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.

Timing criteria

According to Silvertooth, five important timing criteria to follow based on crop and soil monitoring and boll development include:

1 - Boll development requires 600 heat units (HUs) to grow from a flower to a full-sized hard green boll. During that period, adequate soil moisture is needed and irrigations should be managed to support the uppermost bolls on the plants intended for harvest through this period.  An additional 400 HUs are required to develop a full-sized hard green boll into an open boll.

2 - Apply chemical at twice the number of days after the last irrigation. If the last irrigations were 10 days apart, apply the defoliant 20 days after the last irrigation.

3 – Growers can take a soil samples in advance for laboratory analysis to gain texture and other water-holding capacity information. AZSCHED simulation software available online allows the estimation of current evapotranspiration rates. Apply the defoliant when the soil reaches less than 50 percent plant available water.

4 – Apply the chemical when four nodes separate the first (top) cracked boll and the top first harvestable boll.

5 – Apply defoliant after the uppermost boll on the plant intended for harvest has reached maturity.  This is determined by cutting bolls and evaluating the seeds for adequate maturity development prior to defoliation.

In Arizona, historical HU data is available online from the UA’s AZMET meteorological network at http://ag.arizona.edu/azmet.

Defoliant application rates are also weather based. Silvertooth suggests a low chemical rate when the expected HU accumulation two weeks after application is more than 300 and the daytime high temperature averages 90 degrees F. A medium rate is suggested for 200 to 300 total HUs with an 80 degree daytime high. For a high defoliant rate, the expected HUs should total less than 200 with a daytime high of 70 degrees.

“High application rates are needed when there is less heat available,” Andrade said. “This is based on the physiological limitations of the plant and represents the general recommendation for all cotton production areas in Arizona and irrigated cotton in the desert Southwest.”

Follow the product label for rate information regarding weather conditions.

Good season-long pest control is also important to maintain quality leaves and to maximize the defoliant’s effectiveness by distribution over a large leaf area. Defoliant applied to the leaves is not translocated to other plant areas.

There are not many changes in available chemistries for defoliation. Of the defoliant products on the market, the most common active ingredients utilized in Arizona over the last six years include Thidiazuron (e.g. Ginstar, 58.8 percent), Sodium Chlorate (20.3 percent), followed by Tribuphos (e.g. Folex, 12.7 percent), and Endothall (e.g. Accelerate, 8.2 percent). This data is from reported 1080 forms from the Arizona Pest Management Center’s pesticide database.

New defoliation technology

What is coming down the pike in defoliation technology?

Andrade is in the early test stages of proximal sensing technology tests to improve the efficiency and effectiveness of chemical applications in cotton.

For several years, Andrade, a precision agriculture specialist, and UA research specialist John Heun, have conducted trials with proximal sensing using optical spectral sensors to measure canopy color reflectance in field crops including wheat.

The goal is to determine the field areas slower in plant development and then apply extra nitrogen and other inputs to speed the plants to optimal development.

Andrade and Heun are applying the same principle to cotton in the early-mid season, but with a secondary benefit at defoliation time.

During the defoliation spray process, sensors mounted on the front of a rig would record the latest color-based plant development information. The “electronic eyes” detect areas where plants remain physiologically active by looking at the absorption of red light by the green leaves. The defoliation sprayer, located on the back of the rig, would apply higher rates of defoliant so the entire field is defoliated more effectively.

Andrade and Heun are also testing infrared thermometry to detect active growth in the plant by examining cooler canopy temperatures. The goal is to combine canopy temperature and light reflectance information to enhance the real-time detection of active growth in the cotton plant. This research is financially supported by Cotton Incorporated.

“In the early-mid season, our sensor-based approach is looking into ways to improve crop management to help the crop develop healthy growth with high yield potential in terms of defoliation management,” Andrade said. “The greatest benefit of this technology will come from time efficiencies. The chemical defoliation process would be more effective with the first application of defoliant so growers can more quickly move into harvest.”

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