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Timely harvest aid applications and harvests can help maximize the potential for cotton yield, fiber quality, and overall net return. Implementing practices to prevent moisture from penetrating harvested cotton can help maintain fiber quality until it is delivered to the gin.1
Cotton leaves naturally defoliate at physiological maturity. Defoliation may also occur if cotton plants are stressed by disease, nutrient deficiency, drought, or frost. Chemical defoliants are used as a cotton harvest aid to help remove cotton leaves, minimize lint staining and trash quantity, reduce the incidence of boll rot, increase harvest speed and efficiency, and promote earlier harvest timing within the day and the season.
Herbicidal. Works by causing injury to the plants, resulting in the production of ethylene. Ethylene production triggers leaves to drop. If the herbicidal defoliant is applied at rates too high, the leaves may die too quickly, bypassing ethylene production and causing the leaves to remain on the plant (leaf stick).
Hormonal. This type promotes ethylene production in the plant, leading to leaf drop. Hormonal defoliants can be less susceptible to leaf stick than herbicidal defoliants.2
It is important to carefully time defoliant applications because once a crop is defoliated, little to no boll maturation occurs. There are several methods that can be used to determine application timing. Traditionally, the firmness of the bolls (uppermost harvestable boll is difficult to cut with a knife), the color of seed coats (thin, brown), and the percent of open bolls (50-60%) in the field can help determine when to apply a defoliant. Growers can also apply defoliants based on plant growth, which can be a more accurate way to determine when to apply. A defoliant application is recommended after an accumulation of 850 heat units (DD60s) after cutout. Additionally, cotton plants are unable to effectively load more bolls after reaching 4 to 5 nodes above white flower (NAWF); therefore, an application after this maturity is recommended. Regardless of the method or combination of methods used, fields should be visually inspected prior to harvest aid application.2,3
Apply defoliants 10 to 14 days before desired harvest date. Make applications either early in the morning or late in the afternoon when humidity is the highest and with little to no wind. It is best to stagger defoliation to help manage harvest timing and reduce potential yield and quality loss from defoliated cotton weathering in the field. Four days after application, leaves should begin to drop and complete defoliation should occur around ten days post application.2
Drought-stressed cotton may have thick leaf cuticles which can reduce the efficacy of harvest aids, and the addition of a surfactant may be necessary. Cotton that has rank growth can require two defoliant applications due to the thick canopy preventing complete coverage. Increasing the application rate is not effective because a higher rate is just as unable to penetrate the canopy as a normal rate.2 Selecting flat-fan or hollow-cone nozzles coupled with a carrier volume of 15 gallons per acre can help maximize canopy coverage.3