By Charmain Tan Courcelle

MISSISSIPPI STATE -- Producers and pesticide applicators trying to find the safest ways to use pesticides and reduce spray drift may find the answer blowing in the wind, say scientists involved in pesticide drift research.

"We've found that downwind distance is by far the most important variable that affects ground, boom spray drift," said David Smith, Mississippi Agricultural and Forestry Experiment Station agricultural engineer.

Smith led a team that completed a field study in Missouri, and he cooperated with Illinois researchers to look at the factors affecting pesticide drift from ground, boom sprayers. Together these investigations may pave the way for new drift reduction guidelines.

Synthetic pesticides have been used widely since the mid-1940s to protect against pest-inflicted crop damage and losses. But exposure to pesticide residues from off-target spray drift can lead to significant health and environmental consequences, as well as cause damage to sensitive plants.

State and federal agencies regulate the proper use of pesticides and provide recommendations for spray drift reduction. Still, efforts by these agencies and scientists at universities and in industry have not provided all the answers to drift problems because of the number of variables affecting spray drift, and the possible interactions between some of the variables, Smith said.

"Researchers, Extension experts and federal agencies have always told pesticide applicators, 'here is a list of things that affect spray drift,' but these variables have never been placed in priority order," Smith said. "Without information about the variables that are most significantly related to drift, an applicator could focus on controlling one or two factors that have only a small, or no, effect on drift deposits."

To find out what factors have greatest effect on spray drift, Smith evaluated how drift deposits were affected by 14 variables over three years in Missouri and 11 variables over nine years in Illinois. Some of the variables he examined included downwind distance, wind speed, air temperature, spray nozzle height, spray pressure, flow rate, nozzle orientation angle and droplet discharge speed.

"The results from the two studies complemented each other," Smith said. "In both studies, we found that for ground, boom spray drift control, the applicator has to be most concerned about downwind distance.

"The next most important variable is wind speed, but it is clearly secondary in importance to downwind distance. Air temperature also had an effect."

Smith's results are in contrast to a previous theory that held droplet size to be the most influential variable in drift from ground, boom sprayers.

"The most interesting thing that we found in these large-scale field tests is that droplet size doesn't have as significant an effect on ground, boom drift as previously thought," Smith said.

One possible explanation is the time available for the wind to interact with spray droplets during most ground, boom sprays is much less than for aerial and orchard applications, which have been used in the past as models for spray drift.

Smith said he expects the results from these two studies will provide better-defined guidance for ground, boom drift control to producers, applicators and agricultural chemical companies.

"The variables we found to be closely related to drift deposits, like downwind distance to a sensitive area or crop, wind speed and air temperature, can often be managed by making spray applications near sensitive areas at selected times on given days," Smith said.

Smith conducted the research with MAFES experimental statistician Pat Gerard and Loren Bode of the University of Illinois. Results from this research were published in "Transactions of the ASAE" (American Society of Agricultural Engineers).

For more information contact: Dr. David Smith, (662) 325-7351,