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The power of drop size

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.

Another tool that boost application accuracy is the automatic rate controller. This is not a substitute for calibration, however. A plugged nozzle can ax sprayer application accuracy. Ditto for an aged speed sensor with an adhesive magnet that falls off or a mud-caked magnet.

Calibration can boost efficacy and minimize drift. But Wolf recommends another step: requiring that applicators review droplet size charts to choose nozzle types, sizes, and pressure levels. This will help applicators meet a specified droplet classification listed on the label. Droplet size created by a nozzle becomes key when the product efficacy hinges on coverage or when off-target material becomes a priority.

Spray equipment makers include droplet size and nozzle type charts in their catalogs and websites. Droplet size corresponds to six color codes (shown in chart on preceding page). These charts also assign different color codes to nozzle type. Droplet size can differ for each nozzle, depending upon selected pressure. These are American Society of Agricultural and Biological Engineers (ASABE) standards. (Click here for more information).


New-generation venturi nozzles

The advent of venturi (air-induction) nozzles has led to excellent pest control with reduced drift. There is some concern, however, regarding reduced coverage compared to conventional nozzles, says Bob Wolf, Kansas State University Extension agricultural engineer.

Manufacturers have introduced a new generation of venturi nozzles to boost coverage. Two include Hypro's GuardianAIR and AIXR TeeJet air-induction XR flat spray tips. 

"They have been designed to overcome an issue we had with frontline venturi-style nozzles introduced several years ago for good drift control," says Wolf. "Most of those nozzles were lacking in coverage if they weren't used correctly and, in some cases, if they were used correctly. You couldn't get a small enough droplet spectrum for some types of herbicides, the ones that needed coverage."  

The new generation of venturi-style nozzles do. One drawback is they don't reduce drift as much as the earlier generation of air-induction nozzles, says Wolf. But they do reduce drift potential more than conventional nozzles.


Individual nozzle control

First there was boom section control and boom height control for more precise pesticide application. That went a long way in improving efficacy and reducing the chance of off-target pesticide application.

But there's a way to top that to get even better efficacy while reducing unnecessary applications. "There are systems that control pesticide application right down to the individual nozzle," says Bob Wolf, Kansas State University Extension agricultural engineer.

One system is the Smart Nozzle, manufactured by Harrison Ag Technologies, Mobridge, South Dakota. This system uses GPS mapping and tracking to guide sprayer location in the field. It uses a computer-driven valve to control flow based on a computer-prepared map. The computer controls flow at individual nozzles by interrupting flow on areas not intended for spraying. It also prevents overlap by halting application on areas that were previously sprayed.

The amount of chemical saved hinges upon field geography, but it can reduce chemical use up to 15% while maintaining weed control efficacy, say company officials.

The Smart Nozzle can be purchased as an add-on item to most large land-based spray vehicles. For more information, visit

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