Reducing herbicide spray drift

The Problem

Minimising Spray Drift

Before spraying

• Check the product label for legally required spray quality and potential no spray zones
• Always check for susceptible crops in the area, for example broadleaf crops such as grape vines, cotton, vegetables and pulses if you are using a broadleaf herbicide. Satacrop is a tool to mitigate the risk of spray drift by allowing growers to map where sensitive crops are located. Operators can check Satacrop to understand where sensitive crops are located in proximity to their spray operations. The SataCrop tool is an industry initiative developed by Cotton Australia and Precision Cropping Technologies (PCT). Visit https://satacrop.com.au.
• Check sensitive areas such as houses, schools, waterways and riverbanks.
• Notify neighbours of your spraying intentions.

Under the Records Regulation of the Pesticides Act 1999, when spraying you must record the weather and relevant spray details. Wand inversion tower network.

During spraying

• Always monitor weather conditions carefully and understand their effect on 'drift hazard'.
• Don't spray if conditions are not suitable, and stop spraying if conditions change and become unsuitable.
• Record weather conditions (especially temperature and relative humidity), wind speed and direction, herbicide and water rates, and operating details for each paddock.
• Supervise all spraying, even when a contractor is employed. Provide a map marking the areas to be sprayed, buffers to be observed and sensitive crops and areas.
• Avoid spraying during high temperatures (>28C).
• Understand surface temperature inversions.
• Maintain a downwind buffer. This may be incrop, for example keeping a boom's width from the downwind edge of the field.
• Minimise spray release height, aim to maintain double overlap of nozzle patterns (50cm above target for 110 degree nozzles). Reduce ground speed as necessary to ensure maintaining release height.
• Use the largest droplets that will give adequate spray coverage. Increase water volumes if necessary to improve spray coverage with large droplets.
• Always use the least-volatile formulation of herbicide available.
• If there are sensitive crops in the area, use the herbicide that is the least damaging.
• Consider using alternative tactics for managing weeds in sensitive areas and near sensitive crops

Types of Drift

Sprayed herbicides can drift as droplets, as vapours, as particles, or all of these.

Droplet drift is the easiest to control because under good spraying conditions, droplets are carried down by air turbulence and gravity, to collect on plant or soil surfaces. Droplet drift is the most common cause of off-target damage caused by herbicide application. For example, spraying fallows with glyphosate under the wrong conditions often leads to severe damage to neighbouring establishing crops.

Particle drift occurs when water and other herbicide carriers evaporate quickly from the droplet leaving tiny particles of concentrated herbicide. This is most common when spraying under hot conditions and can occur with any herbicide formulations. Instances of this form of drift have damaged susceptible crops up to 30 km from the source.

Vapour drift most commonly occurs with volatile herbicides such as 2,4-D ester and dicamba, but to a lesser extent, may occur with a range of other herbicides. Vapours may arise directly from the spray or from evaporation of herbicide from sprayed surfaces. Use of volatile broadleaf herbicides in summer can lead to vapour drift damage of susceptible crops such as tomatoes, cotton, sunflowers, soybeans and grapes. This may occur hours or even days after the herbicide is applied and is especially significant under surface inversion events

Vapours and minute particles float in the airstream and are poorly collected on plant or soil surfaces. They may be carried for many kilometres in thermal updraughts before being deposited.

Sensitive crops may be up to 10,000 times more sensitive than the crop being sprayed. Even small quantities of drifting herbicide can cause severe damage to highly sensitive plants.

Factors Affecting the Risk of Spray Drift

Any herbicide can drift. The drift hazard, or off-target potential, of a herbicide in a particular situation depends on the following factors.

• Volatility of the formulation applied. Volatility refers to the likelihood that the herbicide will evaporate and become a gas. Generally, ester formulations volatilise (evaporate) more readily, whereas amines are less prone to volatility.
• Proximity of crops susceptible to the particular herbicide being applied, and their growth stage. For example cotton is most sensitive to Group 4.. Many non-target crops are highly susceptible to herbicide damage, for example cotton in the 6 to 12 node stage of crop growthbut can  be damaged by drift of Group 4 herbicides at any growth stage.
• Method of application and equipment used. Aerial application releases spray at 3 m above the target and uses relatively low application volumes, while ground rigs have lower release heights and generally higher application volumes, and a range of nozzle types. Misters produce large numbers of very fine droplets that use wind to carry them to their target. Drift is minimized by applying large droplets at the minimum spray height above the target, which for most broadacre nozzle configurations is 0.5 m and keeping ground speed below 18kms/hr
• Size of the area treated - the greater the area treated the longer it takes to apply the herbicide. If local meteorological conditions change during or even following an application, particularly in the case of volatile herbicides such as 2,4-D ester, then more herbicide is able to volatilise.
• Amount of active ingredient (herbicide) applied - the more herbicide applied per hectare the greater the amount available to drift or volatilise.
• Choosing the correct nozzle is critical. In most cases adding chemical to the spray solution changes the droplet spectrum often with more finer droplets produced, i.e., the output from a coarse nozzle can be shifted to a medium nozzle depending on the chemical added to the spray solution, adding more fine driftable droplets to the atmosphere.
• Efficiency of droplet capture - bare soil does not have anything to catch drifting droplets, unlike taller crops, erect pasture species and standing stubbles.
• Weather conditions during and shortly after application. Changing weather conditions can increase the risk of spray drift.

