Residual herbicides - carryover and behaviour in dry conditions

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

Can we test for herbicide residues?

On-farm pot testing

If you are not sure how much of the herbicide is left in the paddock, it is recommended to conduct an on-farm pot test for residues. When pot testing make sure that the pots remain well drained and not too wet as waterlogging will interfere with your ability to assess whether or not there are herbicide residues.

Triazines

Fill three 5L pots with surface soil from suspect paddock, sow 10-15 wheat seeds in each pot, water the pots and keep in a cool, shady but well-lit spot for about 6-8 weeks. Fill another set of three 5L control pots with surface soil from fence line or an area similar to paddock where suspect herbicides were not applied following the preceding procedures.

At the 5-6 leaf stage, triazine symptoms will show up as tip burning or watery blotches in the middle of the leaves. In such cases, there is a danger of wheat yield penalty.

At low levels of triazine residues, there may be only a general paling of the plants and/or a reduction in height and vigour. In such cases, there will probably be no yield penalty because the residue effects will be more severe in the pots than in the fields.

Sulfonylureas:

Prepare as many sets of pots as required using soils treated with sulfonylureas from 0-10 cm and 10-20 cm depths following the same procedures as described above along with a set of control pots filled with untreated soil for each depth.

Use the legume crops of your choice or a more sensitive species.

Sensitivity ranges from field peas (least sensitive)<lupins<chickpeas<lentils (most sensitive). Corn and canola are also good choices to test for SU residues.

Allow the seedlings to grow up to 2-leaf stage, then carefully pull the seedlings and wash the soil from the roots. If sulfonylureas residues are present, the roots will be thickened and shortened with few or no fine root hairs. Reddening of leaf margins is a characteristic

The MyCrop site has further information on how to diagnose the symptoms of herbicide damage in cereal and broadleaf crops.

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