Figure 1. Herbicide drift injury to alfalfa (left) and cauliflower in northeast Nebraska, May 2014. (Photos by Michael Rethwisch)

Herbicides can cause serious injury to plants when applied improperly or when non-target drift occurs. Here are five things to consider to avoid crop injury when applying herbicides this spring:

1. Types of Drift

Drift occurs in two ways, particle or vapor. Particle drift occurs when small spray droplets travel long distances during periods of high wind and blow droplets from the targeted site. To avoid this, use larger spray droplets with low pressure, and apply herbicides only when wind speed is low. Vapor drift occurs when products volatilize or evaporate and move off the application site. The volatility of some products increases as temperatures rise into the upper 80s and 90s. The product label will provide information on when it's not safe to apply the product based on certain temperatures. The highest potential for drift is when it's hot and dry. For more detailed information on types of drift see Spray Drift of Pesticides (G1773).

2. Nozzle Selection

The nozzle directly affects the size of the spray droplet. The pesticide label may require use of specific nozzles that will produce a coarse- or medium-sized droplet. Coarse droplets resist drift, resulting in a lower drift potential. In addition, when inspecting and calibrating spray equipment, check each nozzle for blockage or wear.  Make sure the output is within 5% of the manufacturer's rating for the nozzle. Clean or replace the nozzle, if necessary to achieve the desired output. In addition, consider boom height. The higher the boom, the higher the drift potential. Keep the boom only as high as it needs to be. For more tips see the UNL publication, Nozzles—Selection and Sizing (EC 141).

3. Wind Speed and Direction

Always measure wind speed and direction before, during, and after the application. Always follow label information, but in general, wind speeds of 3 to 7 mph are preferable. Never spray when wind speeds are more than 10 mph. If wind speed or direction changes during an application, immediately adjust the buffer size or location, or stop the application. Be sure to check DriftWatch to see if a sensitive site is nearby.

4. Temperature Inversions

Applying pesticides during a temperature inversion can result in damaging, long distance drift. Inversions occur when warm air, which is light, rises upward into the atmosphere and cool air, which is heavy, settles near the ground.  With these conditions of warm air above cool air, there is no mixing of air. Spray droplets are not dispersed, but stay in a concentrated mass and move with any subtle airflow that may land off-target. Typically, temperature inversions start at dusk and break up with the sunrise because of vertical air mixing.

5. Applications Plans

Planning is key when applying any pesticides. Many factors influence drift, and applicators must be willing to adjust to particular circumstances. This requires a plan of action. Remember, applicators are legally responsible for spray drift problems, even when Mother Nature is the true culprit.
Making pesticide applications is a substantial responsibility with many consequences if not done correctly. Read pesticide labels, check application equipment, and be aware of environmental conditions to reduce drift and make the best use of each product. When applying pesticides, consider the environment and safety for you, others, and other crops. For more herbicide stewardship information and considerations visit http://pested.unl.edu/herbicide-stewardship.

UNL Pesticide Education Program
Emilee Dorn, UNL Extension Assistant
Pierce Hansen, UNL Extension Assistant
Clyde Ogg, UNL Extension Educator
Erin Bauer, UNL Extension Associate
Jan Hygnstrom, UNL Extension Project Manager