Ian Paulson, Technical Services Lead, Prairie Agricultural Machinery Institute
Pulse Beat 96, Fall/Winter 2022
OVER THE PAST four years, researchers from Agrimetrix Research and Training Ltd., the Prairie Agricultural Machinery Institute (PAMI), and the University of Saskatchewan’s College of Agriculture and the College of Engineering, have been collaborating in a multidisciplinary study investigating the impact of high clearance sprayer operating parameters on pesticide drift. Plot trials, field-scale spray drift measurements, in-field sprayer wake measurements, and numerical modeling were employed to investigate aspects of pesticide drift.
Within this study, a further understanding was found regarding the operational parameters affecting pesticide drift on the Canadian prairies. With the development of new chemical formulations and increasing crop diversity, more scientific data was required on the impact of operational parameters on spray drift as well as a crop’s sensitivity to drift. Additionally, high-clearance sprayers have become large machines that operate at relatively high speeds, so a better understanding of the aerodynamic properties and how they interact with droplets is required to maintain high productivity while minimizing spray drift. By collectively studying these factors, a deeper understanding of the link between machine-induced drift potential and the range of impacts has been gained.
PLOT TRIALS
Plot trials were conducted over two years to study the impact of off- target application of several herbicide formulations on soybeans (Engenia®, Xtendimax® and Enlist™) and canola (Simplicity™, Varro® and Paradigm™). Through the trials it was found that soybeans were extremely sensitive to both dicamba formulations (Engenia®, Xtendimax®) with estimated ED50 (50% yield response versus the control) ranging between 0.242% and 0.590% of the label rate across the two trial years and two formulations. Soybeans showed a higher tolerance to 2,4-D (Enlist™) with an estimated ED50 ranging between 4.34% and 9.02% of the label rate during the two trial years.
Across the two study years of canola, the differences between ranges of ED50 values for Simplicity™, Varro® and Paradigm™ were not statistically significant (p < 0.05), although there was a tendency for canola to be slightly more sensitive to Varro® (ED50 between 7.60 and 12.2% of label rate).
FIELD DRIFT QUANTIFICATION
Another objective of the study was to quantify pesticide drift from a high- clearance, self-propelled sprayer. Travel speed and boom height were varied during these field-scale experiments to represent two operational conditions: “low and slow” (0.64 m boom height at 14 km/h) and “high and fast” (1.14 m boom height at 40 km/h). XtendiMax® with VaporGrip® Technology was applied to a 400 m long strip of Roundup Ready® soybeans during the 2019 and 2020 growing seasons. On-swath and downwind deposits were captured using petri plates at 1, 2, 5, 10, 20, 40, 80 and 160 m downwind. Visual damage was assessed 14 days after treatment, and seed yield and plant biomass were measured at harvest.
The general relationship between deposited drift (as a percentage of applied rate) and the distance downwind is shown in Figure 1. Despite a greater windspeed during the “low and slow” (L&S) pass, greater deposition was measured far downwind in the “high and fast” (H&F) pass.
Figure 1. Drifted dose of dicamba versus distance downwind following application using petri plate samplers.
These trials indicate that dicamba poses a significant drift risk to soybeans without dicamba resistance. The severity of the damage tended to be over- estimated by visual ratings but was nonetheless very significant. The plot trials exhibited a greater sensitivity to herbicide exposure compared to the field- scale experiments, but several possible factors were identified for future study to improve agreement between the two methods.
Figure 2. Turbulence values (TKE) around and behind the sprayer when traveling at 11 m/s with a boom height of 0.64 m.
Figure 3. Velocity vector plots on a vertical plane 0.43 m behind the nozzles. View is from behind the sprayer looking forward traveling at 11 m/s with a boom height of 1.14 m.
The reduction of travel speed and lowering the boom height reduced spray drift by approximately 50% at 40 to 80 m downwind of the spray swath, despite a greater windspeed during the “low and slow” application pass. These improvements may be considered additional to other practices such as the use of coarser sprays, whose benefit is already well documented.
NUMERICAL MODELING OF SPRAYER WAKES AND SPRAY DROPLET MOVEMENT
Finally, extensive computational fluid dynamics (CFD) computer simulations were conducted. First, the changes in sprayer wake features due to different operating parameters were characterized. Then, the movement of spray droplets through the sprayer wake was simulated. These simulations were in addition to field measurements of the airflow patterns around a high-clearance sprayer during operation.1
Regions of high turbulence were identified from the simulations. An overall view of sprayer turbulence
is shown in Figure 2. Complex flow patterns were present behind the tractor unit due to the large blockage that it created, as well as behind the rear tires. The turbulence that results from these complex patterns increases the variability of the aerodynamic forces that act on the spray droplets. Furthermore, increased turbulence downstream of large obstructions on the spray boom (hydraulic cylinders, larger structural elements) was observed.
Numerical modeling enabled multiple operating conditions to be evaluated. In Figure 3, the vertical and horizontal components of air flow are visualized in an operating scenario with a high boom position (1.14 m). This scenario was compared to an operating condition with a low boom position (0.64 m). Larger vertical and horizontal components were present with the high boom, particularly near the rear tire. These components can contribute to spray droplets being directed off-target.
KEY TAKEAWAYS: REDUCING DRIFT AND ITS IMPACT
Depending on the herbicide and off-target crop, notable crop damage can occur even at small fractions of the herbicide label rate. Boom height and travel speed were both identified as influential operational factors that affect spray drift. Detrimental wake features increased in size and severity with airspeed, especially when considering the contribution of headwind. A greater boom height further increased the risk potential of spray drift due to the longer droplet travel time, the impact of the boom position on the wake structure of the sprayer and the introduction of droplets into a more severe wake near the sprayer tractor. Reducing travel speed generally reduces boom oscillations, which further enables a lower boom position to be used. As measurable yield effects were noted multiple sprayer widths downwind of the application pass, great diligence should be paid to environmental and operational conditions when spraying the headlands of a field, particularly when adjacent to a highly sensitive crop (of course, in addition to observing other buffer zone requirements per label instructions). The specifics that inform the choice of exact operational conditions of the sprayer will vary depending on a multitude of factors (wind, temperature, humidity conditions and the crop type and staging of both target and adjacent fields in combination with the applied pesticide). However, the important conclusion is that these factors be considered and adjusted for throughout the application process.
1 Paulson, I. W., Gerspacher, J., Gagnon, B. J., Landry, H., Sumner, D., & Bergstrom, D. J. (2022). Experimental Characterization of the Aerodynamic Wake of a High-clearance Sprayer with Changing Operational Parameters. Journal of the ASABE, 65 (3).
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