Avoiding spray drift

How to reduce spray drift

Small droplets have a large surface area in relation to their mass. They are therefore easily blown by wind. The higher the wind speed at the time of spraying, the more likely are droplets to be blown away from the target.

Moreover, the liquid carrier may evaporate in hot dry conditions, thus reducing the droplet size in transit from spray nozzle to target.

All spraying systems produce a range of droplet sizes, although the range produced by Controlled Droplet Applicators is much narrower than that of conventional hydraulic nozzles. Therefore, spray drift is impossible to eliminate but it may be reduced to acceptable levels. This can be done by avoiding spraying in adverse conditions (for example, during high winds and temperature inversions) and adjusting the boom spray operation.

As a rule of thumb, droplets with a mean diameter of 250µm (0.25mm) or larger do not normally drift. So, by aiming for large droplet sizes, drift is reduced.

Smaller droplets are, however, more economical. They give better coverage for a given volume of spray and provide better penetration of foliage and attachment to leaf surfaces.

Large droplets have other disadvantages; they give an uneven cover of the target plant surface and tend to bounce off leaves. They are also less likely than smaller droplets to stick to vertical surfaces and the underside of leaves.

A recent development with nozzle design is air induction or injection. The air induction nozzle draws air into the system using a venturi action and then forms large droplets that are filled with air. These droplets because they are large do not drift as much as the small droplets. When they hit a target they then shatter into a large number of smaller droplets. This way the there is less drift, coverage of the target is still achieved when the large drops shatter into many smaller ones and the air filled drops do not bounce once they hit a target.

Misters and boom sprays produce many small droplets. Misters should never be used up-wind close to susceptible crops. Drift from boom sprays may be minimised by manipulation of the sprayer nozzles (for example, air induction nozzles), spraying height, reduced spraying pressure and tractor speed. Airfoils above booms have also directed the droplets downwards towards the targets.

A long droplet trajectory from nozzle to crop increases the chance of droplets evaporating. To minimise droplet travelling distance, choose wide spray angle flat fan nozzles (110º) rather than the narrow fan angle, for example, 80º, angled backwards at 45º and run the boom as low as possible above crop height.

Vapour drift is a problem with certain chemicals, mainly the volatile ester formulations. The spray vaporises from the soil or plant surface after spraying and may drift many kilometres to damage susceptible crops. This mainly occurs under hot, dry conditions. It can be avoided by use of amine formulations or low volatile esters.

Weather conditions affect both droplet drift and vapour drift. Avoid hot dry conditions and windy days. The best time to spray is in cool, moist conditions in the morning or early evening, but not when the weather is calm because then temperature inversion is likely which can lead to unpredictable spread of chemical droplets. A slight breeze blowing away from the susceptible crop is ideal.

Further information:

• Bulletin 4243 ‘Management of Agricultural Weeds in Western Australia’.
• Reducing Herbicide Spray Drift, by Andrew Storrie, Weeds Agronomist, Tamworth Agricultural Institute New South Wales Agriculture Agnote DPI-477, January 2004 issn 1034-6848.

Legislation 

Use of phenoxy (hormone-like) herbicides are restricted in Western Australia under the Aerial Spraying Control Act 1966 and the Agriculture and Related Resources Protection (Spraying Restrictions) Regulations 1979.

The herbicides covered are: Dicamba, MCPA, MCPB, Picloram, 2,4-D, 2,4-DB and 2,4,5-T.

Triclopyr (for example, Garlon™600 also contained in Grazon™DS) is a phenoxy herbicide but is not restricted by legislation. Nevertheless you should take similar precautions when using it.

Aerial Spraying Control Act, 1966 

Restrictions apply in three areas of the State:

• within 19km of Geraldton
• the Swan vineyard area
• the Ord River irrigation area.

Within each of these areas:

1. Pilots are not permitted to undertake aerial spraying with the chemicals listed above unless:
   1. they have obtained written approval from the Director General of the Department of Agriculture and Food, Western Australia (DAFWA)
   2. the spraying is carried out according to the conditions imposed at the time approval is given.
2. A pilot is not permitted to transport the listed chemicals in a spray aircraft.
3. A pilot is not permitted to fly the aircraft unless the spray tanks and spraying equipment have been cleansed of the chemicals.

ARRPA Regulations, 1976 

Under the Agriculture and Related Resources Protection Act 1976 (ARRPA) Restrictions are imposed in different areas. These restrictions involve:

1. use of volatile esters or approved low volatiile esters or other approved phenoxy amines or salts
2. storage of volatile esters
3. spray contaminated vehicles and
4. permits to use approved phenoxy herbicides within a restricted area.

The use of any kind of ester formulations of phenoxy acid herbicides is prohibited.

The use of amine or salt formulations is permitted subject to prior written approval from the Director General of DAFWA.

The use of an approved low volatile ester is allowed.

Storage restrictions

The storage of any volatile ester of phenoxy acid is totally banned within 19km of the Geraldton post office. Storage beyond 19km but within 50km radius of the Geraldton Post Office requires prior written approval from the Director General of DAFWA.

Contamination restrictions: vehicles and open containers

No opened container of volatile esters or equipment or vehicle contaminated with these chemicals may be stored or parked. These restrictions do not apply to a moving vehicle or to approved low volatile esters.

Permit to spray with amine or salt

The spraying of an amine or salt form of a phenoxy acid herbicide within certain areas is subject to prior written approval from the Director General of DAFWA. To obtain approval to spray, a written application must be made setting out the following information:

• approximate area to be sprayed
• location of the area to be sprayed
• the amine or salt form to be used
• the method of spraying and the equipment to be used
• the proposed date of spraying.

Elsewhere in the State no restrictions apply. For exact definitions refer to the Act and its Regulations.

Further Information

Further information on controlling declared plants can be found through the Declared plant control handbook link.