How To Prevent Spray Drift

What Is Spray Drift?

Spray drift is the term used to describe when chemicals are sprayed, and droplets of chemicals hang in the air and are carried away from the target spray area by the wind. Spray drift can occur in three different ways, as droplet drift, particle drift or as vapour drift.

Droplet drift: The most common cause of off-target drift, droplet drift occurs when poor weather conditions carry droplets away from the spray area.

Particle drift: This occurs when water in the spray droplets evaporates, creating smaller droplets of concentrated chemicals. These smaller droplets remain airborne for longer and can drift kilometres away from the spray target area – in some cases damage has occurred to crops thirty kilometres away.

Vapour drift: Vapour drift relates to volatile herbicides that will turn into a gas after application. With volatile herbicides, vapours can rise from the surface for hours or even days after spraying. The best way to avoid vapour drift is to choose a herbicide with low volatility. 

What Causes Spray Drift?

As we have already discussed, spray drift occurs due to droplet drift, particle drift or vapour drift. These are caused by factors such as the weather conditions and chemical volatility but there are a few other factors that affect the risk of spray drift, for example, the height of the spray release, size of the target area and the application method.

Let’s discuss the factors that influence the risk of spray drift in more detail.

Factors that increase the risk of spray drift.

• This one is simple, the more herbicide applied per hectare the more herbicide available to drift away from the target.

• Aerial application generally sprays at three metres above the target but at low application volumes. Conversely, a ground rig will release the herbicide at a lower height but with higher application volumes. Misters can also be used, these produce a high volume of fine droplets that are carried by the wind over the target area.

Whatever the application methods and equipment used, if used incorrectly or in the wrong conditions, they can contribute to a spray drift situation. For example, with an aerial application, high air speeds can cause air shear which shatter droplets into smaller sizes. 

• While ‘droplet capture efficiency’ sounds rather complex it is fairly straightforward and refers to the ability of the target area to capture the drifting droplets. For example, established leafy crops have a much better droplet capture efficiency compared to bare soil or seedling crops – so it is important to consider the type of surface of your target area, and the surface area between the target area and susceptible crops.

• The droplet size influences how far the spray drift will spread. This is because a small droplet will fall more slowly than a larger droplet and will be carried further by air currents before settling on the ground.  

• The spray release height influences how much chemical is left in the air – the higher the chemical is released, the more it will be carried before it settles. 

• A large target area can increase the risk of spray drift simply because a larger area will take longer to be sprayed, meaning there is a higher chance that weather conditions will change in this time. 

• Also known as an atmospheric inversion, surface temperature inversion occurs at night or early in the morning when conditions are still. In these conditions the spread of droplets becomes unpredictable. This document from Agriculture Victoria helps explain surface temperature inversions and how to recognise when it is occurring.

• When there is high ambient temperatures and low humidity, the rate of evaporation increases in turn increasing spray drift.

• As we have already mentioned, vapour drift can occur with volatile chemicals.

• The faster the wind speed, the further spray droplets will be carried before they drop and land on the target surface area. 

Obviously, a combination of these factors can combine to provide conditions conducive to spray drift and that can have serious implications, which leads us to our next question – why is spray drift a problem?