Grains Convo

Mitigating pre-emergent herbicide damage on ameliorated soils

Department of Primary Industries and Regional Development research scientist Tom Edwards
DPIRD research scientist, Tom Edwards

Project name

Increasing farming system profitability and longevity of benefits following soil amelioration

Funding partners

Grains Research and Development Corporation (GRDC) and DPIRD
GRDC code: DAW1901-006RTX

Sandplain soils and herbicide toxicity

Increased herbicide toxicity following strategic deep tillage can have a major production impact with a 70 per cent yield reduction recorded in some circumstances, however new research has led to improved knowledge to help growers select the best herbicide strategy for their soil type post-amelioration.

In Western Australia, an increase in herbicide use is correlated with a decrease in regular use of tillage, so how the two interact is poorly understood.

The aim of this research, led by Department of Primary Industries and Regional Development research scientist Tom Edwards as part of his PhD research, was to evaluate the performance of a range of herbicides on sandplain soils to estimate their risk to crop production following strategic deep tillage.

Ultimately, the project showed pre-emergent herbicides can be more active on soils which have been inverted and as a result, significant yield reductions can occur when some commonly used pre-emergent herbicides are combined with shallow sowing, furrow infill, dry soil at sowing or intense rainfall shortly after sowing.

Mr Edwards presented his findings this week at the 2023 Soil Science Australia Conference in Darwin.

The impact of deep tillage

The sandplain soils of WA are naturally fragile with surface layers that have very low content of organic matter and clay.

Strategic deep tillage, such as soil inversion and deep soil mixing, have been shown to increase crop production on these soils, however the reduction in surface organic matter and associated changes in soil structure potentially alter how herbicides behave and may increase their toxicity.

A survey of more than 200 growers, estimated one million hectares of sandplain soils had been strategically deep tilled with either soil inversion or deep mixing. However, following tillage more than 25 per cent of those growers observed some level of herbicide damage, leading them to change their herbicide strategy post amelioration.

Field and glasshouse trials

To support growers in making these decisions Mr Edwards and his team undertook 13 field trials from Geraldton to Esperance, as well as six intensive screenhouse experiments with a range of commonly used pre-emergent herbicides.

The field trials replicated across the sandplain demonstrated soil inversion and deep mixing can lead to an increased risk of herbicide toxicity, but the frequency and severity of production loss was very dependent on the soil and environmental conditions.

Further analysis in glasshouse and laboratory experiments confirmed strategic tillage can reduce the soil’s capacity to adsorb some herbicides, increasing their potency.

Strategic deep tillage weed management tips


  • Test soils to depth to identify all production constraints, as well as what machinery and depth will work best.
  • Identify the weed species and densities and use this to estimate the likely weed pressure after deep tillage, considering what depth each weed species will emerge from.


  • Undertake tillage when there is good soil moisture, improving the depth of weed burial and reducing the risk of erosion and furrow infill.
  • Select an appropriate pre-emergent herbicide with good crop safety that is not sensitive to changes in organic matter or the amount of crop residue on the surface, and not reliant on separation from the crop.
  • Establish a vigorous cereal crop as soon as possible to mitigate acute erosion risk and maximize crop competitiveness. Increased seeding rates and one-pass amelioration and seeding approaches are useful strategies to improve crop establishment.
  • Long coleoptile wheat varieties sown deeper may ensure better separation from some pre-emergent herbicides.


  • Assess weed densities and whether the dominant weeds have changed to best design future herbicide strategies and use the best post-emergent herbicides available for the target weed.
  • Check sandiest parts of the paddock for any possible instances of crop damage from herbicide, using these observations to change future herbicide strategies.
  • Even if weed density is reduced, use an integrated weed management program, including in-crop control and harvest weed seed control, to maintain weeds at low density.

More information

Journal article: The phytotoxicity of soil-applied herbicides is enhanced in the first-year post strategic deep tillage


Tom Edwards
DPIRD research scientist
M: 0472 849 393