Spray drift is in operator's hands

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Herbicide Target-selectable sprayers

A WeedSeeker® selectively spot-spraying ‘green’ targets. Target-selectable sprayers can reduce drift potential by reducing
the amount of active ingredient sprayed over a paddock.

PHOTO: McIntosh Distribution 

Key messages

Spray operators need to able to change their set-up and operating parameters for different situations. Things that the spray operator has direct control over, and therefore the ability to change, include:

  • tank mix, including choice of product and formulation type;
  • nozzles and operating pressure;
  • spraying speed;
  • boom height; and
  • deciding when to spray and when to stop.

Many of the drift-management strategies and techniques mentioned in this article are expanded on in
other articles throughout this Ground Cover Supplement.

Spray operators can change their set-up and operating parameters to improve spray coverage and minimise spray drift

Almost every pass of a spray rig over a paddock can result in a small amount of the applied product remaining in the air after the spray has been released from the nozzles.

When the weather conditions are suitable for spraying, the majority of this airborne product will usually dilute and settle back to the ground within a few hundred metres from where it was released.

The distance required before such dilution occurs increases if too much of the product remains in the air through poor nozzle choice, booms set too high, spraying at high speeds or spraying under the wrong conditions.

The spray operator can reduce this off-target movement of spray – which can damage sensitive areas or crops – through decisions such as: sprayer set-up, operating parameters, tank mix, and when they start and stop spraying.

These factors, which can be changed by the operator to reduce the potential for off-target movement of product, are often referred to as drift-management strategies. Product labels refer to some of these strategies and others have been proposed for inclusion on future labels.

Spray quality

Shielded herbicide sprayers

A Crop Stalker 10-metre shielded sprayer. Shielded sprayers can reduce spray drift by more than 90 per cent and help prevent spray droplets from depositing onto the crop.

PHOTO: Southern Precision 

Spray quality (droplet size) is one of the simplest factors the spray operator can change to have a large effect on drift potential.

Operators should always try to select the coarsest spray quality that will provide an appropriate level of control for the tank mix used. Operators should check product labels for the recommended spray quality.

In many situations where weeds are of reasonable size and the product being applied is well translocated, coarse spray qualities (or coarser) could be used without seeing a reduction in efficacy.

However, moving to very large droplet sizes, such as an extremely coarse (XC) spray quality, can create situations where efficacy is reduced. These include:

  • using contact-type products;
  • using low application volumes;
  • targeting very small weeds;
  • spraying into heavy stubbles or dense crop canopies; and
  • spraying at higher speeds.

If spray operators intend to use a coarser spray quality than recommended on the label, they should seek trial data to support this use. If data is not available, operators should spray small test strips to compare the larger spray droplet sizes to their regular nozzle set-up and carefully evaluate the control obtained. Farm advisers or agronomists can assist in evaluating the efficacy.

Boom height

herbicide boom sprayer

Ensuring that booms are stable and height control systems can maintain boom height is critical to obtaining good spray coverage and for reducing drift.

PHOTO: Graham Betts 

Boom height above the target is critical to ensuring an even overlap of nozzle spray patterns.

For most broadacre spraying, the aim is to achieve a double overlap of the spray patterns from each nozzle. Double overlap occurs when the outer edge of the spray patterns arrive at the target area in alignment with the adjacent nozzle.

Wider fan angles can allow for lower boom heights, but the trade-off will be an increase in drift-prone droplets produced compared with fans of the same type and orifice size.

Increasing the boom height above what is required to achieve the overlap will increase the amount of chemical that remains in the air, because the smaller drift-prone droplets lose their downward velocity very quickly.

Research has shown that when using the same nozzle type at the same pressure, increasing boom height from 50 centimetres to a height of 70cm above the target can increase the amount of drift-prone droplets by as much as four times. An increase in boom height from 50cm to higher than 100cm can result in a 10-fold increase in the amount of drift-prone droplets from some nozzles.

The amount of increase in drift potential is related to the size of the droplets, their initial velocity and the rate at which they slow down.

Product and formulation choice

The choice of active ingredient and formulation can influence the droplet sizes produced by a nozzle, as well as the level of damage that may occur.

An operator can reduce drift or potential damage by:

  • substituting the product: in some situations an alternative active ingredient may have a lower impact on sensitive areas – for example, using 2-methyl-4-chlorophenoxyacetic acid (MCPA) in place of 2,4-D where it will provide an appropriate level of control on the target weeds; and
  • selecting a different formulation type: for example, choosing a salt-based product in place of an ester-based formulation, or choosing an emulsion-type formulation over an aqueous concentrate.  

