Residual herbicides - carryover and behaviour in dry conditions

Page last updated: Tuesday, 9 May 2023 - 11:59am

Decay of pre-emergent herbicides in dry soils

Dry sowing crops has become a popular method of crop establishment in WA. The technique not only relies on having paddocks that are as weed free as possible but also on the longevity of soil applied herbicides. It is the intention that the opening rains will commence soon after the paddocks have been sprayed and sown to optimise the length of the growing season and to minimise the possible decay of applied herbicides.

Dr David Minkey (Western Australian No Tillage Farmers Association, WANTFA) has been studying how the efficacy of residual herbicides change with the length of the dry period following application and exposure to moisture between application and crop emergence.

Sakura®, Boxer gold® and TriflurX® was sprayed onto various soil types, dates and moisture regimes over a two year period.

Results indicate that:

  • Sakura®, Boxer Gold® and TriflurX® decay slowly under dry soil and can be applied early under these conditions
  • Under wet conditions decay of these herbicides was rapid and was highest under warm conditions (earlier application).
  • Sakura® had the slowest decay rates followed by Boxer gold® and then TrilfurX®
  • Soil type had little effect on the decay rates.
  • Do not rely on pre-emergent herbicides to give you adequate control under wet conditions when sowing in April.

Under dry soil conditions the herbicides used in the study all persisted until the 5th or 6th week. A small decline in efficacy was observed but it was concluded that this was due to a slight increase in soil moisture due to humidity and seepage. Under these conditions all pre-emergents used would be deemed suitable for dry seeding purposes.

The risk with early dry seeding wheat is when a rainfall event occurs before dormancy of annual ryegrass is broken and the residual life span is not long enough to last until the major flush of germination occurs – usually mid to late May. To minimise these risks:

  1. Use a longer residual herbicide such as Sakura® (followed by Boxer Gold®, then TriflurX®)
  2. Delay dry seeding until late April or early May with a one week dry period to follow
  3. Ensure weed seed bank is low
  4. Use forecast services and the weed seed wizard to predict when germination over laps with active pre-emergent herbicides
  5. If early rain occurs wait until surface and sub surface has dried out before dry seeding

This research was presented at the 2017 Research Updates, Perth. A full paper on this work is availabe on the GIWA website Decay of pre-emergent herbicides in dry soils.

Dr Minkey has also discussed the behaviour of residual herbicides in a Podcast available on the WeedSmart website.

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