What to do with emerging weeds?

As a result of wide-spread recent rains (August 2017), weeds are now emerging and becoming a challenge for growers to manage in paddocks with patchy crops, staggered emergence or no crop emergence.

Annual ryegrass and wild radish are the most problematic weeds as they continue to emerge with late rainfall events. The following information provides an overview of control options to better manage weeds in different cropping situations.

Weed control in chemical fallow, patchy or bared paddocks

In situations where the soil is bare because no crop emerged or was sown, one way to minimise soil erosion is to let weeds grow and accumulate biomass before controlling them.

Determining when to control weeds is important as they can provide cover to protect paddocks from wind erosion, particularly sandy surface soils; however, waiting too long to control weeds can result in weed seed set that creates more issues in subsequent crops.

Plan to kill all weeds at the early flowering stage of weeds (1-5% flowering) before weeds set any seed.

Spray glyphosate in late August, in the northern and central agricultual regions and late September in the southern agricultural region (rate should be consistent with label recommendation).

Addition of oxyfluorfen, SUs, phenoxy spike or carfentrazone-ethyl, as appropriate, will increase weed control efficacy.

Respray with Spray.Seed® or equivalent 2-7 days later.

Weed control in patchy and staggered germinated paddocks

In low yield potential cereal, legume and canola paddocks, consider light grazing, if stock are available (but keep minimum soil cover), otherwise apply a double knockdown herbicide at the early flowering stage of weeds to kill all weed plants before they set any seed.

In high yield potential cereal and legume paddocks, apply selective herbicides, post-emergence, for in-crop grass and broadleaf weed control as appropriate.

In high yield potential canola paddocks, herbicide can be applied to patchy canola crops having plants at different growth stages (already flowering and 4-6 leaf stage).

The safe window for application of glyphosate on RR or TR canola is from emergence to six leaf stage of the crop. After the six leaf stage of plants, tolerance of canola to herbicide starts to decline which could result in seed yield loss. However, if the majority of canola plants are within the safe window of herbicide application, then it will still be beneficial to apply herbicide to control seed set of emerged weeds to reduce weed number in the succeeding crops in rotation.

Suggested post-emergent weed control in four different herbicide-tolerant canola systems are:

Triazine Tolerant (TT) canola 1.1kg/ha atrazine + 0.5% Hasten™ + 500mL/ha Select® + 1.0% Liase at 2-6 leaf stage
Roundup Ready (RR) canola Roundup Ready herbicide with PLANTSHIELD® by Monsanto 0.9kg/ha at cotyledon-2 leaf stage, followed by 0.9kg/ha at 4-6 leaf stage
Triazine Tolerant Roundup Ready (TTRR) canola Roundup Ready herbicide with PLANTSHIELD® by Monsanto 0.9kg/ha at cotyledon-2 leaf stage, followed by 0.9kg/ha at 4-6 leaf stage and 1.1kg/ha atrazine + 0.5% Hasten™ + 500mL/ha Select® + 1.0% Liase before or after PLANTSHIELD®
Clearfield (CL) canola 750mL/ha Intervix® + 0.5% Hasten™ + 500mL/ha Select® at 2-6 leaf stage

The late season pre-harvest herbicide application to prevent weed seed-set (or to desiccate crops) needs to be carried out with extreme caution and in line with herbicide label recommendations.

Weedmaster®DST® is the only glyphosate formulation registered now for pre-harvest use in canola, wheat, chickpea, field pea, faba bean, and lentils. For more information refer to the GRDC Pre-harvest use of herbicide fact sheet.

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Abul Hashem
+61 (0)8 9690 2136
Page last updated: Monday, 21 August 2017 - 12:05pm

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