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Windy Conditions and Herbicide Applications

April 26, 2002
Looking back through previous years' issues of the Bulletin, I find it's been about 5 years since I've mentioned the problem of herbicide drift. During these 5 years, and continuing this year, the problem of herbicide drift remains with us. Soil conditions in many areas of the state are favorable for corn planting, but windy conditions during the week of April 15 were much less favorable for herbicide applications. Indeed, several days this spring have seen wind speeds greater than 25 miles per hour, far in excess of wind speeds indicated on many herbicide labels for safe application. Pesticide applications made when wind speeds are sufficiently high favor off-target movement and are obviously not advisable.

Spray drift is a function of droplet size and wind speed. Spray droplet size is primarily determined by nozzle type and size and by spray pressure but can also be modified by air temperature and relative humidity. Smaller spray droplets are more prone to move outside the intended target area than larger droplets, but even large spray droplets can move outside the target area when applied during windy conditions. Much interest has recently been focused on drift-reduction nozzle technology, as well as utilizing drift-reduction agents to reduce the amount of spray drift. Although these technologies can change droplet size and reduce spray drift, they may sometimes give a "false sense" of security such that applications are made when wind speeds are, in reality, too high.

We understand that, with the number of acres covered by commercial applicators, applications are sometimes made under less than ideal conditions. However, there are not many "good things" that come from herbicide drift complaints, not to mention the potential weed control problems if too much of the herbicide moves out of the intended target area. Increasing gallons of carrier per acre and lowering spray pressures are two options to help reduce the possibility of drift. Precluding drift may require keeping the sprayer in the shop until the wind dies down.--Aaron Hager and Christy Sprague

Author: Aaron Hager Christy Sprague

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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