When applying pesticides, the aim is to maximise the amount reaching the target and to minimise the amount reaching off-target areas. This results in:
In areas where a range of agricultural enterprises coexist, conflicts can arise, particularly from the use of pesticides. All pesticides are capable of drift.
When spraying a pesticide, you have a moral and legal responsibility to prevent it from drifting and contaminating or damaging neighbours' crops and sensitive areas.
Sprayed herbicides can drift as droplets, as vapours or as particles.
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 establishing crops.
Particle drift occurs when water and other herbicide carriers evaporate quickly from the droplet leaving tiny particles of concentrated herbicide. This can occur with herbicide formulations other than esters. Instances of this form of drift have damaged susceptible crops up to 30 km from the source.
Vapour drift is confined to volatile herbicides such as 2,4-D ester. Vapours may arise directly from the spray or evaporation of herbicide from sprayed surfaces. Use of 2,4-D ester in summer can lead to vapour drift damage of highly susceptible crops such as tomatoes, cotton, sunflowers, soybeans and grapes. This may occur hours after the herbicide has been applied.
In 2006, the federal regulators of pesticide use, the APVMA, have restricted the use of highly volatile for of 2,4-D ester. The changes are now seen with the substitution of lower volatile forms of 2,4-D and MCPA. Products with lower 'risk' ester formulations are commonly labelled with LVE – short for low volatile ester. These formulations of esters have a much lower tendency to volatilise, but caution still remains as they are still prone to droplet drift.
Vapours and minute particles float in the airstream and are poorly collected on catching 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.
Any herbicide can drift. The drift hazard, or off-target potential, of a herbicide in a particular situation depends on the following factors.
Changing weather conditions can increase the risk of spray drift.
Many ester formulations are highly volatile when compared with the non-volatile amine, sodium salt and acid formulations.
Table 1 is a guide to the more common herbicide active ingredients that are marketed with more than one formulation.
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 are unsuitable for targets such as seedling grasses that present a small vertical target.
In 2010, the 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 could use, wind speed suitable for spraying and the downwind buffer zone between spraying and a sensitive target. These changes should be evident on current herbicide labels. Hand held spraying application is exempt to these regulations.
When large areas are treated relatively large amounts of active herbicide is applied and the risk of off-target effects increases due to the length of time taken to apply the herbicide. Conditions such as temperature, humidity and wind direction may change during spraying.
Applying volatile formulations to large areas increases the chances of vapour drift damage to susceptible crops and pastures.
Targets vary in their ability to collect or capture spray droplets. Well grown, leafy crops are efficient collectors of droplets. Turbulent airflow normally carries spray droplets down into the crop within a very short distance.
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.
The most hazardous condition for herbicide spray drift is an atmospheric inversion, especially when combined with high humidity.
Do not spray under inversion conditions.
An inversion exists when temperature increases with altitude instead of decreasing. An inversion is like a cold blanket of air above the ground, usually less than 50 m thick. Air will not rise above this blanket; and smoke or fine spray droplets and particles of spray deposited within an inversion will float until the inversion breaks down.
Inversions usually occur on clear, calm mornings and nights. Windy or turbulent conditions prevent inversion formation. Blankets of fog, dust or smoke and the tendency for sounds and smells to carry long distances indicate inversion conditions.
Smoke generators or smoky fires can be used to detect inversion conditions. Smoke will not continue to rise but will drift along at a constant height under the inversion 'blanket'.
|Form of active||Full name||Product example|
|MCPA dma||dimethyl amine salt||MCPA 500|
|2,4-D dma||dimethyl amine salt||2,4-D Amine 500|
|2,4-D dea||diethanolamine salt||2,4-D Amine 500 Low Odour®|
|2,4-D ipa||isopropylamine salt||Surpass® 300|
|2,4-D tipa||triisopropanolamine||Tordon® 75-D|
|2,4-DB dma||dimethyl amine salt||Buttress®|
|dicamba dma||dimethyl amine salt||Banvel® 200|
|triclopyr tea||triethylamine salt||Tordon® Timber Control|
|picloram tipa||triisopropanolamine||Tordon® 75-D|
|clopyralid dma||dimethylamine||Lontrel® Advanced|
|aminopyralid K salt||potassium salt||Stinger®|
|MCPA Na salt||sodium salt||MCPA 250|
|MCPA Na/K salt||sodium & potassium salt||MCPA 250|
|2,4-DB Na/K salt||sodium & potassium salt||Buticide®|
|dicamba Na salt||sodium salt||Cadence®|
|MCPA ehe||ethylhexyl ester||LVE MCPA|
|MCPA ioe||isooctyl ester||LVE MCPA|
|triclopyr butoxyl||butoxyethyl ester||Garlon® 600|
|picloram ioe||isooctyl ester||Access®|
|2,4-D ehe||ethylhexyl ester||2,4-D LVE 680|
|fluroxypyr M ester||meptyl ester||Starane® Advanced|
|Distance downwind to susceptible crop||< 1 km||1 to 30 km >|
| Preferred droplet size (British Crop Protection Council)|
(to minimise risk)
|coarse to very coarse||medium to coarse|
|Volume median diameter (microns)||310||210|
|Flat fan nozzle size #||11008||11004|
| Recommended nozzles|
Yamaho Turbodrop® Hardi Injet® AI Teejet® LurmarkDrift-beta®
DG TeeJet® Turbo TeeJet® Hardi® ISO LD 110 Lurmark® Lo-Drift
|CAUTION||Can lead to poor coverage and control of grass weeds. Require higher spray volumes||Suitable for grass control at recommended pressures. Some fine droplets.|
Volume Median Diameter (VMD): 50% of the droplets are less than the stated size and 50% greater.
# Refer to manufacturersí selection charts as droplet size range will vary with recommended pressure. Always use use the lowest pressure stated to minimise the small droplets. (Adapted from P. Hughes, DPI, Queensland.)
Original publication produced by Andrew Storrie, former NSW DPI Weeds Agronomist.
Revised July 2015 by Tony Cook, Technical Specialist Weeds, Tamworth Agricultural Institute.