Residual herbicides - carryover and behaviour in dry conditions

Page last updated: Tuesday, 4 May 2021 - 1:36pm

Planting options for herbicide treated paddocks

If you suspect that there will be an issue with herbicide carryover from the previous year, it may be prudent to plant a crop that is more tolerant to the residual herbicide. Planting options for the common carryover problems are listed in Table 1.  

Table 1 Planting options for soils with suspected herbicide residue problems
 

Previous

crop

Herbicide

residue

Crops

at risk

Crop

options

Comments
1a TT canola or Lupins

Simazine/atrazine

Cereals Lupins Following canola, lupins are the most tolerant crop, but lupins after canola are often poor.  Volunteers are a big problem.  Delay sowing until volunteer canola germinates then control with glyphosate.  Remaining volunteers will compete until Eclipse® can be used.
1b       TT Canola Following lupins, if the soil is suitable and disease not an issue, this is the best option by far.  
1c       Peas/Chickpeas As for lupins.  Delay sowing as above.  Delayed sowing is a better prospect in peas than in lupins.  Use Broadstrike® to control canola volunteers.
1d       Barley

Risky option.  Barley is the most triazine tolerant of the cereals.

Sow as late as possible to allow some breakdown. 

Atrazine will be more damaging than simazine. 4 L/ha in the previous season is unlikely to be tolerated unless sowing is delayed by 6 weeks.

Sow with knife points, putting the seed below the herbicide as much as possible.  Sow after a rain event.  A drying topsoil will help as the plants will tend to produce deep roots and so minimise residue absorption.

Sow the shortest season variety possible.  Not an option if the break is delayed.
1e       Wheat

The riskiest option.

Machete is very sensitive, and should NOT be grown.

Newer varieties have not been tested.

Sowing rules as for barley, 1c.
1f       Pasture Grasses and clover may struggle, but there will be plenty of canola.  Some lupin seed could also be top dressed to fix N2.  Take the opportunity to prevent grass seed set and the paddock will be set up for wheat the following season.
2a

Clearfield ® canola

Imidazolinone tolerant

Imazapyr + Imazapic (OnDuty®)

Cereals Field Peas

Safe option.  Canola volunteers are a problem. 

Chickpeas are not an option, as Broadstrike® will not control volunteers.

Faba beans may be an option.
2b       IT wheat

Safe option.  For sound resistance management do not apply a Group B herbicide in this season.

Use phenoxy and/or diflufenican (e.g. Tigrex®, Jaguar®) based products and mixes for control of canola volunteers.
3a Pulses Imazethapyr (Spinnaker®) Cereals IT wheat As for 2b above.
3b       IT canola

If the soil is suitable and disease not an issue, this is a good option.

Use Lontrel® if volunteers are a problem.
4a Cereal

Sulfonyl ureas –

Chlorsulfuron, triasulfuron, metsulfuron

Legumes Cereal The easy and safe option with wheat being the safest choice.
4b     Non IT canola IT canola

If canola was the planned rotation crop, IT (Clearfield®) varieties will be safe.  The ability to tolerate soil residues of sulfonyl ureas does not imply that these herbicides can be used in-crop.

TT and conventional varieties are not as sensitive to low SU residues as legumes, and may be safe. Do a pot test to check.

Futher information on safe plant back periods is available on all herbicide labels. Note: the plant back period refers to the period of time, usually in months, that is deemed safe to plant sensitive crops following the application of a residual herbicide.

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