Managing Herbicide Drift

There are very few things that can turn good neighbourly relations as sour as herbicide drift can. Whether it is on a large farm or in a small neighbourhood, there is no shortage of instances where friendships are strained because someone’s favourite grape tree or tomato plant was killed after a neighbour sprayed dandelions. But herbicide drift is not only a concern for the owner of sensitive crops, it is also a loss to the applicator. The goal of herbicide use is to maximize the amount reaching the target weeds, so minimizing amount moving to off-target areas ensures the herbicide is being applied at the right rates for effective weed control, while protecting neighbouring crops and friendships.

What is herbicide drift?

Herbicide drift is the movement of herbicide at the time of application or soon thereafter, through the air to any site other than that intended for application. There are two main types of herbicide drift: particle and vapor. Particle drift occurs when droplets of herbicides are carried by wind during application, to non-target crops or site. Vapor drift happens when already-applied herbicides evaporate from sprayed surfaces and move from the application site to an off-target area. The potential for vapor drift is high in hot and dry conditions and for volatile herbicides. Drift can result in crop damage and yield loss for non-target sensitive plants, as well as contaminate the environment.

How to reduce herbicide drift

Typically, there is a small window to apply herbicides and the weather doesn’t always cooperate. But although we can’t change the weather, we can adjust for it. A good place to start is to read product labels and select appropriate nozzle type and pressure while also keeping an eye on weather conditions such as wind speed, wind direction and temperature inversions. Large droplets fall faster than small droplets thereby reducing the time the droplets are exposed to wind activity. For example, experiments showed that for a falling height of 10 feet, large droplets (diameter: 1000 microns) required 1 second to land, and drifted 4.7 feet in 3 mph (4.8 km/h) wind, while a foggy droplet (diameter: 5 microns) required 66 minutes to fall and drifted for 3 miles (4.8 km) under the same wind conditions.1 Droplet size can be increased by reducing spray pressure, increasing the size of nozzle opening, and adding suitable drift retardants. Herbicide drift is also influenced by boom or flying height. The higher the height from which the droplet is released, the further the wind can move it away from the target area before it lands. Keep the boom only as high as it needs to be for adequate coverage. It is also important to be aware of plants grown close to herbicide application and to leave an adequate buffer zone between treated fields and sensitive plants.

Large droplets fall faster than small droplets. That’s why the time the droplets are exposed to wind activity is reduced. For example, experiments showed that for a falling height of 10 feet, large droplets (diameter: 1000 microns) required 1 second to land, and drifted 4.7 feet in 3 mph (4.8 km/h)  wind. Meanwhile, a foggy droplet (diameter: 5 microns) required 66 minutes to fall and drifted for 3 miles (4.8 km) under the same wind conditions.¹ Droplet size can be increased by reducing spray pressure, increasing the size of nozzle opening, and adding suitable drift retardants.

Herbicide drift is also influenced by boom or flying height. The higher the height from which the droplet is released, the further the wind can move it away from the target area before it lands. Keep the boom only as high as it needs to be for adequate coverage. It is also important to be aware of plants grown close to herbicide application and to leave an adequate buffer zone between treated fields and sensitive plants.

-Ikenna Mbakwe, PhD, PAg
Head of Research
SynergyAG

Reference

1. Dexter AG, 1995. Herbicide spray drift. North Dakota State University Extension Service, Fargo, ND.