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The power of drop size

Gil Gullickson Updated: 07/28/2010 @ 2:09pm Crops Technology Editor for Successful Farming magazine/Agriculture.com

Remember that old Maxwell House coffee tagline, "Good to the last drop?"

Well, focusing on drop size can help boost pesticide efficacy and reduce the chances of off-target application, too.  This joins steps like calibration, boom height control, section control, and even individual nozzle control to help boost application accuracy. It also coincides with widespread use of postemergence herbicides that are more environmentally friendly these days.

Optimum droplet size varies between products. "If it's glyphosate, we can be safe with larger droplets for efficacy," says Bob Wolf, Kansas State University Extension agricultural engineer. "Large droplets are also good for drift control."

It's different for herbicides like paraquat. Medium-size particles are recommended for this non-selective herbicide. Meanwhile, differences magnify for other types of pesticides. 

Here you'll find how you can incorporate optimum droplet size into the calibration process. Be sure to use the chart below; it shows optimum droplet size for each pesticide class. A few developments on new-generation venturi nozzles and individual nozzle control can go a long ways in boosting efficacy and reducing off-target pesticide applications.

"The goal in all this is to apply crop-protection material to get the best efficacy possible," says Wolf. "But we want to do it only in the field we are treating. So we want drift reduction, but we still need to have good efficacy."

1: Calibrate, Calibrate, Calibrate

Sure, it takes some time at season's start. Yet, calibration is a key step to ensure pesticide application. First, step back to your high school math class and review the calibration formula. (This one actually applies to what you do!) Below are components to consider when you calibrate your sprayer.

  • Gallons per minute (GPM).
  • Gallons per acre (GPA). This is the desired application volume.
  • Miles per hour (MPH). This is speed.
  • Nozzle spacing (W = inches). Nozzles spaced 20 inches apart would have the value of 20.
  • Plug these components into this formula to achieve the desired GPM rate:
  • GPM = GPA x MPH x W

2: The nozzle comes next

"The best way to achieve efficacy is to consider the nozzle type," says Wolf.

Wolf and Scott Bretthauer, University of Illinois Extension agricultural engineer, have outlined steps to aid applicators in nozzle orifice size. This is based on the nozzle type and pressure that spray manufacturers recommend. Most applicators are familiar with using flow rate charts from spray equipment catalogs and websites to determine optimum nozzle orifice size, says Wolf.

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