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ISSUE 5  May 31, 2001



The spraying season is here and with it comes the fine spray drops that occur with any spraying operation. Eliminating all the fine drift prone droplets (under 150 microns) is impossible, but applicators do have considerable control over the spray drops they are applying.

Some applicators are reducing the spray volume for foliar application of herbicides. In some cases, applicators who have been applying 10 to 12 gallons per acre are reducing this to 5 to 10 gpa or even less and increasing pressures to improve coverage. These actions are usually not recommended. Lowering application rates (gpa) reduces drop size, which can reduce deposition on the target and increase drift potential.

Research has shown that control of some weeds with contact herbicides may be reduced when spray volume is reduced. However, reduced spray volumes have little affect on weed control with most systemic herbicides as long as the chemical is applied properly at the recommended rate.

To compensate for reduced spray volume, some applicators will increase operating pressure from 30 to 40 psi to 50 to 60 psi. They believe they can drive small droplets into the crop canopy to increase coverage. In reality, the opposite occurs. A small drop will usually lose its velocity and evaporate before it reaches the plant. For example, a 50 micron drop will lose its velocity in 3 inches from the nozzle and evaporate the water carrier in 1.8 seconds reducing the spray drop down to a pure chemical drop of 17 microns. To give and idea of drop size, the thickness of a human hair is about 100 microns. In addition, small drops have low momentum and very little energy to be driven into the plant canopy. Therefore, increasing spray pressure should not be used as a substitute for spray volume. It is recommended to maintain pressures under 40 psi and if more coverage is needed, increase spray volume.

Vern Hofman
Extension Ag Engineer
Ag & Biosystems Engineering

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