Volatility

Many ester formulations are highly volatile when compared with amine formulations, sodium salt and acid formulations. Nevertheless, all herbicides can drift and many have some degree of volatility.

Minimising Drift

A significant part of minimising spray drift is the selection of equipment to reduce the number of small droplets produced. However, this in turn may affect coverage of the target, and therefore the possible effectiveness of the pesticide application.

This aspect of spraying needs to be carefully considered when planning to spray. As the number of smaller droplets decreases, so does the coverage of the spray.

A good example of this is the use of air-induction nozzles that produce large droplets that splatter. These nozzles produce a droplet pattern and number that at low water rates, is unsuitable for targets such as seedling grasses that present a small vertical target. However, issues of poor coverage with these nozzles can be addressed by increasing the water rate.

In 2010, the Australian Pesticides and Veterninary Medicines Authority (APVMA) announced new measures to ensure the number of spray drift incidents are minimised. The changes are restrictions on the droplet size spectrum an applicator can use, wind speed suitable for spraying and the downwind buffer zone between spraying and a sensitive target.

Spray Release Height

• Operate the boom at the minimum practical height. Drift hazard more than doubles as nozzle height doubles. Where boom height can be reduced consider  angle nozzles, e.g. forward 30° to allow lower boom height with double overlap or consider rear and forward facing twin nozzles. Be aware that lower boom heights, however, can lead to more striping, as the boom sways and dips below the optimum height.
• 110° nozzles produce a higher percentage of fine droplets than  80° nozzles (where equivalent nozzle type and orifice size are used), but they allow a lower boom height while maintaining the required double overlap.
• Operate within the pressure range recommended by the nozzle manufacturer. Production of driftable fine droplets increases as the operating pressure is increased.
• Be aware that each active ingredient added to the spray solution may negatively affect the droplet spectrum producing more fine and medium size droplets.

Size of Area Treated

When large areas are treated, relatively large amounts of the active herbicides are applied and the risk of off-target damage increases due to the length of time taken to apply the herbicide and the total amount of herbicide applied. Conditions such as temperature, humidity and wind direction are likely to change during spraying.

Applying volatile formulations to large areas increases the chances of vapour drift damage to susceptible crops and pastures. Products such as 2,4-D amine, that have relatively low volatility, can become problematic when applied over large areas in the landscape as commonly occurs when herbicides are applied to fallows following widespread summer rainfall. Small amounts of herbicide vapour from a large number of applications made within a small-time space can result in a high environmental loading within a river valley leading to widespread damage to sensitive crops.

Spray operators need to consider not only the potential drift from their own applications, but the environmental loading from the combined applications of their local area.

Capture Surface

Targets vary in their ability to collect or capture spray droplets. Tall, well grown, leafy crops are efficient collectors of droplets. Turbulent airflow normally carries spray droplets down into the crop within a very short distance.

Bare fallow paddocks or seedling crops have poor catching surfaces. Drift hazard is far greater when applying herbicide in these situations or adjacent to these poor capture surfaces.

The type of catching surface between the sprayed area and susceptible crops should always be considered in conjunction with the characteristics of the target area when assessing drift hazard.

Weather Conditions to Avoid

Inversions

Midday turbulence

• Up-drafts during the heat of the day cause rapidly shifting wind directions. Spraying should be avoided during this time of day.

High temperatures

• Avoid spraying when temperatures exceed 28°C.

Humidity

• Avoid spraying under low relative humidity conditions, i.e. when the difference between wet and dry bulbs (Delta T, ∆Τ) exceeds 10°C.
• High humidity extends droplet life and can greatly increase the drift hazard under inversion conditions. This results from the increased life of droplets smaller than 100 microns.

Wind

• Avoid spraying under still conditions.
• Ideal safe wind speed is 3-10 km/h, a light breeze. (Leaves and twigs are in constant motion.)
• 11-14 km/h (a moderate breeze) is suitable for spraying if using low drift nozzles or higher volume application, say 80-120 L/ha. (Small branches move, dust is raised and loose paper is moving.)

Further resources

• Grains Research & Development Corporation
• Cotton Info

Further education

Tocal College has face-to-face and online courses for growers, sprayers and agronomists/advisors to increase skills in spray planning and overall chemical application management.

Acknowledgements

Original publication produced by Andrew Storrie, former NSW DPI Weeds Agronomist. Revised Graham Charles Australian Cotton Research Institute and Eric Koetz Wagga Wagga Agricultural Institute.