The Pulse Width Modulation spraying system (PWM) is a technology developed to improve precision application of pesticides. PWM flow control involves switching an electrically actuated spray nozzle on and off very quickly in order to control the flow rate of the nozzle. This cycling takes place so rapidly the flow often appears to be constant, and the coverage remains uniform.
Controlling flow rate by adjusting the duty cycle (opening and closing of the nozzle), and cycling the frequency of an electric nozzle while maintaining a constant pressure, provides advantages over simply controlling flow by adjusting pressure. Normally, increasing spray pressure results in increased flow rate; however, increased pressure also changes the spray angle and drop size and may result in increased particle drift (Sidebar). PWM flow control provides an extremely wide range of flow rates from a single nozzle, maintaining a consistent spray angle and droplet size without having to adjust pressure.
Objectives of this study were two-fold:
1) To evaluate PWM sprayer technology on pasture and rangeland for reduction of off-target herbicide movement (drift) while maintaining herbicide efficacy.
2) To incorporate knowledge gained from field studies into practical use for land managers.
Five experiments were established to evaluate leafy spurge and Canada thistle control using PWM. Various nozzles were used with the PWM sprayer to apply herbicides so that the majority of the spray pattern consisted of 150, 300, 450, 600, 750, or 900 micron droplets (Figures 1 and 2). A tractor-mounted boom sprayer with 8002 nozzles was used as the control treatment for comparison. All herbicide treatments were applied at 17 gallons per acre (gpa) and 35 pounds per square inch (psi) pressure. Tordon® 22K herbicide at 1 pint per acre (pt/A) plus 2,4-D at 1 quart/A was applied in the leafy spurge study while Milestone® herbicide at 5 fl oz/A was applied for Canada thistle control. Separate spring or fall studies were established on June 23, 2016 and September 14, 2016 for each weed species. Leafy spurge was at the true flower or fall-regrowth stage at the time of the spring or fall treatment, respectively. Canada thistle was at the rosette to bolting stage for spring applications and at the rosette growth stage for fall.
Results and Conclusions
Herbicide applications with the PWM system on leafy spurge and Canada thistle resulted in control comparable to applications from a standard boom sprayer at all droplet sizes, except 150 microns. For example, Canada thistle control averaged about 98 percent 12 months after application (MAA) with either spring or fall application, except with the use of nozzles applying 150 microns (Table 1 and 2). Control averaged about 36 percent with the 150 micron nozzle.
Results were similar for leafy spurge (data are not shown). Tordon 22K plus 2,4-D applied with the pulse sprayer nozzles provided the same control as the broadcast sprayer, except when the 150 micron droplet size was used.
The fifth study evaluated leafy spurge control with quinclorac, applied at 12 oz/A with the PWM sprayer set at three application speeds, 5, 10, and 15 mph. The droplet size was held constant at 600 microns for all application speeds. Applications were made in June, 2017 when leafy spurge was in the flowering growth stage. Leafy spurge control averaged 95 percent 2 MAA, regardless of application speed.
The PWM sprayer can be used to apply herbicides in pasture and rangeland at a variety of travel speeds while maintaining medium-sized or larger droplets. This allows for reduced drift and more uniform coverage compared to traditional boom sprayers. Land managers adopting this technology also reduce herbicide over- and under-application resulting in more consistent invasive weed control on a variety of terrain.
For More Information
LINK TO MORE INFORMATION REGARDING PULSE WIDTH MODULATION SPRAYERS:
Published April 2018; reviewed and updated June 2019
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