Planning late summer and pre-seeding weed control

Author: | Date: 01 Jan 2019

Key Points

  • Weed management is a long-term investment requiring multiple tactics
  • Failure to control weeds increases cropping costs
  • Lack of weed control also heightens risks of herbicide resistance
  • It is vital to mix and rotate herbicides with different modes-of-action
  • Rotate crops and pastures
  • A double-knock herbicide tactic can be highly effective
  • Stopping weed seed set and destroying seeds at harvest will drive down the weed seedbank
  • Seeding systems and practices can set crops up to be competitive with weeds
  • One-off deep tillage can improve weed control.


Weed control tactics will vary widely across the Western Australian grainbelt going into season 2019 due to the vagaries of the past year. Conditions saw some areas contend with a dry start, warm or wet winter, out of season rainfall, spring frost or late hail.

But the key message for all regions is to act early and stay on top of emerging weeds during summer and autumn for optimum soil moisture and nutrient conservation and to minimise the ‘green bridge’ for carry-over of pests and diseases.

Weed species identification is key to successful control and herbicides are best used at weed seedling stages. This also reduces the need for the highest label rates of a knockdown herbicide before seeding.

GRDC is investing in research to understand and control WA’s most problematic weeds and has produced a range of resources for growers to find up-to-date information and advice. 
Links can be found in its latest GroundCover™ ‘Summer Weeds’ supplement.

During February, GRDC is offering growers free, half-day workshops focused on the value of early and effective summer weed control in WA grain cropping systems and tactics to achieve this.

Speakers include Department of Primary Industries and Regional Development (DPIRD) research officer Dr Catherine Borger, adviser Andrew Storrie, of AGRONOMO, and local growers.

GRDC invested in a series of glyphosate sustainability workshops last year and a series of summer weeds workshops will be held in February 2019. PHOTO: AGRONOMO.

Control of small weeds, particularly seedlings, in summer and autumn is achieved with IWM practices that include herbicides, strategic cultivation and/or grazing tactics.

Past research with GRDC investment has indicated controlling weeds early and conserving an extra 20 millimetres of stored soil moisture could be worth an extra 0.5 tonnes a hectare in crop yield.

It has found if summer weeds are allowed to establish in the fallow, these will start extracting water from depth (of more than 100 millimetres in some regions) within about 12 days.

Tips about achieving effective early weed control in WA can be found on the DPIRD website in the article 'Crop weeds: controlling small weeds'.

Further information about pre-emergent herbicide application is also available in the ‘Spray Application Manual for Grain Growers’.

It outlines various spraying systems, components and how to manage operations for optimal effectiveness, operator safety and minimal spray drift and contains latest advice about:

  • sprayer set-up, including self-propelled sprayers
  • tools for determining sprayer outputs
  • advice for assessing spray coverage
  • improving droplet capture by the target
  • drift-reducing equipment and techniques
  • effects of adjuvant and nozzle type on drift potential
  • surface temperature inversion research.
DPIRD research officers Abul Hashem and Catherine Borger are working to identify strategies to control summer weeds in WA. PHOTO: DPIRD

Optical sprayers

Optical spot spraying of isolated or small patches of weeds before seeding, or in wide-row cropping systems early in the growing season, can reduce herbicide use and help ensure sustainability of key active ingredients.

There are two main units available - WEEDit and Weedseeker®. The key difference is that WEEDit is typically a tow-behind machine and uses one sensor ‘eye’ with five lenses controlling five nozzles per one metre spacing on the bar. The Weedseeker® can be fitted to tow behind, or is self-propelled, and uses one sensor for each nozzle.

Research with GRDC investment has found the use of this technology can significantly cut herbicide use and costs, allow timely control and - in some cases - allow higher rates of herbicide to be used strategically or for hard-to-control weeds. But there is a high start-up cost of investment to consider.

Updated 2,4-D requirements

From October 2018, new 2,4-D label instructions came into effect, with a focus on reducing the likelihood of spray drift damage.

Users of 2,4-D must comply with the new label instructions, even if they are using products with the old labels. A useful resource is the GRDC Fact Sheet ‘Maintaining efficacy with larger droplets - new 2,4-D application requirements’.

The Australian Pesticides and Veterinary Medicines Authority (APVMA) has issued a permit to allow use of Amicide® Advance 700 and Trooper 75-D products through optical sprayer units, such as the WEEDit technology.

APVMA permit (PER87570) conditions include the use of optical sprayer technology, which delivers only a coarse spray quality with a 75cm boom height above the ground if only 10 per cent of the application area is treated.

More information is available from the Australian Pesticides and Veterinary Medicines Authority website.

