In just a few months, cotton pickers will bear down crop rows to reap the benefits of a good growing season so far amid the highest crop prices in years.

Now is a good time to review defoliation techniques to ensure the plants are in good shape before harvest.

“There is an art to defoliation that comes from understanding cotton physiology, years of experience, and monitoring crop field conditions and the weather,” said Pedro Andrade, University of Arizona assistant professor and specialist at the Maricopa Agricultural Center (MAC) in Maricopa, Ariz.

Timely and effective defoliation is a critical part of late-season cotton management. Defoliation is a two-step process utilizing the plant’s incredible physiological resources to self defoliate; combined with effective chemical defoliants which allow mechanized pickers to quickly and efficiently harvest cotton.

“In modern agriculture, we want to terminate the crop, harvest, and move on,” Andrade told growers and pest control advisors at a UA mid-season cotton management workshop in Yuma, Ariz. “A quick harvest is especially important when double cropping.”

Farmers push plant development during the growing season by maximizing applications of nitrogen, water, and other inputs to attain good yields. After maturity, inputs are reduced to end the growing cycle.

Andrade shared important defoliation criteria developed by Jeff Silvertooth, head of the UA Department of Soil, Water, and Environmental Science.

Physiological changes move the plant naturally into senescence (biological aging) where the leaves change from an intense green to a purplish color due to the buildup of the pigment anthocyanin. The natural process involves the accumulation of the enzymes pectinase and cellulase in the abscission zone which digest the cell walls where the stem and the petiole join.

While the leaf stops growing, stem growth continues. This physical imbalance separates the leaf and it falls to the ground. Andrade warns farmers not to allow the soil and the plant to get too dry during this critical period since dryness can cause dead leaves to stick to the plant and create problems at harvest.

Environmental factors including ambient temperatures and relative humidity also impact the natural defoliation process.

Ground-applied defoliation chemical helps maintain cotton quality during the mechanized harvest. Keys to chemical use include the application timing and the rate of the active ingredient and carrier dilution.