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2,4-D herbicide use under scrutiny

Friday 4 December, 2009

Farmers who plan to use 2,4-D or other phenoxy herbicides including triclopyr (eg Garlon®) for summer weed control are being urged to exercise extreme caution.

2,4-D and other phenoxy herbicides can cause off-target damage to sensitive crops through droplet or vapour drift.

The use of 2,4-D high volatile ester is prohibited from 1 September to 30 April when the risk of volatilisation is high and sensitive crops such as grapevines, tomatoes, oilseeds and other fruits and vegetables are growing actively.

Mr John Kassebaum, PIRSA Biosecurity – Rural Chemicals, says the use of low-volatile (LV) esters is not restricted, but there is still a risk of volatilisation with these products, particularly when the temperature at or soon after application rises above 28° C.

Mr Kassebaum urges all farmers and contractors to make sure they use herbicides according to directions and are mindful of the weather conditions during and after application.

"2,4-D amine is practically non-volatile and farmers should use this formulation instead of LV ester in situations where vapour drift might be a risk," he said.

"To reduce the chance of spray drift, it is important to only spray when the wind is between 3 and 15 km/hour at the application site.

"All 2,4-D products must be applied with no smaller than coarse droplet size. Air-induction nozzles are best because they produce coarse spray quality over their entire pressure range. Low drift nozzles 03 size or higher are also suitable.

"Standard flat fan nozzles are not suitable, because the droplet size under normal operating conditions for broad-scale farmers is not large enough to comply with label requirements."

Last summer, symptoms of off-target 2,4-D damage were observed on grapevines in the Clare Valley, where there was widespread use of 2,4-D and other herbicides following a significant rainfall event in December 2008.

Mr Kassebaum said a significant number of growers who applied herbicides around Clare last summer did so at night or in the early morning.

"There are some benefits in spraying when temperature is lower and relative humidity is higher, but there are also risks – particularly from temperature inversions."

Temperature inversions frequently form overnight, being strongest near sunrise. Inversion layers are characterised by calm, light or variable winds that make it difficult to predict the movement of spray droplets or vapour.

PIRSA Biosecurity is also urging growers to make and keep records of all herbicide applications.

"We recommend all chemical users get into the habit of keeping application records now, before it becomes a mandatory national requirement in 2010. If off-target damage from herbicide use occurs, PIRSA Biosecurity will request application records from users."