Types of Drift
- Droplet Drift
Sprayed whole droplets of herbicide are carried off target by air turbulence or gravity. Normally only travels a short distance off-target.(e.g. next paddock)
- Particle Drift
The water component of the spray evaporates from the sprayed droplet, leaving a tiny particle of concentrated herbicide. Particles can travel several kilometers off-site under some conditions.
- Vapour Drift
Volatile herbicides (especially esters) can form vapours that can drift directly from the spray or even by evapoartion of herbicide from sprayed surfaces. This can occur for several hours after application, and may result in herbicide being transported many kilometers by updrafts, before being deposited off-site.
Some crops are highly susceptible to certain herbicides, and can be damaged by concentraions as much as 10,000 times lower than "normal" application rates. In particular, tomatoes, grapes and many horticultural species are highly sensitive to volatile herbicides like 2,4-D ester.
Factors Affecting Spray Drift Risk
- Volatility of the herbicide. Esters are riskier than amines, salts and acids.
- Proximity to susceptible crops, and their stage of growth.
- Method of application - air, ground, mist blower etc.
- Size of target area and application rate of the herbicide.
- Droplet capture efficiency of the target species - leafy crops are less risky than bare ground, pasture, fallow paddocks and seedling crops.
- Weather conditions during and until shortly after application.
How to Minimise Spray Drift Risk
- Check first for sensitive crops and sites in your area. Notify neighbours.
- Choose non-volatile formulations when possible.
- Don't spray when the termperature is over 28oC.
- Never spray under termperature inversion conditions.
- Avoid excessive wind but also avoid dead calm. 7-10 km/hr is ideal.
- Maintain a downwind buffer zone (e.g. booms width from downwind edge).
- Minimise spray release height (e.g. boom height).
- Use the largest droplet size that will give adequete spray coverage.
- If sensitive crops are nearby, choose the least damaging herbicide.
- Maintain a log of dates, times, herbicides, rates, wind (approxomate speed and direction) and temperature.
- If favourable conditions change, stop spraying.