Double-knock approach

Controlling herbicide resistant weeds is essentially all about double knockdowns, according to weed expert Peter Newman, from the Australian Herbicide Resistance Initiative (AHRI) and WeedSmart.

He says this can be one herbicide/mix followed by another herbicide/mix, or a herbicide/mix followed by a non-herbicide tool.

Peter says the aim is to kill any resistant survivors to the first herbicide/mix treatment with another control measure so that the weeds don’t grow and/or set seed.

He says common herbicide double knockdown tactics in WA include using:

  • a full label rate of glyphosate (Group M) followed by paraquat (Group L) - especially if herbicide resistance risk is low
  • glyphosate followed by a mix of paraquat plus diquat (Group L)
  • a mix of glyphosate plus an effective alternative herbicide in the first knock, then a robust rate of paraquat in the second treatment.

DPIRD’s Catherine Borger says glyphosate is the most important herbicide for controlling summer and early season weeds in WA, but several common species have evolved resistance to it.

She says all growers are advised to reduce resistance risks by mixing and rotating between alternative herbicide modes-of-action and integrating non-chemical weed control methods.
More information is available on the WeedSmart website.

Getting the pre-emergents options right

It is advised that growers always use robust herbicide rates in accordance with product labels and adhere to best management spray application recommendations for water rate, environmental conditions, droplet size and boom height.

Peter says a good tip is to invest in premium pre-emergent products and mix two together where possible to get a good rotation of active ingredients.

He says combining robust pre-emergent treatments with tactics to ensure a competitive crop, including seeding rate, row spacing and orientation, variety and soil health, would set crops up for optimal yields.

Crop competition

A competitive crop will typically suffer less yield loss and help reduce weed seed set, compared to an uncompetitive crop, according to Peter.

He says common tactics to boost crop competition include considering environmentally-appropriate:

  • seeding rates
  • row spacings
  • crop planting orientation (north-south versus east-west)
  • crop types and varieties
  • soil health amelioration measures
  • time of sowing.

More information is available on the WeedSmart website:

Narrow row spacing

DPIRD and AHRI research is finding narrow row crop spacing can deliver average cereal yield increases of about 5 per cent, mostly in medium and high rainfall areas, and at the same time almost eliminate annual ryegrass seed heads.

This can be achieved with single rows, of about 15cm apart, or paired row seeding systems - where a single seeding boot typically creates paired crop rows that are 75-100mm apart.
Dr Borger says DPIRD trials at Merredin from 1987 to 2013, using crop row spacings of 9, 18, 27 and 36cm and crop residue management (residue burnt prior to crop seeding or unburnt), found:

  • average wheat yields fell as row space increased
  • yields dropped from 1.66t/ha at 9cm to 1.49t/ha at 36cm
  • this was an average production loss of 0.4 per cent per 1cm increase in row width
  • average annual ryegrass seed numbers at harvest increased from 58/m2 at 9cm rows to 333/m2 at 36cm rows
  • residue burning reduced average crop yield to 1.53t/ha compared to not burned at 1.64t/ha
  • but burning residue reduced annual ryegrass seed at harvest to 57/m² from non-burned at 297/m².

More information and results from this long-term study can be in the update paper 'Eleven years of narrow row spacing'.

Run the seedbank down

Peter says growers who have been using a multi-pronged IWM program for the past 10 years are achieving very high levels of weed control and have slashed herbicide costs.
He says it is vital to run the seedbank down over the long-term to get on top of weedy paddocks.
Integral to success is stopping weed seed set using tactics such as:

  • crop topping canola, pulses and feed barley (currently under permit) in weedy paddocks
  • considering hay, brown manure or long fallow in high-pressure paddocks
  • spray topping pasture or use a spray fallow prior to the cropping phase
  • capturing and destroying weed seeds at harvest (Harvest weed seed control).

More information is available on the WeedSmart website.

Delving into weed control where deep tillage is used

Each weed species has a different depth from which it can emerge and the duration of which its seeds persist in the soil.

These ecological factors are important to understand when considering the potential role and consequences of using tillage.

Deep tillage can bury weed seeds to a depth from which they cannot emerge. However, they may persist and remain viable in these conditions and so secondary cultivation - even after two or three years - can bring them to the surface again.

Peter says growers using deep tillage systems, such as mouldboard ploughs, to combat soil health issues and boost crop yields are also getting good weed control on responsive soils.

He says they are smashing the weed seedbank and making extra profit following a one-off deep tillage operation

More Information

Dr Catherine Borger, DPIRD, 08 9690 2220,
Dr Abul Hashem, DPIRD, 08 9690 2136,
Alexandra Douglas, DPIRD, 08 9821 3246,

Useful resources

GRDC Project Code: UA00156, UA00149, DAW00257, CSP00111, BGC00003

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