Spray drift of phenoxy compounds (for example, 2,4-D, MCPA, dicamba) and the subsequent damage to susceptible crops grown close by, is a major concern in vineyard and vegetable growing districts. More recently it has become important to broadacre farming due to the wider scale sowing of lupins, canola, faba beans, and peas.
The knockdown herbicides paraquat, diauat, and mixtures of these (for example, Spray.Seed®) and glyphosate sometimes damage crops and native vegetation by spray drift.
How to reduce spray drift
Small droplets have a large surface area in relation to their mass, so they are easily blown by wind. The higher the wind speed at the time of spraying, the more likely droplets are to be blown away from the target.
Moreover, the liquid carrier may evaporate in hot dry conditions, reducing the droplet size in transit from spray nozzle to target.
Droplet size
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 may be reduced to acceptable levels by:
- avoiding spraying in adverse conditions (for example, during high winds and temperature inversions)
- adjusting the boom spray operation.
Droplets with a mean diameter of 250 µm (0.25 mm) or larger do not normally drift, so drift is reduced by aiming for large droplet sizes.
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 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.
Air induction or injection nozzles can assist in spray-drift reduction. The air induction nozzle draws air into the system using a venturi action and forms large droplets that are filled with air. Because they are large, these droplets do not drift as much as small droplets. When they hit a target, they shatter into smaller droplets. This results in less drift, coverage of the target is achieved when the large drops shatter into many smaller ones, and 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 manipulating the sprayer nozzles (for example, using air induction nozzles), spraying height, reduced spraying pressure and tractor speed. Air foils above booms that direct droplets down toward the targets may also help.
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 narrow fan angle (such as 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 using 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 temperature inversion is likely, which can lead to unpredictable spread of chemical droplets. The ideal wind speed for spraying is between 3 and 15 km/h, with wind direction away from sensitive areas.
Delta T is a key indicator for assessing optimal spray conditions, calculated as the difference between the dry bulb and wet bulb temperatures. The ideal Delta T range for spraying is between 2 and 8. Although applicable year-round, it is especially valuable in summer when high temperatures and low relative humidity limit effective spraying time.
Delta T indicates evaporation rates and droplet survival; values below 2 enhance droplet survival, increasing the risk of spray drift. Conversely, higher Delta T values accelerate water evaporation, reducing droplet size and making them more prone to drift.
Herbicides restricted under legislation
Use of phenoxy (hormone-like) herbicides are restricted in Western Australia under the Agriculture and Related Resources Protection (Spraying Restrictions) Regulations 1979 under the Agriculture and Related Resources Protection Act 1976.
Herbicides covered by the legislation are:
- Dicamba
- MCPA
- MCPB
- Picloram
- 2,4-D
- 2,4-DB
- 2,4,5-T.
Phenoxy herbicide Triclopyr (for example, Garlon™600, which is also contained in Grazon™DS) is not restricted by legislation, but similar precautions should be taken when using it.
Spraying restrictions
Restrictions under the Agriculture and Related Resources Protection (Spraying Restrictions) Regulations 1979 are imposed in different areas for:
- use of volatile esters or approved low volatile esters or other approved phenoxy amines or salts
- storage of volatile esters
- spray contaminated vehicles
- 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 department’s Director General.
The use of an approved low volatile ester is allowed.
Key P = permit required, X = not permitted, ok = ok to spray, store or transport
|
Prescribed areas (distance from commercial vineyards and tomatoes) |
Salt and amines |
Low volatile esters |
Esters |
||||||
|---|---|---|---|---|---|---|---|---|---|
|
- |
spray |
store |
transport |
spray |
store |
transport |
spray |
store |
transport |
|
Commercial crops |
P |
ok |
ok |
X |
ok |
ok |
X |
ok |
ok |
|
Commercial crops |
ok |
ok |
ok |
ok |
ok |
ok |
X |
ok |
ok |
|
Geraldton post office (0-19 km) |
P |
ok |
ok |
X |
ok |
ok |
X |
X |
X |
|
Geraldton post office (19-50 km) |
ok |
ok |
ok |
ok |
ok |
ok |
X |
P |
ok |
|
Kununurra (0-10 km) |
P |
ok |
ok |
X |
ok |
ok |
X |
X |
X |
|
Kununurra (10-50 km) |
ok |
ok |
ok |
ok |
ok |
ok |
X |
X |
ok |
|
Swan Valley |
P |
ok |
ok |
X |
ok |
ok |
X |
ok |
X |
Swan Valley boundaries for Table 1
All the land bounded by lines starting from the junction of Ida Street and Anzac Terrace in the town of Bassendean, extending east to the southeast corner of Swan location 3218; then north to the southeast corner of Swan location 1339; then west to the northwest corner of Swan location 277; then south to the starting point.
Storage restrictions
The storage of any volatile ester of phenoxy acid is banned within 19 km of the Geraldton post office. Storage beyond 19 km, but within 50 km radius of the Geraldton post office, requires prior written approval from the department’s Director General.
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 in certain areas is subject to prior written approval from the department’s Director General. To obtain approval to spray, a written application must be made, including 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 equipment to be used
- the proposed date of spraying.