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Working with the Weather to Avoid Drift

May 18, 2001
This spring has been windy. For anybody who has tried to find a good day to spray, this has been obvious. In the challenge to get crop protection products applied in a timely and responsible way, a key component is drift control. The two main factors that affect drift are very different in an important way: the amount of influence the applicator has over them. The first factor is droplet size in the spray pattern and is to a large extent controlled by the applicator's decisions when setting up and operating the spray equipment. The second is the wind, which has so far escaped anyone's control. It must be worked with as is, and the applicator's choices are when to spray or not spray.

In general, wind speed increases as the atmosphere warms during the day into the afternoon and then decreases from evening into the night. If the wind increases to unacceptable speeds or gusts during the day, the window for spraying may be split between early morning and evening. In recent years there has been interest in spraying at night if it is too windy during the day.

Nighttime spraying can be an option if certain conditions are met. The most obvious need for nighttime spraying is adequate lighting. Many sprayers or tractors may need additional lights since the entire swath of the sprayer should be illuminated. The operator must be able to clearly see to the end of the boom and far enough ahead to avoid obstacles. The equipment should also be lit well enough to see that all the components are working right. This is important for the safety of both the operator and the environment.

If seeing the equipment is one concern, seeing where to drive is another. Using a smaller sprayer in row crops, it may be possible to count the rows for each swath. A foam marker at the end of the boom can help the operator keep track of rows when turning. Row counting may not be an adequate guide for very wide sprayers in row crops and is of little value in solid seeded crops when it is dark. A foam marker probably won't provide enough help in solid-seeded fields at night either. In these cases GPS and a lightbar can be used for guidance if the GPS equipment is accurate.

Some special weather conditions may not favor spraying during the night. One such condition is nighttime inversions, which can contribute to drift. An inversion is a layer of cooler air near the earth that is calm and doesn't mix with the air above it. The problem with dead calm air is that the small droplets in a spray pattern may become suspended in a concentrated cloud whose movement is unpredictable. This adds to the risk of off-target movement. A light breeze of 3 mph can provide the necessary mixing of the air to prevent such a problem.

Finally, equipping a sprayer for night work should not be used as a means to turn a 10-hour workday into an 18-hour workday. Safety and alertness must always be top priorities for the applicator. Also, be sure to check if the product you are spraying is still effective if sprayed at night.--Mark Mohr

Author: Mark Mohr

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

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