Managing weeds in emerging crops

Page last updated: Monday, 2 July 2018 - 8:25am

Following the extended dry conditions this autumn, weeds are now emerging and becoming a challenge for growers to manage in paddocks that may have patchy crops, crops with staggered emergence or no crop emergence at all.

Where the main flush of weeds germinate at the same time as the crop, strong early competition can severely reduce potential crop yield.

Annual ryegrass and wild radish are the most problematic weeds, as they continue to emerge with later rainfall. This page provides an overview of control options to better manage weeds in different cropping situations.

Weed control in chemical fallow, patchy or bared paddocks

Where the soil is bare because no crop emerged or was sown, one way to minimise soil erosion – especially on sandy-surfaced soils – is to let weeds grow and accumulate biomass before controlling them.

However, waiting too long to control weeds can result in weed seed-set that creates more problems in subsequent crops.

Plan to kill all weeds at the early flowering stage of weeds (1–5% flowering), before any seed is viable.

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).

Adding oxyfluorfen, SUs, phenoxy spike or carfentrazone-ethyl, to the mix 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 potential-yield cereal, legume and canola paddocks there are 2 options:

  • light grazing if stock are available (but keep minimum soil cover)
  • apply a double knockdown herbicide at the early flowering stage of weeds to kill all weed plants before they set any seed.

In high potential-yield cereal and legume paddocks:

  • apply selective herbicides, post-emergence, for in-crop grass and broadleaf weed control as appropriate. It is important to do this when the weeds are still small to reduce the impact of weed competition with the crop.

In high potential-yield 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 6 leaf stage of the crop. After the 6 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 for herbicide application, then it will still be beneficial to apply herbicide to control seed set of emerged weeds to reduce weed numbers in succeeding crops.

Suggested post-emergent weed control in four different herbicide-tolerant canola systems are shown in Table 1.

Table 1 Suggested post-emergent weed control in four different herbicide-tolerant canola systems
Herbicide tolerant canola Suggested post-emergent weed control
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 0.9kg PLANTSHIELD® by Monsanto/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 0.9kg PLANTSHIELD® by Monsanto/ha at cotyledon–2 leaf stage, followed by 0.9kg/ha at 4–6 leaf stage and 1.1kg atrazine/ha + 0.5% Hasten™ + 500mL Select®/ha + 1.0% Liase before or after PLANTSHIELD®
Clearfield (CL) canola 750ml Intervix®/ha + 0.5% Hasten™ + 500ml Select®/ha 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.

Contact information

Abul Hashem
+61 (0)8 9690 2136