Residual herbicides may have an impact on crop choices in 2007.

Carryover of residual herbicides in soil is worse after a drought; the lack of moisture reduces herbicide breakdown.

The levels of herbicide remaining in soil are affected by the date of herbicide application, rate and type.

Soil texture and pH also affect the rate of breakdown, hence the label has plant-back periods for different soil acidities and often lists the level of cumulative rainfall needed for effective breakdown.

The chemical and biological processes that degrade herbicide residues are highly dependent on soil temperature and moisture levels.

The surface often contains the residue and if dry, despite high soil temperatures, residue breakdown is often slow in summer.

If conditions remain dry up to sowing, consider a crop species or variety suited to a later sowing period.

For example, switching from lupins to field peas, results in a much later sowing window.

Totally changing the crop species, for example using wheat in a paddock with suspected glean residue, is another alternative.

There are a number of ways to test for residues.

There will often be damage to weeds or volunteer plants growing in a paddock, so examining weeds germinating in a paddock before sowing is probably the easiest way to get some indication.

Symptoms are delayed where herbicides have leeched deeper into the soil – it can take time for roots to pick up the herbicide.

If there is an early break before the sowing window, sowing a strip of sensitive species across the paddock can give an indication.

This is a more reliable method than using a pot test, however, it can lead to sowing delays.

Pot tests need to be carried out at least three to four weeks before sowing.

This involves planting sensitive species into pots of the soil with a suitable control for comparison.

A more detailed description of the pot test method can be found in the NSW Department of Primary Industries publication, Weed Control in Summer Crops.