Experts encourage careful pre-em use in fire zone

Author: | Date: 17 Mar 2016

Nozzle on a boomspray

University of Adelaide's Dr Gurjeet Gill says growers in the Pinery fire zone will need to seed quickly following the sprayer to avoid problems with drifting.

Growers with bare soils affected by fire are being asked to be mindful of pre-emergent herbicide rates ahead of seeding in 2016, with a lack of organic matter on the surface meaning chemicals will penetrate the soil more readily than normal.

University of Adelaide Associate Professor – weed and crop ecology Dr Gurjeet Gill says some pre-emergent herbicides, namely Treflan™ and Sakura®, bind tightly to organic matter such as stubble, clay and ash or carbon on the surface.

However, with strong winds likely to have blown ash and carbon away following the November 2015 Pinery fire in South Australia’s lower north, Dr Gill says growers need to be aware of the risk of herbicide damage ahead of pre-emergent spraying.

“Even though the mode of action of different pre-emergent chemistries can be very different, they share common physical properties in that they are highly insoluble and therefore would tend to bind to the residues of plant material,” he says.

“Where growers have been affected by fire and the soil is bare that won’t be the case, but that won’t reduce the activity of those herbicides. If anything, it will make them more active.”

Dr Gill says growers will need to seed quickly following the sprayer to avoid problems with drifting.

“The thing for growers will be to ensure these chemicals are left on the soil for as little time as possible before seeding,” he says. “There can be a problem after spraying pre-emergent herbicides where strong winds come in and with no residue there to protect the soil there could be chemical drifting and accumulating in furrows. In that situation we would end up with crop toxicity.

“The important thing is to seed fairly soon after spraying to minimise that risk, because once seeded the soil should be more stable than before.”

Dr Gill says bare sandy soils carry more risk of herbicide damage than others.

“Where growers are using products such as triazines and metribuzin, they can be pretty ‘hot’ and can damage crops on sandy soils,” he says. “When there was a heavy stubble load on the surface then growers may have needed to use a higher water rate, but that will not be the case this season on fire-affected soils with no cover.

“While the circumstances are far from ideal, the fires would have burnt a lot of the weed seeds, which will help in reducing weed infestations this season.”

More information

Dr Gurjeet Gill,
(08) 8313 7744,

Dr Chris Preston,
08 8313 7237,

View and download a GRDC pre-emergent herbicide use fact sheet 

View and download the GRDC publication, Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers

Region South