Herbicide application to a 'T'

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Consultant Bill Gordon (left) and GRDC Southern Panel member Allan Mayfield examine pre-emergent spraying programs at a recent GRDC Update in Adelaide.

Consultant Bill Gordon (left) and GRDC Southern Panel member Allan Mayfield examine pre-emergent spraying programs at a recent GRDC Update in Adelaide.

A consultant specialising in application technology and spraydrift management, Bill Gordon, is urging growers to be vigilant about three main considerations – the “three Ts” – in pre-emergent herbicide spraying programs this season. Mr Gordon says these are target, timing and technique, and emphasises how each is important for optimising the effects of pre-emergent herbicides.

At the recent GRDC Update in Adelaide, Mr Gordon recommended that growers focus on both maximising and uniformly distributing pre-emergent spraying on the soil surface.

“The evenness required will depend on the solubility of the product and its ability to move within the soil,” he said. He explained how the time that elapses between the application and incorporation of spraying could be significant, with more volatile products requiring incorporation soon after application.

The target
Mr Gordon said there needed to be knowledge of the target weeds to determine whether a selected product was the most appropriate for both the weeds and the on-farm situation.
The previous weed burden, potential resistance, the product’s residual activity, solubility and soil moisture requirements, ability to tank mix and the target area, including stubble load and ground cover, were among a range of relevant considerations highlighted at the Update.

The timing
Mr Gordon said the timeframe for applying pre-emergent herbicides often needed to be considered in concert with the most appropriate conditions for spraying.
“Extremes of temperature, relative humidity and wind speed all affect the amount of product available to deposit onto the target area, and often these factors will interact with droplet size, speed and boom height.”

The technique
Small adjustments to the sprayer set-up could improve how herbicides are deposited onto the soil and minimise losses in stubble and the atmosphere.
“The heavier the stubble load, the greater the need for a coarse spray quality or larger droplets.”
He said applying larger droplets usually required greater volumes (more than 80 litres per hectare) for uniform spraying.
He added that this was particularly important for pre-emergent herbicides with a low solubility in the soil.
“There are many ways to deliver the desired spray quality and application volume which can require thinking about more than just the nozzle type.”
Mr Gordon said the best way to maximise spraying deposits was to conduct a test using water-sensitive paper.
“Growers can check the impact of variations in nozzle type, application volume, boom height and travel speed to compare the effects on deposition in and around stubble to determine what works best in their situation.”

GRDC Research Code CRD00002
More information: Bill Gordon, 0429 976 565,
email; GRDC Weedlinks,

GRDC Project Code CRD00002

Region South