Volatile vapour drift risk
Some herbicides can turn into vapour after spraying. This vapour can move away from the target area and damage nearby crops, gardens and other sensitive plants. In some conditions, vapour can travel several kilometres from where it was sprayed.
Not all herbicide types have the same risk. Ester formulations can produce vapour, while amine and salt formulations generally do not at normal temperatures.
Before you spray, check the product type, weather conditions and nearby sensitive crops. Avoid spraying during atmospheric inversion conditions and follow all label requirements. These steps can help reduce the risk of vapour drift and off-target damage.
Phenoxy herbicide formulations
Many chemical users know that phenoxy herbicides such as 2,4-D and MCPA, and other herbicides, such as triclopyr and picloram come in amine, salt or ester formulations. They behave differently when applied and even afterwards.
Understanding vapour risk
The main difference between these herbicide types is their ability to produce vapour. Amine and salt formulations do not usually produce vapours at normal temperatures. Ester formulations can produce vapour. Some easter formulations produce more vapour than others, but all esters have some risk.
Both High Volatile Esters (HVE) (ethyl, butyl and isobutyl esters) and Low Volatile Esters (LVE) (hexyl, octyl etc) can produce volatile vapours – it’s simply a matter of scale.
Amine formulations may produce vapours if the temperature rises around 50°C. However, spraying should not occur in these conditions.
How vapour drift happens
Spray drift happens when a chemical moves away from the target area. Some herbicides move only as spray droplets. These droplets usually travel a few hundred metres, depending on weather conditions.
Ester herbicides can move in 2 ways:
- as spray droplets
- as vapour that cannot be seen
Vapour can travel much further than spray droplets. In some cases, it can travel several kilometres. This creates a risk for nearby sensitive crops (Figure 1) such as:
- tomatoes
- grapevines
- melons
Some ester herbicides can continue to release vapour after spraying. This means damage can occur long after the application has finished.
To minimise drift, there are restrictions. You must apply all phenoxy herbicides using VERY COARSE droplets. This follows the American Society of Agricultural Engineers Standard (ASAE SS72). Also, apply them when the wind speed is between 3 and 15 km/h.
Weather and vapour drift
Weather conditions play an important role in vapour drift.
Wind can increase the risk of spray droplets moving off target. Vapour drift can occur even in conditions that seem suitable.
Some people think calm days are the best time for spraying. Yet, calm conditions, especially in autumn or spring, can lead to a weather event called an atmospheric inversion. During an atmospheric inversion, vapour can remain suspended in the air and move long distances. Vapours can travel several kilometres from the treated area and settle in unexpected locations.
Avoid spraying ester herbicides when inversion conditions are present. You should also avoid spraying when extreme temperatures are forecast. Vapours may continue to form days after application, particularly during hot weather.
Restrictions on certain ester herbicides
In Victoria, anyone using ester formulations of:
- 2,4-D
- 2,4-DB
- MCPA
- triclopyr
must either:
- hold a valid Agricultural Chemical User Permit (ACUP), or
- work under the direct supervision of someone who holds a valid Agricultural Chemical User Permit.
These products are classified as ‘Restricted use’ chemicals.
‘Restricted use’ chemicals include products that:
- are Schedule 7 Poisons (Dangerous Poisons)
- contain atrazine
- contain metham sodium
- contain ester formulations of 2,4-D, 2,4-DB, MCPA or triclopyr.
Agricultural Chemical Control Areas
Additional restrictions apply when an Agricultural Chemical Control Areas (ACCA) is operating. These restrictions help protect valuable crops during their growing seasons.
Record keeping requirements
You must keep records when using agricultural chemical products.
You must:
- make specified records within 48 hours of using an agricultural chemical product
- keep the records for two years.
More resources
- Buffer zones and vegetative barriers – Buffer zones and vegetative barriers are useful tools. They help reduce the risk of spray drift when using agricultural chemicals.
- Living in harmony pesticides and bees – Information on managing risks associated with agricultural spraying near apiary sites.
- Recognising surface temperature inversions – It's unsafe to spray when surface temperature inversions may occur. This increases the chance of spray drift. Learn how to identify weather conditions associated with surface temperature inversions.
- Spray drift restraints and mandatory buffer zones – This page explains how to manage spray drift risks. It covers the correct use of mandatory buffer zone labels.
- Visual Guide to estimating wind speed – This guide is not a replacement for hand-held weather meters. You can find these at spray equipment suppliers.