Researchers to help determine 'safe' paddocks

In the course of a chemical fallow there are often several applications of herbicide and some residues may still be present on or near the soil surface when it is time to sow the next crop.

NSW Department of Primary Industries soil scientist Mick Rose said concerns about the effect these residues on soil microbial activity and on the establishment and growth of crops following the fallow, even after the plant back period, were still high.

"Glyphosate has been the most commonly used knockdown herbicide in northern fallows for several decades and more recently growers have been looking to use more diverse programs that include chemicals with residual activity on weeds," he said.

"The increased use of imazapic and diuron have been of most concern to growers when choosing the next crop, particularly after a low rainfall fallow period.

With investment from GRDC, Dr Rose has been working on a project led by the Department of Agriculture and Fisheries in Queensland Michael Widderick to develop a soil test for imazapic and diuron residues that will indicate damaging residue levels and help growers to decide which crops would be safe to plant in a paddock.

"We are determining the threshold levels of residues of these two herbicides at which crop damage is likely for six crops, both winter and summer growing, in a range of soil types," Dr Rose said.

In earlier work he also looked at the level of glyphosate residue in soils around the country at planting time and the impact these residues have on soil biological processes.

"We found that residues of glyphosate were commonly detected in the soil at planting but there was no indication that the herbicide was adversely affecting soil biological activity," Dr Rose said.

"This suggests that the label recommendations are suitable and the proper application of glyphosate in Australia is not posing a threat to soil health.

"For growers to be able to keep using glyphosate they need to implement the WeedSmart Big 6 strategy, including using diverse chemistry in fallows," he says. "Residual herbicides are a useful tool for growers but there are some gaps in our knowledge about how these herbicides break down in different soils and under different seasonal conditions."

The herbicide itself can inhibit germination and growth, or it can exacerbate other factors, such as root disease, too.

Most recently, a project is establishing phytotoxicity thresholds for six summer and winter crops in a range of soil types, for two herbicides - imazapic and diuron.

By mid-2021 the aim is to have established the thresholds so that soil could be tested pre-plant to determine what crops would be safe to plant.

This will give growers confidence to use these herbicides in a diverse strategy to manage weeds like feathertop Rhodes grass in the fallow, while avoiding germination or establishment failures in the following crop.

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