Apply Defoliants Properly

When possible, apply defoliants in late afternoon or early morning when humidity is high and winds are calm. Defoliants work best when nighttime temperatures are 60 °F or higher and the plants are not drought-stressed. You can often have better defoliation by delaying application until periods of cool weather have moderated. Defoliation materials do not move within the plant, so adequate coverage of the foliage is essential.

Aerial Application

Good aerial applications begin with the pilot’s having thorough knowledge of equipment, climatic conditions, the material being applied, and the target organism. Aerial applicators should first pattern test their planes to determine the effective swath width and uniformity. Removing all active nozzles from the outer 30 percent of the aircraft wing is recommended to reduce drift and improve swath uniformity by reducing the amount of material being entrained into the wingtip vortex. Pilots should review and practice pre-1986 phenoxy herbicide application recommendations as a guide to reduce drift. The pilot and the customer must understand the most uniform application occurs when the aircraft’s boom is operated at a height of approximately one-fourth of the plane’s wing span above the target ; that is, 8 to 12 feet above the crop for most ag spray planes. Modify this height only for special crop and weather conditions. To reduce drift, avoid applications during periods of high winds, high temperatures, and low humidity or dead calm conditions. Be very careful when making applications near susceptible crops, commercial fish ponds or other bodies of water, and densely populated areas. Some defoliants have a characteristic offensive odor, but consideration of materials and timing of the applications might prevent complaints from neighbors. Read the product label carefully before making applications. The most effective application occurs when all of the intended rate of active ingredient is delivered to the target with reasonable plant coverage and canopy penetration and with a medium droplet size (275-400 microns VMD spray at the release point). Because of coverage and droplet size requirements of defoliants, total application volumes by air usually are in the 4 to 8 gallons/acre range. Larger finished spray volumes generally improve coverage and result in better defoliation, especially on larger plants with lush foliage (caused by heavy insect pressures, excessive or late-applied nitrogen, or untimely irrigations and/or rainfall). Most applicators use a D6-46 or D8-46 nozzle and core combination to apply these volumes and to get good coverage, droplet size, and canopy penetration. For aircraft operating 115 mph or faster, the nozzle should be oriented spraying straight back (i.e., with the airstream). For fixed-wing planes operated below 115 mph, nozzles should be smaller (D6-46) or oriented 45 degrees down and back into the airstream. You can also use a larger nozzle (D12-46) with straight-down orientation to generate the desired droplet size by shear; however, this causes variation in droplet size and contributes significantly to driftable fines in the spray cloud.

Ground Applications

In recent years high volume (15-20 GPA) and pressure have enhanced boll opening and defoliation activity and have allowed growers to obtain acceptable results with only one application. However, rank cotton may be an exception. Conventional high-clearance spray rigs with wheel shields are capable of good defoliant applications with minimum crop damage. Small cone tips (Spraying Systems TX-6 to TX-18), flat fan nozzles (Spraying Systems 80015 to 8004), and twin jet tips (TJ60-8002EVS to TJ60-8004EVS) produce the desired droplet size and coverage for nozzle spacings of 18 to 20 inches. Maintain boom height during application at 80 percent of nozzle spacing above the crop for proper overlap. Sprayer speed normally will be 3-7 mph but should be dictated by crop and field conditions, equipment capabilities, and operator skills. Pressures should be within the manufacturer’s specified range for the nozzle tip used, usually 40-60 psi for small cone tips and 20-40 psi for fan tips. These operating conditions will result in finished spray rates of 5-15 gallons/acre, with most applicators targeting for a finished spray volume of 10 gallons/acre. Calibrate the equipment before spraying begins; check and clean nozzles and strainers daily. Flush and clean spray equipment daily according to the label directions for the material being applied. Read and follow label directions and recommended application and safety practices before, during, and after applications.

Problem Situations

When it becomes apparent certain areas in the field (borders and low areas) are not defoliating as well as the majority of the field, re-treat those areas with a standard defoliation approach. To decide on re-treatment, determine if there are enough fully developed leaves on the plants to justify defoliation.

Regrowth

Second growth after first defoliation, referred to as regrowth is frequently a problem. Reapplying any of the recommended defoliants is permitted, but it may provide less than desired results due to poor coverage on small leaves and the continuing emergence of new leaves. If the regrowth problem is field wide, consider treating with a desiccant such as sodium chlorate or Gramoxone Max/Boa. In this case, treat at the earliest possible date to avoid new leaves’ reaching size enough to cause downgrading of the sample grade. Under conditions where second growth is a potential problem, consider Dropp/Freefall in the initial treatment, since it inhibits regrowth.