Watch out for residuals

Summer weed control is essential and widely used, but the use of residual herbicides can compromise your options for broadleaf crops in winter.

Residual herbicides, while good at knocking down troublesome summer weeds, can damage broadleaf crops such as faba beans, lentils, chickpeas and other pulses or canola, in the winter.

If you are planning a pulse or canola crop in winter, or are at least leaving the option open, make sure you only apply non-residual summer weed herbicides in those paddocks.

Group B herbicides, such as metsulfuron-methyl (Ally), and Group I herbicides, such as Dicamba and Lontrel, have extensive plant back restrictions.

These herbicides need between six and 12 months to breakdown, along with suitable amounts of rainfall.

Crop failure is likely if susceptible crops are sown before the plant back period is complete.

These herbicides will effect the germination and vigour of all grain legume crops including field peas, faba beans, chickpea and lentils, along with canola.

While extensive summer rain can hasten the breakdown of these herbicides, the risk for susceptible crops remains very high.

Non-residual herbicides such as glyphosate, 2,4-D LV ester or 2,4-D Ester 800 and Garlon, are all preferred herbicides for the summer weed spraying program.

All can be used very close to the break of the season.

Plan your summer spraying program carefully to ensure that herbicide residues do not affect your broadleaf crop options.

Also, keep in mind your spraying program from winter 2013.

Some herbicides used for in-crop weed control in cereals have residual properties that present a significant risk to susceptible crops 12 months later.

Each pulse crop differs in its sensitivity to residual herbicides.

Check each herbicide label and assess the rainfall received since the herbicide was applied.

There is a significant risk to grain legumes and canola in paddocks if you applied some of the following herbicides to cereals.

Lentils and chickpeas are most susceptible to group B residues (e.g. Glean, Logran), with field peas and faba beans the least sensitive.

However, the risk for field pea and faba bean is still high.

Faba bean is more sensitive to Monza residues at low soil pH (< 6.5) than chickpea, lentil, lupin and field pea.

All pulse crops are sensitive at higher soil pH (> 6.5).

Chickpea, faba bean and field pea are least sensitive to the group B imidazolinones (e.g. Spinnaker, Raptor, Midas), with lentil extremely sensitive. Lupin and vetch are intermediate.

Raptor has no minimum re-cropping interval if field pea is being sown.

All pulses are vulnerable to Group I pyridine residues (e.g. Lontrel), but faba bean appears to be more susceptible than lupin.

This article was published online at Farm Weekly. More information is available from Pulse Australia.