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2018 - Doing the Dicamba Math to meet the XtendiMax, FeXapan, and Engenia Label Application Requirements for Speed, GPA, and PSI

For the 2018 dicamba application season, the EPA has suggested changes to the labels for XtendiMax, FeXapan, and Engenia.  Specifically, for the purposes of this post, the recommended minimum GPA has been adjusted/suggested to be 15 GPA.  The increased GPA will help achieve better coverage while at the same time helping keep application speeds lower. The goal being to improve efficacy and help mitigate spray drift.

The maximum application speed remains at 15 MPH.  Speed in combination with GPA will influence the pressure used during the application. Though not specifed on the 2018 version of these labels, it remains essential to maintain a higher pressure when using venturi nozzles. Research supports better efficacy at the higher pressures (30-60 PSI), while still having a very low driftable fines content.  When using a rate controller, the speed changes will infuence pressure.  As we know, higher speeds will result in higher pressure and lower speeds will result in lower speed, given the same nozzle orifice size.  What is critical with the application of these products when using venturi type nozzles (as approved by the EPA), is maintaining a pressure  that is high enough to maintain a sufficient droplet size to maximize coverage.  If the pressures are allowed to drop too low, then the droplets will become larger, which is good for drift mitigation, but not necessarily supportive of the weed control needed.  On the flip side of that is when the speeds are increased causing higher pressure, more drift is likely and the application will become off-label due to the smaller droplets being created.  Every nozzle, chemical, adjuvant, and DRA on the approved list has been placed there based on wind tunnel data and using the AGDISP Model to ensure a low drift profile as established by the EPA.  Any variations from the label specified requirements are potential off-label applications.

It is also important to understand the influnce of using DRA's as required for the application of these products.  It is known that certain nozzle types in combination with a DRA and low pressures may negatively influence the spray pattern quaility, resulting in bigger droplets and poorer coverage with less control.  This is another reason to maintain the higher pressures needed to effectively operate a venturi designed nozzle with these products.

To help applicators understand the limitations imposed with the 15 MPH and 15 GPA requirements, I have prepared the two following documents doing the math to show the relationship between GPA, MPH, and PSI.  The calculations are for all the approved nozzle orifice sizes (03, 04, 05, 06) and are based on 15-inch and 20-inch nozzle spacings.  The results will be different for each nozzle spacing.  These charts are for conventional spray booms only and will not reflect any of the pulse width modulation (PWM) sprayers.  All the formulas used to determine GPM and PSI calculations are footnoted in the documents.  The acceptable combinations are based on a pressure range considerd appropritate (by the author) or otherwise specified on the label or in the manufacturers literature.  I am basically using a pressure range of 40-63 PSI (exceptions are noted).  The charts also reflect a precaution about the recommended slowdown around the edges of the field.  Note that the pressures in those situations are well below what would be necessary to maintain a quality application.

It is my recommendation that as sprayer operators, instead of using your speedometer to monitor your travel speed, pay close attention to your pressure gauge and use it as your speedometer, maintaining the PSI range that has been suggested for the nozzle being used.  Know your nozzles operating "Sweet Spot".  That may not be the manufacturers advertised pressure range. 

As mentioned in the documents, I have utilized the University of Illinois Smart Phone or Tablet App - SPRAYER CALIBRATION CALCULATOR  to make all the necessary calculations for these charts.  That app is available through Google Play or the App Store on your phone or tablet.

Please report any calculation errors.

LINKS TO THE CHARTS:

15-inch nozzle spacing - MPH & PSI Options for selected GPA's

20-inch nozzle spacing - MPH & PSI Options for selected GPA's