Australian Pulse Bulletin

Residual herbicides and weed control

Pulse growers need to be aware of:

• Possible herbicide residues impacting on crop rotation choices where rainfall has been minimal.
• Herbicide residues could possibly influence crop rotations more than disease considerations.
• Weed burden in the new crop will depend on the seed set from last year.
• Herbicide efficacy and crop safety of the new crop can suffer if the soil is dry at application time.

Residual effects

Herbicide breakdown: Herbicides applied to paddocks in previous years may not have broken down adequately because of insufficient rainfall. Summer rainfall is not necessarily as effective as growing season rainfall in breaking down herbicide residues, so needs to be substantial and to keep the soil wet for a specified time. Read the herbicide label. It will be extremely important to know the chemical type used, as well as the plant-back periods, and the soil pH, rainfall and other requirements for breakdown. Herbicides applied two years ago could still have an impact too, as could the presence of cereal stubble with herbicides like Lontrel®.

Withholding periods for dicamba or similar ‘spikes’ to knockdown sprays used pre-sowing may need to be longer if there is no rainfall to activate chemical breakdown, otherwise poor establishment can occur. Note that dicamba plant-backs only commence after 15 mm of rain. Lentils, faba beans are not listed on label. Alternative products with lesser or no residual may be more appropriate (e.g. carfentrazone-ethyl, oxyfluorfen).

Crop sensitivities: Pulse types differ in their sensitivity to residual herbicides, so check each herbicide used:

• Lentil and chickpea are the worst for group B sulfonyl urea residues (e.g. Glean®, Logran®), with peas and faba/broad beans the least.
• Faba/broad beans are more sensitive to Monza® residues at low pH (<6.5) than chickpea, lentil, lupin and fieldpea. All are sensitive at higher pH (>6.5).
• Faba/broad bean, lentil and lupin are more sensitive to group B sulphonamide residues (e.g. Broadstrike®), and be aware of the impact on shallow duplex soils.
• Chickpea, faba/broad beans and fieldpeas are least sensitive to the group B imidazolinones (imis, e.g. Spinnaker®, Raptor®, Midas®), with lentils extremely sensitive, and lupins, vetch intermediate.
• Raptor® has no minimum re-cropping interval if field peas are being sown.
• All pulses are vulnerable to group I pyridine residues, and the rate applied (e.g. Lontrel®), but beans appear more vulnerable than lupins.
• Lentil cannot follow straight after chickpeas if Balance® (group F) has been used in the chickpea.
• Lentil cannot immediately follow after beans or field peas if some group B’s were used (e.g. Spinnaker®, Broadstrike® or Raptor®).

In areas that receive minimal summer-autumn rains and delayed opening rains, then the herbicide residual effect becomes far more pressing on rotation choices. Pulse following cereal could then become a higher risk situation than pulse following a pulse!

Weed burden: Weed burden in 2016 will depend on past history and how effective seed set was prevented, and will influence sowing choice and strategies. There was minimal weed germination after poor rainfall events of 2015, but weed seed set may have been minimised through drought, hay cuts, desiccation, crop top or grazing of crops, unless the crop was let go or weed control failed.

Pulses can fit a rotation to ensure herbicide diversity and the ability to crop top or desiccate to prevent weed seed set. Resistant ryegrass has been a major target, but Giant Brome resistant to group A herbicides also poses a new threat in areas like the Wimmera and Mallee.

Herbicide efficacy in the dry: Instances of herbicide damage that occurred under dry conditions be avoided in future, eg metribuzin and diuron in field pea and lentil, and with Balance® in chickpea.

• If applied to dry soils, the risk of herbicide damage is increased greatly when rains occur, particularly if the soil is left ridged.
• PSPE herbicides should be applied to moist soil regardless of the sowing time.
• IBS may be more appropriate in dry conditions, or a split application to minimise risk in future.

The decision as to which pulse to grow, and where, will be based on risk and rotation need. Assess the disease risk versus residual herbicide risk. Assess any quality issues that arise from re-cropping, e.g. chickpeas after field peas, lentils after vetch. The rotation solutions do not become fully apparent until the summer-autumn rainfall has fallen.

Disease implications

Despite the very low starting disease levels from pulse stubble after a dry year, it is the rainfall conditions in the new growing season that will have the major influence on the severity of pulse diseases.

Paddock selection is still critical, and sowing time along with canopy management is always important in disease management, and the cheapest to implement.

If considering growing a pulse again on a failed pulse crop, then consider what pulse to choose. Disease risk is one of the main considerations, and whether it can be managed. Botrytis grey mould (BGM), phoma and sclerotinia are common across most pulse species, but ascochyta blight (AB) is not. There is an ascochyta blight specific to each pulse species that do not cross infect. Assess any quality issues that arise from re-cropping, eg chickpeas after field peas, lentils after vetch.

Data taken from herbicide labels as viewed from APVMA web site, or company web sites.

Wayne Hawthorne, formerly Pulse Australia

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Last updated: 20 November 2015