Droplet Size Determines Product Efficacy

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Crop protection chemical manufacturers and application equipment manufacturers are both looking for a simple system to establish parameters for what equipment to use with what chemicals for optimum pest control and least drift potential.

Success in determining how to better apply products will result in reduced financial/legal exposure for chemical trespass by custom applicators, ag retailers, chemical manufacturers and farmers. Additionally, government regulators will be pleased with less potential negative environmental impact from off-target chemicals.

Work focusing on determining least drift potential is being done by researchers at a couple universities and collaboration by representatives of basic ag chemical manufacturers, adjuvant and surfactant manufacturers, distributors, spray equipment manufacturers, U.S. Department of Agriculture and U.S. Environmental Protection Agency.

The EPA is expected to issue its “Verification of Pesticide Application Spray Drift Reduction Technology” in 2013. Representatives of the various stakeholders provided recommendations for the verification protocol as participants of the Spray Drift Working Group. 

Glyphosate Velvetleaf chart Research to establish data relevant to reducing risk is not on hold waiting on the EPA. The most sophisticated research is being done at North Platte, Neb., where two wind tunnels were put in place by the University of Nebraska with financial support from private corporations. A high-speed wind tunnel can evaluate aerial application technology in winds up to 210 miles per hour, and the low-speed wind tunnel can evaluate ground rig application products in winds up to 15 miles per hour.

“We’re really trying to understand what the (pesticide) droplet size looks like under various application conditions—different nozzle selection, different orifice sizes, varied applicator pressures—and how that translates into off-target movement. And then taking those same parameters, how does that translate into pesticide efficacy?,” explained Greg Kruger, University of Nebraska, assistant professor, cropping systems specialist, working at the West Central Research and Extension Center.


Boiled down to the simplest terms, the majority of work being done by Kruger, a university team and private company representatives is aimed at determining what droplet size provides the best efficacy for pesticides currently on the market. Besides there being hundreds of registered pesticides, the research is complicated by how adjuvants change the droplet size and weed control efficacy. Pesticides in mixture, in the majority of cases, come through spray nozzles in different droplet sizes than water sprayed alone.

Kruger admitted it is a daunting task to think that every product on the market could be tested under different scenarios.

“I think the key is to try and find a trend in the data. We are starting to get there. The reality is that we will never be able to address every individual applicator situation because there are so many variables. But we think there are things we can do to understand how flat fan nozzles behave compared to turbo chamber nozzles versus air-induction nozzles. I think there are trends we can pick out in terms of adjuvants, too. Every company has their own adjuvant, but if you break it down all those adjuvants fall into a few similar classes—non-ionic surfactants, crop oils, methylated seed oils, polymers, micro-emulsions and others.” Kruger said.

The latest work spurred ahead when the EPA became frustrated with the number of drift complaints they were hounded with from chemical trespass complaints.


Kirk Howatt, weed scientist, teaching researcher at North Dakota State University, Fargo, noted how the EPA was headed toward demanding pesticides, and especially herbicides, be applied with coarse to very coarse droplets to mitigate drift potential. But Howatt has determined that those large droplets don’t make sense in some situations.

“We started looking at systems and tried to identify if we increase the droplet size what will happen to the performance of a herbicide. We realized with some herbicides as you increase the droplet size the weed control decreases,” Howatt said. “For some herbicides small droplets are the best, and as they get bigger, the worse the control.”

He continued, “We have had some research plots where the weed control was about 50 percent of the maximum possible. Instead of getting 95 percent control, we were only getting 45 percent to 50 percent, and that is a big concern.”

He has primarily worked with growth regulator products such as 2,4-D and dicamba. The research of the last couple years has shown for some herbicides that small droplets are not optimal and large droplets are not optimal either. With herbicides, when droplets are categorized into five different sizes—very fine, small, medium, coarse and very coarse—very fine droplets aren’t often effective in weed control, although small droplets are quite often effective.

The research has shown that some herbicides alone or mixes containing adjuvants do not show much variance in weed control as the droplet size changes, and in those cases, use of the very coarse droplet makes the most sense to limit drift risk.

