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Understand the Differences between Volatility and Spray Drift

Applicators have plenty to consider before, during and after making an herbicide application. Proper handling procedures, mixing rates, safe disposal of containers and personal protective equipment (PPE) are just a few. Each step can affect the desired result: targeted control of unwanted vegetation.

What is not desired is damaging off-target vegetation, such as nontarget trees on a right-of-way or ornamental vegetation on public or private property.

Besides applicator error, there are two main causes of off-target damage: volatility and spray drift. Understanding the differences between the two is essential to minimize their effects, and should be a matter of annual training for experienced and newly hired applicators.


Herbicide volatility: causes and ways to minimize

Herbicide volatility is the result of movement after application when the herbicide converts to a gas and moves from the application site. Volatility can occur when spray solution settles on-site and then changes to a vapor and moves off-site. Herbicide vapor can be carried off-site by wind. Potential for volatility is greatest from inert surfaces, such as rocks and pavement, where the herbicide is not absorbed.

Volatility is a characteristic of the formulation of the herbicide and, in some cases, the active ingredient, so not all herbicides have the potential for volatility. When the conditions for volatility are higher, such as higher temperatures and low humidity, using a nonvolatile herbicide will help prevent off-target vegetation damage.

As a general rule of thumb, the lower the vapor pressure of the chemical, the less susceptible it is to volatility.


Best practices to minimize herbicide volatility

  • Check weather conditions, and avoid applying in high temperatures and low humidity.
  • When spraying, avoid applying to impermeable surfaces, such as rocks or pavement.
  • Know which herbicide formulations are potentially volatile, such as ester formulations and certain active ingredients.


Spray drift: causes and ways to minimize

Spray drift is a more common concern for off-target injury. Drift occurs when small droplets or droplet fines from the application solution move to nontarget vegetation without ever landing on the target site. When this happens, it can lead to off-target damage to desirable vegetation or sensitive crops, with unintended environmental and financial consequences.

Weather is an important factor contributing to drift potential, including wind speed and direction, air temperature and humidity. Selecting the proper equipment and treatment method can help minimize spray drift, especially in relation to the droplet size produced.

As droplet size increases, the potential for drift decreases. Applicators should always start by referring to the herbicide product label for application guidelines. For example, all Dow AgroSciences herbicide labels include a “Precautions for Avoiding Spray Drift and Spray Drift Advisory” section.

Applicators are responsible for applying herbicides in a manner that minimizes potential risk to people and the environment. This includes exercising good judgment and erring on the side of caution, which will always help mitigate that risk, especially when trying to avoid spray drift.


Best practices to minimize off-target damage due to spray drift

  • Determine wind direction, and try to avoid spraying when sensitive plants are downwind and adjacent to the site.
  • Keep nozzle heights as low as possible when spraying.
  • Avoid treating taller vegetation upwind when sensitive crops (soybeans, tobacco, grapes, etc.) or water are close or adjacent to the application site.
  • Use nozzles that apply coarse, large and uniform droplets (about 400 μm or greater).
  • When using an adjustable nozzle, use the coarse (as opposed to the fine) spray setting.
  • Don’t rapidly wave herbicide spray wands or guns back and forth, as this shears spray droplets and creates more fine droplets that can drift off-site.
  • Avoid using worn or improper nozzles and equipment.
  • Consider the use of internal or external pressure regulators such as constant flow valves.
  • Avoid using higher pressures, which tend to generate smaller droplets.
  • Consider using drift control additives, which can dramatically reduce drift potential.
  • Always follow herbicide label directions.


Article provided by Dow AgroSciences.

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