Product labels with downwind buffers or no-spray zones provide a useful basis for comparing products to see which present a lower risk to a sensitive area.

Rate of product used

Robust product rates are important for control and to minimise resistance. However, the rate of product can also influence the level of damage that may occur if the product moves away from the target area.

The rate of product applied per hectare can influence:

  • the total amount of the active ingredient available to move off target;
  • the concentration of the active ingredient within each droplet; and
  •  the concentration of adjuvants and additives within the spray solution that can affect droplet size and drift potential.
warning sign reads 'avoid spray dift'

Many communities take spray drift very seriously. Drift-management strategies become critical in areas where sensitive crops such as grapes and cotton are grown.

PHOTO: Bill Gordon 

Higher product rates increase the amount of active ingredient released into the environment. When more active ingredient is released, a greater buffer distance downwind is required for sufficient dilution and reduced concentration of deposited droplets.

The total volume of spray product should be adjusted accordingly. If water rates are not increased, droplets will contain a higher concentration of active ingredient and can cause increased damage if they land on a sensitive area or crop.

Lowering water rates has a similar effect on droplet concentration as increasing product rates, as this produces more concentrated droplets.

Increasing the concentration of the active ingredients also increases the concentration of other additives in the spray solution.

When products have a high surfactant loading, increasing the rate of product can also increase the number of drift-prone droplets produced.

In other situations, where the product is formulated as an emulsion, increasing the rate of active ingredient may have little effect on droplet size or may actually reduce the number of drift-prone droplets.  

It is important to evaluate the impact that changes in rate or additions to the tank mix may have on droplet size. Spray operators should carefully assess claims made by manufacturers and decision-making tools.

Adjuvant choice and tank-mix partners

The addition of some adjuvants and other products to the tank mix can change the spray quality in ways the spray operator may not anticipate.

Adding wetters and spreaders that reduce surface tension will generally produce smaller droplets, whereas the addition of oils and products that increase viscosity will generally increase the droplet size. However, not all nozzles will respond this way.

To understand how the addition of an adjuvant or other products will affect droplet size it is important to understand the terminology used to describe nozzle outputs (see Spray speak droplet and drift terminology) and use this to evaluate the information supplied by product manufacturers. All claims about droplet size or drift reduction should be supported by statements on the Australian Pesticides and Veterinary Medicines Authority-approved label.

Sprayer design

Some spraying systems have the potential to reduce the amount of spray drift produced, provided they are operated correctly and during appropriate conditions.

In general terms, systems that increase productivity during favourable conditions for spraying can help reduce the risk of product moving away from the target area.

Increasing boom width, provided height is maintained, can increase the number of hectares sprayed per hour. This can be a positive outcome when the weather conditions are good, but can also have negative consequences when conditions are poor.

Spraying systems have been shown to reduce the potential for spray drift in different ways.

A Hardi Twin Force air-assisted herbicide boom sprayer

A Hardi Twin Force air-assisted boom sprayer. Air assistance can be used to constrain and transport droplets to the target, reducing the number of small droplets that can become airborne.

PHOTO: Bill Gordon 

  • Shielded or shrouded sprayers: shielded and shrouded sprayers can reduce spray drift by preventing droplets from becoming airborne. Many designs can reduce spray drift by more than 90 per cent, provided they are operated at appropriate speeds and the shield is maintained close to the ground.
  • Target-selectable sprayers: target-selectable sprayers, such as the WeedSeeker® and the WEEDit®, are selective spot sprayers that use cameras to detect and target-spray weeds. Usually the weed cover in a paddock will not be more than about 30 per cent, so this can reduce the total amount of active ingredient released over an area, which can reduce the potential for spray drift during favourable conditions.
  • Air-assisted sprayers: well-designed and operated air-assisted sprayers can help constrain droplets within the airstream, reducing the potential for smaller droplets to become airborne. In many European countries, air-assisted sprayers can legally operate nearer to sensitive areas than conventional booms. However, if they are not correctly operated (such as using finer droplets and high air speeds when there is little or no crop canopy to catch the droplets) they can increase the amount of drift produced compared to a conventional boom sprayer.

Vegetative barriers and hedges

Vegetative barriers are areas of vegetation deliberately planted by the landholder to intercept and filter airborne droplets.

Generally, one or two rows of an appropriate species can intercept up to 70 per cent of the airborne droplets, which may reduce the distance required of a downwind buffer.

To be fully effective, these buffers need to be designed to allow for air movement through the foliage and have leaf types that are effective at catching small droplets.

More information:

Bill Gordon,
0429 976 565,


Research goes under cover to reduce drift


Spray speak: droplet and drift terminology

GRDC Project Code BGC00003

Region National