FirstRate Velvetleaf chart Howatt’s research has focused more on the weed-control efficacy in the field after application with different droplet sizes, and he has not gotten into the engineering of new nozzles or other equipment. He has relied on the TeeJet nozzle manual for approximation of droplet sizes being applied by different nozzles.

He noted because of the original simplified view from the EPA that larger droplets are better, application technology companies have been focused on promoting nozzles producing coarse to extremely coarse droplets. But bigger droplets, it is now thought, might allow more herbicide to roll off the leaves, especially depending on the plant species. There is no confirmation of this supposition as yet. 


Kruger is doing some of the same droplet size and weed control efficacy work as Howatt, but Kruger has the advantage of knowing precisely what droplet sizes are being applied based on wind tunnel testing of nozzles and specific herbicide mixes.

Representatives of all phases of the industry know that coarse droplets will in general limit drift, but if a pesticide is not going to perform when applied with the wrong droplet size, there is no justification for the farmer using the product.

Industry participation in testing of pesticides and equipment to be classified as drift reduction technology is originally going to be a voluntary EPA program for companies. It is anticipated by many who know how the EPA works that not participating will eventually result in consequences. If a company cannot reformulate a product that performs best applied with small droplets, to one appropriate for application with coarse droplets, then the dreaded buffer zone restrictions could be quite substantial.

The first emphasis by the EPA is on classifying nozzles by their drift potential so that they can be assigned a drift reduction technology rating of one to four stars. This rating of nozzles is the easiest aspect of the whole drift reduction issue to standardize.

Kruger is hopeful that the EPA protocols for reducing drift are based on research already begun. He hopes the final protocol recommendations from the EPA follow the working group-submitted information to a large degree, although, he gives the EPA credit by saying it has “good insight and has done a lot of research on drift.”


Having applicators informed and educated is a major aspect of what Greg Kruger, University of Nebraska, wants to happen. The data his team is developing is relevant to the real world, and that is why the research group created an iPhone and Android phone app so that “any pesticide applicator can easily access some of the data we have generated,” Kruger said. “What we have done with this app is basically establish a droplet size calculator.

“An applicator can enter what nozzle type is being used, what the operating pressure is, what orifice size is being used and what spray solution is to be applied. The app will provide the droplet size.”

Currently, there are limited herbicides referenced in the app. As research is completed in 2013, new spray solutions data will be added. The goal is to add different adjuvant classes and different products. He wants the app to have a “robust database behind it” so that applicators know what droplet size is being sprayed and to perform their application appropriately with limited risk of off-target drift.

The app can be found by searching the iTunes or Google Play Store with the key words of ground spray.

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Alan McCracken    
Kansas City, MO  |  February, 13, 2013 at 07:36 AM

Very interesting work, however what about the changes to the droplet size under varying conditions of temperature, relative humidity and airspeed. My conviction is that we must first control the rate of evaporation of the droplets otherwise the applicator has to change droplet size constantly to allow for changes due to evaporation losses. My research and commercial applications on millions of acres in S.America we have developed mixtures to control evaporation and hence maintain droplet size until the drops hit the target. Coincidentally EVERY product tested including herbicides, insecticides, fungicides on corn, soybeans, cotton etc have worked better and even at lower rates of active. In addition we operate with a MAXIMUM spray volume of 4 gals/acre and often as low as 1.0 gals/acre using ground rigs. By air the standard with my clients is 0.5 gals/acre which gives best performance especially so for fungicides on corn, and rust control in soybeans under conditions of very high temperatures.

Pune, India  |  August, 01, 2014 at 04:28 AM

Mr. Alan McCraken, could you please tell me, how you follow label claims . Do you simply use application rates given by pesticide manufacturer for high volume spray ( i.e dose / ha ) in very small quantity of water ( 1-4 gallons / acre) . I would be grateful , if you could tell me how you control evaporation. We are using Electrostatic Sprayers which deliver droplets of 30 -40 micron size. We use about 70 lit / ha as water volume and generally spray only during good Delta T conditions . Most formulations are WP and its is difficult to use less water with some formulations.

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