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, usually resulting from the 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.
Understanding its causes is essential to minimizing its affects, according to Travis Rogers and Rich Hendler, both IVM specialists with Dow AgroSciences.
“Applicator training on the causes and effects of spray drift is critical,” said Rogers. “This is especially true since each spray season can bring with it significant numbers of new applicators.”
Understanding equipment factors
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. For example, Milestone specialty herbicide includes a “Precautions for Avoiding Spray Drift and Spray Drift Advisory” on page three of its label.
“Also, whether using a backpack sprayer or a high-volume spray gun, selecting the correct nozzle is crucial,” said Hendler. “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.”
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 are carried further. In very calm conditions, 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,” said Rogers. “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 the strip later when the wind shifts or conditions improve. Otherwise, change application methods from a foliar spray to a more targeted method such as a basal application.
Droplet size and nozzle height
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 pressure, climatic conditions and nozzle size. This is important, because, as shown in the figure below, smaller droplets can drift considerably longer distances when released at the same height.
Image provided by Dow AgroSciences LLC
Nozzle height is another factor to consider during application. The following 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 8 times or more — even under relatively calm conditions.
Source: Ohio State University Extension
Weather’s role in drift
“Applicators need to be aware of other weather conditions besides wind speed and direction prior to performing spray applications,” said Hendler. “Temperature and humidity can also 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.
Another weather condition that should be noted is a temperature inversion, which can occur when the air temperature is coolest at ground level, warms with an increase in elevation, and then gets cooler again. The calm air caused by an inversion makes it easier for spray droplets to suspend in the air and move off-target.
Responsibility rests with applicators
“It ultimately rests with applicators to apply herbicides in a manner that minimizes potential risk to people and the environment,” said Rogers. “But exercising good judgment and erring on the side of caution will always help mitigate that risk, especially when trying to avoid spray drift.”
Additionally, if an applicator is ever in doubt about whether or not to spray, it’s best to check in with a supervisor for additional guidance.
Article provided by Dow AgroSciences LLC. This article originally appeared in “Vistas,” a publication of Dow AgroSciences LLC.
Note: Milestone is a trademark of Dow AgroSciences LLC. 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.
Best Practices to Minimize Spray Drift
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 regulating pressure with either internal or external pressure regulators such as constant flow valves.
Avoid higher operating 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 dramatically reduce downwind drift deposits.
Always follow herbicide label recommendations.