300 (~90 F day and ~70 F night), medium rates when the heat units will be 200 to 300 (~80 F day and ~60 F night), and high rates when the heat units will be less than 200 (~70 F day and ~40 F night).
For the related tables associated with this article, click on this link: http://ag.arizona.edu/crops/cotton/files/HarvestAid2vF.pdf