Minimizing Spray Drift in Utility Herbicide Applications in Utility Herbicide Applications

Spray drift is one of the most common causes of off-target injury. This occurs when the herbicide being applied moves or drifts to areas that are not targeted by the applicator, as a result of physical movement of very small droplets, or fines from the target area at the time of application.

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Spray drift is one of the most common causes of off-target injury. This occurs when the herbicide being applied moves or drifts to areas that are not targeted by the applicator, as a result of physical movement of very small droplets, or fines from the target area at the time of application. When this happens, even in small amounts, it can lead to off-target damage to desirable vegetation or sensitive crops, or unintended environmental and financial consequences.

According to Travis Rogers and Rich Hendler with Dow AgroSciences, understanding the causes of spray drift is essential to minimizing its effects.

"Applicator training on the causes and effects of spray drift is critical," Rogers said. "This is especially true because each spray season can bring with it significant numbers of new applicators."

Understanding Equipment's Role in Prevention

There is a variety of spray equipment used to apply herbicides, and being familiar with and choosing the right equipment and treatment method can help minimize spray drift. Applicators should always start by referring to the herbicide's product label for application guidelines. As an example, all Dow AgroSciences herbicides include a "Precautions for Avoiding Spray Drift and Spray Drift Advisory" on their labels.

"Also, whether broadcast spraying or using a backpack sprayer or a high-volume spray gun, selecting the correct nozzle is crucial," Hendler said. "Nozzle types vary and control critical variables such as spray volume per minute, droplet size, spray angles and patterns. Always refer to nozzle manufacturing guidelines for information."

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Spray drift when applying herbicides can cause off-target damage to desirable vegetation on rights of way.

Two Critical Factors to Consider: Wind Speed and Direction

Wind speed is one of the most critical factors affecting drift, which is the leading cause of off-target damage. As wind speeds increase, spray droplets can be carried farther. In calm conditions, small spray droplets can remain suspended in the air and move with wind gusts. Before spraying, determine wind direction relative to desirable vegetation, especially sensitive crops, or bodies of water.

"It's also important to remember that wind direction can change during an application," Rogers said. "So it should be checked periodically-not just at the beginning of a project."

If current wind speed and direction make drift into sensitive areas likely, applicators should consider a couple of options. First, think about rescheduling spray activities to another date or time. Or determine a buffer zone at the edge of the spray area-50 to 100 feet-and spray that strip later when the wind shifts or conditions improve. Otherwise, consider changing application methods from a foliar spray to a more targeted application method such as a basal application.

The Influence of Droplet Size and Nozzle Height on Drift

After wind speed and direction, spray droplet size is the second most important factor affecting drift. Droplet sizes can vary based on a number of things such as nozzle size, pressure and climatic conditions. This is important because smaller droplets can drift considerably longer distances when released at the same height.

Nozzle height is another factor to consider during application. The previous table shows the effects of release height and wind velocity on a 300-micron droplet with a temperature of 70 degrees and a relative humidity of 50 percent. In general, doubling the nozzle height during an application can increase drift distances by eight times or more-even under relatively calm conditions.

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Applicators should be trained regularly on proper herbicide application techniques.

The Effect Weather Can Have on Drift

"Applicators need to be aware of other weather conditions besides wind speed and direction prior to performing spray applications," Hendler said. "Temperature and humidity also can affect drift, mainly through the evaporation of spray particles."

This evaporation reduces the size of the droplets in the air, making them more susceptible to movement. In general, higher temperatures and lower humidity accelerate evaporation of spray droplets and reduction in their size.

Another weather condition that should be noted is a temperature inversion, which can occur when the normal temperature gradient is inverted and a cool layer of air is trapped-usually near the ground level-below warmer air. The transition layer between the cool and warm air, caused by the inversion, can trap spray particles, stopping their fall and making them more available to drift off the application site.

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The Responsibility Rests With Applicators

"It is ultimately the applicators' responsibility to apply herbicides in a manner that minimizes potential risk to people and the environment," Rogers said. "But exercising good judgment and erring on the side of caution will always help mitigate that risk, especially when trying to avoid spray drift."

In addition, if an applicator is ever in doubt about whether to spray, it's best to check with a supervisor for additional guidance.

Keep Spray Drift to a Minimum With These Tips

• Avoid using higher pressures, which generate smaller droplets;
• Avoid treating taller vegetation near sensitive crops (soybeans, tobacco, grapes, etc.) or near water;
• Consider using drift control additives, which can greatly reduce drift potential;
• Determine wind direction;
• Periodically check weather conditions;
• Avoid using worn or improper nozzles and equipment;
• Keep nozzle heights low when spraying;
• Don't rapidly wave herbicide spray wands or guns back and forth;
• Use nozzles that apply larger droplets;
• Consider use of internal or external pressure regulators such as constant flow valves; and
• Always follow herbicide label recommendations.

When treating areas in and around roadside or utility rights of way that are or will be grazed, hayed or planted to forage, important label precautions apply regarding harvesting hay from treated sites, using manure from animals grazing on treated areas, or rotating the treated area to sensitive crops. See the product label for details.

Always read and follow label directions.

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