Iowa State University
of nozzle type on spray retention and absorption
by Bob Hartzler
May 8, 2003 - Increasing concerns over the negative effects of drift has led to the rapid adoption of new nozzle tips that reduce drift potential by reducing the formation of small spray droplet. While converting to nozzles that produce large droplets is an effective drift management strategy, it raises concern whether the shift in droplet size may negatively affect target coverage. A recent article appearing in Weed Science describes experiments in which Roundup was applied using three nozzle types (Feng et al. 2003). The researchers measured spray retention and determined the amount of glyphosate that was absorbed and translocated within the treated plant. Roundup Ready corn was used as the target in order to eliminate the effects of phytotoxicity on movement of glyphosate within treated plants.
The three nozzles evaluated were XR Flat Fan TeeJet (fine droplets), Turbo TeeJet (medium droplets) and the Air Induction TeeJet (coarse droplets). Roundup was applied in a volume of 20 GPA at a spray pressure of 34 PSI. The Flat Fan nozzle produced a VMD of 175 microns (50% of the spray volume contained in droplets less than 175 microns), whereas the AI nozzle had a VMD of 491 microns (Table 1). The paper did not give a VMD value for the Turbo TeeJet, simply stating it was intermediate to the others. Droplets of 150-200 microns or smaller are considered to be at risk to drift, thus at least 50% of the volume emitted by the Flat Fan nozzle was in droplets having the potential to leave the target site.
Table 1. Effect of nozzle size on retention and absorption of glyphosate by corn.
Volume Mean Diameter
Spray Retention on
|% of glyphosate absorbed|
|XR Flat Fan||175||47%||30|
1Spray retention based on maximum retention on a flat surface versus amount of glyphosate retained per unit of leaf area of corn.
The Flat Fan nozzle provided the most uniform coverage of the corn, achieving 47% of the theoretical maximum coverage (Table 1). The Turbo TeeJet and AI TeeJet resulted in approximately 10% less coverage of the corn than the Flat Fan nozzle. The authors speculated that the reduced retention with nozzles producing medium and coarse droplets may have been due to a greater likelihood of the larger droplets to bounce off leaves. It was also reported that a higher percentage of the glyphosate contacted the lower leaves when applied with the Flat Fan nozzle than the other nozzles, indicating better penetration of the canopy with small droplets.
The amount of glyphosate absorbed was determined by harvesting plants, washing herbicide off the leaf surface, and quantifying glyphosate present inside the plant. The majority of absorption occurred within the first 24 hrs of application. While less glyphosate was retained with the coarse droplets, a higher percentage of the herbicide was absorbed into the plant when applied with AI tips versus the Turbo TeeJet or Flat Fan nozzles (Table 1). It is not known why the large droplets increased absorption of glyphosate compared to smaller droplets. The glyphosate formulation used (Roundup Original) contains a tallowamine-based surfactant. These surfactants disrupt the integrity of the cell cuticle, and this disruption is believed to be at least partially responsible for the enhanced absorption when the surfactant is included in the formulation. The authors speculate that the larger droplets would create larger areas of cuticle disruption, therefore increasing absorption.
Glyphosate translocation within the corn was also examined. Glyphosate was translocated from the sites of absorption to actively growing plant parts. Thus, concentration of glyphosate in new leaves and roots increased over time. A higher percentage of the absorbed glyphosate was translocated when applied in coarse droplets (79%) than with fine droplets (70%). With coarse droplets, 23% of the glyphosate was present in new leaves three days after treatment, whereas with fine droplets only 12% was found in new leaves.
These studies document that the use of newer nozzles designed to reduce drift potential may decrease coverage and retention of the herbicide on the target. However, with Roundup the decrease in retention was countered by enhanced absorption and translocation. Thus, with a systemic herbicide such as glyphosate one would not expect a reduction in weed control due to the reduced spray retention. However, the reduced coverage and retention with coarse droplets could impact performance of contact herbicides such as the diphenyl ethers (UltraBlazer, Phoenix, Flexstar, etc.) or Liberty.
Feng, P.C.C., T. Chiu, R.D. Sammons and J.S. Ryerse. 2003. Droplet size affects glyphosate retention, absorption, and translocation in corn. Weed Sci. 51:443-448.
Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University
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