Harvest cotton as early as possible to avoid yield and quality losses due to insects, weathering of cotton fiber, moisture, and poor drying conditions. Harvest operations should start only after an acceptable percentage of bolls are mature and open, and after effective defoliation is achieved.
Defoliation is the last operation where management decisions can have a large impact on profit. Defoliants are applied to cotton to increase the rate of leaf loss and desiccation, allowing for timely and efficient harvesting.
Timely crop termination is often suggested when populations of whiteflies or aphids are threatening. Be sure to control threshold and greater-than-threshold populations. An additional consideration of timing defoliation and harvest is the regulation of root dislodging, residue shredding, and residue incorporation for the pink bollworm control program in the San Joaquin Valley.
Make defoliation decisions on a field-by-field basis. You will see a wide range of crop maturity and impacts of weather and crop conditions among fields. Note that fields with even vegetative cutout and good boll load will be far easier to defoliate than fields with a non-uniform fruit set. Remember that there is substantial late vegetative growth due to factors like late plantings, late irrigations, or high late-season soil nitrogen. Plan for early crop termination where whitefly infestations are severe.
To time application of defoliants, count the nodes above cracked bolls (NACB) and follow these guidelines: