Section 2: Herbicides and killing weed seedlings

Section 2.1 - Introduction
Section 2.2 - Killing weeds with herbicides
Section 2.3 - Weed control in wide-row cropping
Section 2.4 - Spot spraying, chipping, hand roguing and wiper technology
Section 2.5 - Weed detector sprays
Section 2.6 - Killing weeds with tillage
Section 2.7 - Biological control
Section 2.8 - Further information

2.1 Introduction

Killing weeds with cultivation has been the focus of weed management since agriculture was first developed. Since the release of products such as trifluralin, 2,4-D, glyphosate and Group A and B herbicides, herbicides have been the primary tool for controlling weeds due to their cost effectiveness, non disturbance of soil and crop residue, high levels of control and ease of use. However this approach to controlling weeds has led to the development of herbicide resistance. Despite herbicide resistance, herbicides remain an important tool, but require support from a range of non-herbicide tactics to remain effective.

Tactics that assist include fallow and pre-sowing and interrow cultivation, double knock, alternate pre and post-emergent herbicides, rogueing individual plants and weed detector spraying.

2.2 Killing weeds with herbicides

The rapid development of resistance in glyphosate in several weeds has placed increased reliance on in-crop weed management.  With many selective herbicides already with their own resistance issues, there is far greater reliance on pre-emergent herbicides as well as increasing reliance on several non-herbicide tactics to keep weed seedbanks in decline.

With the last significant new mode of action groups released into the Australian Herbicide market being Group B when chlorsulfuron was launched in 1982 (over 30 years!!) and Group H in 2001, and with no awareness of any new post emergent herbicides anywhere near commercialisation, it is plain to see that the supply of new chemistries is limited.

The only new modes of action on the horizon at the time of writing are all pre-emergent chemistries and they are not great in number.

The bottom line is that we need to look after what we currently have for as long as we can, as the cavalry is just not coming!

Further information on registered chemicals can be found from APVMA and CropLife Australia. Regional weed control references are available from a range of sources.

2.2.1 Knockdown (non-selective) herbicides for fallow and pre-sowing control

Knockdown herbicides are key tools to enable no-till fallows to be economically and efficiently managed. More recently, they have also been used in the crop – especially glyphosate in Roundup Ready crops.

Knockdown herbicides also represent a key component of other weed management tactics, including:

Since its release in the late 1970’s, glyphosate is now the most widely used herbicide in the world.  Prior to this, paraquat was more commonly used. Developed to deal with capeweed in southern Australian farming systems, Spray.Seed® (paraquat + diquat) also improved the control of Erodium spp. Capeweed (Arctotheca calendula) and black bindweed (Fallopia convolvulus) over paraquat used alone.

In unselected weed populations, genes carrying resistance to glyphosate are rare.  Some 15+ years of selection are required before the frequency of resistant individuals is likely to lead to a spray failure.

The Australian Glyphosate Sustainability Working Group website provides up-to-date information on glyphosate and paraquat resistance.

With widespread use over a prolonged period and often few if any other measures taken to control weed escapes, populations of weeds resistant to glyphosate have increased exponentially in recent years.  This increase is forecast to continue.

In Northern Australian cropping systems, rotations to summer crops have regular winter fallows with no weed seed set allowed. This has been a simple and highly effective method to reduce the weed seed bank of annual winter weeds with low seed bank dormancy.  In systems with regular weed free winter fallows, the weed seed banks of many key winter weed species such as wild oats, annual ryegrass and Phalaris, have been driven so low, that in-crop herbicides are often not required for these weeds.  Often glyphosate has been the only weed management tactic controlling these and other grass weeds in fallow. Not surprisingly, this system has selected for glyphosate resistance in annual ryegrass.

In winter crop no-till rotations, the selection pressure for resistance to glyphosate is placed more on summer weeds. Here we have developed glyphosate resistance in multiple grass weeds as well as fleabane and more recently, milk/sow thistle. No-tillage has enabled the wheat belt to expand into lower rainfall rangeland country as it has enabled far better management and storage of limited rainfall.  Increasingly widespread resistance to glyphosate threatens the base technology of many current cropping systems.

Cultivation is returning to many no-till fields as a result of resistance to glyphosate.  Other issues can also drive the decision to spray or cultivate.

Table 1. Fallow situations – suggestions for weed control (Somervaille and McLennan, 2003)




Weeds small and fresh


Weeds stressed


Soil too wet for machinery


Weather unsuitable for spraying


Weeds grazed but have not regrown


Excessive wheel tracks after harvest


Excessive stubble in windrows/disease control


Soil conditions suitable for planting


Build-up of weeds not well controlled by herbicides – too large and/or resistant


Wind erosion – paddock starting to blow


With widespread use of herbicides comes increased potential for spray drift.  Weather conditions, droplet size, proximity to adjoining crops are critical issues.


Somervaille, A. and McLennan, B. (2003). The 2nd Fallow Weed Management Guide. Conservation Farmers Inc.

Also see the Summer fallow weed management reference manual (2014) and GRDC Adjuvants booklet: Oil, surfactants and other additives for farm chemicals (2015) for more information.

GRDC 'Tips and Tactics': Keeping sorghum safe (2016)

-- top--

2.2.2 Double knockdown or ‘double knock’

‘Double knock’ is ideally defined as the sequential application of two different weed control tactics where the second tactic controls any survivors from the first tactic. From a resistance perspective, a double knock works when either of the two tactics used would kill the weed population by itself. However, with increasing resistance to glyphosate, double knock applications are often used just to obtain a commercially acceptable level of weed control.

A double knockdown technique in common use is the sequential application of glyphosate (Group M) followed by paraquat/diquat (Group L), at an interval of between one and 14 days. For resistance management benefits, each herbicide in the double knockdown must be applied at a rate which would be sufficient to control weeds if it was used alone. The second herbicide is applied to control any survivors from the first herbicide application. Control of weeds germinating between the two applications of herbicide is a secondary benefit.

Other double knock strategies include following a herbicide with burning or grazing, or harvest weed seed capture followed by removal, seed destruction, or burning. Increased crop competition also provides a partial double knock to reduce the number of seeds set by weeds surviving other management tactics.

Double knock strategies delay the onset of herbicide resistance.  However, modelling by Thornby et al (2008) shows that if many years of selection take place where survivors of glyphosate applications are allowed to set seed before double knock strategies are used, the benefit of double knock as a delaying strategy for the onset of resistance to glyphosate is greatly diminished.

A secondary benefit of using a double knockdown or double knock strategy is general improvement of control of those weeds where glyphosate alone may not provide commercially acceptable levels of control.

Some key grass and broadleaved weeds can only be reliably controlled using double knockdown sprays.  Some key weeds include:

Populations of weeds that have developed resistance to glyphosate:

At the time of writing, permits or registrations were available in NSW and Queensland for a double knock of specified Group A mode of action herbicides followed by paraquat for the control of windmill (NSW) and feathertop Rhodes grass (Qld).  On these weeds, a double knock with glyphosate followed by paraquat has not been sufficiently robust to provide control. Note there are residual / recrop issues for following crops when using Group A herbicides in fallow. 

In addition to the standard glyphosate followed by paraquat doubleknock, a number of other herbicide strategies are recommended for specific situations:

  • For control of fleabane in fallow, mixing glyphosate with either FallowBoss® Tordon® (aminopyralid + picloram + 2,4-D) or Amicide® Advanced (2,4-D amine) then followed by an application of a paraquat based herbicide
  • The group A herbicide Shogun® (propaquizafop) followed by paraquat/diquat for control of feathertop Rhodes grass in fallow.
  • A permit (PER 12941 Expires 31 Aug 2019) is in place to allow the use of the Group A herbicide haloxyfop followed by paraquat to control feathertop Rhodes grass in fallow in Queensland only, preceding mungbean planting.

Key issues for double knocking

Where glyphosate and/or paraquat are appropriate products to use, glyphosate should be applied first and followed by paraquat or paraquat/diquat.

The ideal time between applications will vary with the main target weed species.

Almost all annual species benefit from at least 1 day or more between applications. In some species, longer delays of 1-2 weeks are beneficial, but delaying too long can lead to regrowth of weeds and result in poorer results.

For very small weeds, adequate glyphosate may translocate to the roots within 1 day however for many tillering weeds herbicide translocation to the roots continues for approximately 3 days. In some species, particularly larger or woody type plants, longer delays of 1-2 weeks are beneficial, but delaying too long can lead to regrowth of weeds and result in poorer results.

Apply the first herbicide when the weeds are most likely to be killed, i.e. small and actively growing.

Maximum control of annual ryegrass results from an application of herbicide at the three- to four-leaf stage. Annual ryegrass sprayed at the zero- to one-leaf stage can potentially regrow from seed reserves (Borger et al 2003, 2004).

Later application, when the annual ryegrass is tillering, risks an incomplete control by the bipyridyl application. Paraquat and Spray.Seed® are contact herbicides which result in little translocation taking place within the plants.

When applying contact herbicides or Group A mode of action herbicides (which are also relatively poorly translocated), increase spray carrier volume and avoid very coarse droplet sizes as excellent spray coverage is needed for success. (See the summer fallow weed management reference manual).

Be aware that double knocking a percent of land each year will add logistical stress to spray operations. This needs to be planned for.

Further information: Effective double knock herbicide applications fact sheet (2012)

-- top--

2.2.3 Pre-emergent herbicides

Pre-emergent herbicides control weeds at the early stages of the life cycle, between radical (root shoot) emergence from the seed and seedling leaf emergence through the soil.

Some pre-emergent herbicides also have post-emergent activity through leaf absorption and can be applied to newly emerging weeds.

The residual activity of a pre-emergent herbicide controls the first few flushes of germinating weeds (cohorts) while the crop is too small to compete. As a result, pre-emergent herbicides are often excellent at protecting the crop from early weed competition.

There are four main factors to consider when using pre-emergent herbicides.

1. Weed species and density - Knowledge of what weeds are expected is critical.  Pre-emergent herbicides are particularly beneficial at stopping early weed competition – especially if high weed densities are expected.

2. Crop or pasture type - How competitive is the crop and what herbicide and non-herbicide options exist?

3. Soil condition - Cloddy soil surfaces, large amounts of stubble or excesses of charcoal from stubble burning can all affect the performance of some pre-emergent herbicides.

Less soluble herbicides such as simazine need to be mixed with the topsoil for best results. While more mobile herbicides such as the sulfonylureas and imidazolinones may not need mechanical incorporation, as they move into the topsoil with water (rain or irrigation). Some herbicides need incorporation or coverage to prevent UV losses (e.g. atrazine) or volatilisation (e.g. trifluralin).

4. Rotation of crop or pasture species - All pre-emergent herbicides persist in the soil to some degree. Some post-emergent herbicides may also persist in the soil. Consequently, herbicides may carry over into the next cropping period. The time between spraying and safely sowing a specific crop or pasture without residual herbicide effects (the plant-back period) varies, depending on herbicide, environmental conditions and soil type.

The following factors all influence herbicide behaviour in the soil. All these issues are dealt with in some detail in the GRDC publication 'Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers'.

  • how strongly a herbicide binds to organic matter or the soils cation exchange sites (reflected in K equilibrium coefficients)
  • the herbicides solubility in water
  • herbicide mechanism of breakdown (i.e. chemical/microbial)
  • herbicide persistence as reflected by their DT50 values (days of time for 50% breakdown or half-life)
  • soil texture
  • soil pH (for some herbicides)
  • organic matter
  • previous herbicide use
  • soil moisture
  • initial application rate
  • soil temperature
  • volatilisation
  • photo degradation

For further information see: Integrated Weed Management Manual:  Herbicide uptake by plants


Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers. This link provides detailed information on the soil behaviour of all key pre-emergent herbicides used in Australian field crops as at 2015.

Table 2. Soil attributes that contribute to herbicide availability

Higher herbicide availability

Lower herbicide availability

Sandy soils

Clay soils

Low organic matter

High organic matter

Wet conditions

Dry conditions

When using pre-emergent herbicides, it is important to think about how the herbicide kills weeds, how it gets into the weed zone and where it will be when weeds are germinating. Typically, situations that reduce availability will require higher application rates to achieve equivalent control. Properties that reduce availability also tend to increase the length of herbicide persistence in the soil, thus increasing rotational crop constraints.

A pre-emergent herbicide that is sitting on a dry soil surface at the time of weed emergence is unlikely to have sufficient soil moisture for uptake by the weed or sufficient contact with the emerging weeds to kill them.  Such a scenario might occur if a herbicide was applied immediately post sowing while weeds were already germinating and there was no rain or mechanical incorporation to take the herbicide into the germination zone where it can be taken up by the young weeds.  Weed escapes in such situations are likely.

Crop safety is also an important issue when using pre-emergent herbicides.

Crop tolerance of several pre-emergent herbicides (i.e. trifluralin) is often related to spatial separation of the young crop from the herbicide.  This in turn is related to the level of solubility and potential movement in the soil of the herbicide, the crop establishment process, the level of soil displacement over the crop row, follow-up rainfall and physical nature of the seed furrow.

Table 3. Positive and negative aspects of using pre-emergent herbicides



Relatively inexpensive

Strongly dependent on soil moisture

Optimises crop yield through control of early weed germinations

Because weeds are not yet visible, must have paddock history/knowledge of previous weeds/weed seedbank

Potentially different modes of action to post-emergent herbicide options;

Plant-back periods limit crop rotation

Timing of operation – different application timing to post-emergent options may assist

Crop damage if sown too shallow or rainfall or other actions move excessive quantities of herbicide into the root zone

Best option for crops with limited post-emergent options (e.g. grass control in sorghum and broadleaved weed control in pulse crops)

Seed bed preparation – soil may need cultivation and herbicide may need incorporation, which can lead to erosion, soil structural decline and loss of sowing moisture

Effective on some weeds that are hard to control with post-emergent herbicides (e.g. wireweed and black bindweed)

Not suitable when dense plant residues or cloddy soils are present

Extended period of control of multiple cohorts - good for weeds with multiple germination times

Varying soil types and soil moisture across paddock can be reflected in variable results

Further information: Soil behaviour of pre-emergent herbicides in Australian farming systems: a reference manual for agronomic advisers

-- top--

2.2.4 Selective post-emergent herbicides

Selective post-emergent herbicides control emerged weeds in the crop or pasture.

The first selective post-emergent to be developed was a Group I MOA herbicide 2,4-D – released in around 1945. More recently, Group A and B herbicides were released in the 1980’s.

Selective post-emergent herbicides belong to a wide range of herbicide MOA Groups.

While there have been many new selective post-emergent herbicides released in recent years, all of these are from known/existing mode of action groups.

Selective post-emergent herbicides give high levels of control (often >98 per cent) when applied under recommended conditions on susceptible populations. When used early in crop development, at recommended rates and timings, selective post-emergents also result in optimum yield with potential for significant economic returns.

Early use on small susceptible weeds both improves control and removes weeds before significant crop yield loss occurs.

In addition to post emergent activity, some post-emergent herbicides also have pre-emergent activity on subsequent weed germinations.  This is particularly the case with some Group B, C, F & I MOA herbicides. Group A products have sufficient residual activity that they may affect cereal crops if sown too soon after use.

When choosing a selective post-emergent herbicide for a particular situation, consider the following factors:

  • target weed species and growth stage
  • herbicide resistance status of target weeds
  • crop safety (variety, environmental conditions, effect of previously applied herbicide on crop)
  • grazing and harvest withholding periods and plant-back periods (minimum recropping intervals after application)
  • cost
  • spray drift risk
  • mix partners

Application of post-emergent herbicides to stressed crops and weeds can result in reduced levels of weed control and increased crop damage.

Normally tolerant crops can be damaged when stressed due to waterlogging, frost or dry conditions because they cannot produce sufficient levels of the enzymes that normally break down the herbicide.  Examples are when sulfonyl ureas are applied to cold and waterlogged crops and high levels of crop impact are seen.  Group A herbicides often fail to kill weeds if applied too soon after a severe cold stress (frost).

Good crop competition improves the efficacy of post-emergent herbicides. Marley and Robinson (1990) found that barley was more competitive than wheat against black bindweed (Fallopia convolvulus) and turnip weed (Rapistrum rugosum) and that higher crop populations improved the effectiveness of herbicides against these species (Figure 1).

In a study by Walker et al (1998) on the effect of crop type and herbicide rate on annual phalaris seed set, it was shown that barley was more competitive than wheat at all rates of herbicide (Figure 2).

Figure 1. Effect of wheat and barley population and herbicide rate on the dry matter production of turnip weed and black bindweed measured at crop anthesis (Marley and Robinson 1990).

Bar chart showing effect of wheat and barely population and berbicide rate on the dry matter production of turnip weed and black bindweed measured at crop anthesis.

Figure 2. Effect of crop type and herbicide rate on paradoxa grass seed production (Walker et al 1998)

Bar chart showing effect of crop type and herbicide rate on paradoxa grass seed production.

When using selective post-emergent herbicides it is important the correct application technique is used.  Particular attention should be paid to:

  • equipment (nozzles, pressure, droplet size, mixing in the tank, boom height, ground speed) to maximise the efficiency of herbicide application to the target
  • meteorological conditions. Suitable conditions are indicated by Delta T (ideally <8°C) when air movement is neither excessively windy nor still. (Delta T is an indication of evaporation rate and droplet lifetime and is calculated by subtracting the wet bulb temperature from the dry bulb temperature).
  • spraying should not be conducted in inversion conditions. DO NOT SPRAY during the day when wind speed is less than 3 km/hr, or at night when wind speed is less than 13 km/hr. These wind speeds are the minimum required to break-up layers of air at different temperatures.
  • spraying should be ideally done when temperatures are less than 28°C.

It is important to use the adjuvant recommended on the herbicide label to get the best performance from the herbicide being applied. Because plants have different leaf surfaces, an adjuvant may be needed to assist with herbicide uptake and leaf coverage. Some adjuvants can also increase performance by lowering the pH, hard water, compatibility, rain fastness or drift. For more detailed information on adjuvants, see Somervaille et al. (2012).

Selective post-emergent herbicides applied early and used as a stand-alone tactic are likely to be insufficient to put the weed seedbank into decline over time.

Early post-emergent herbicide use is aimed at maximising yield by removing weed competition in crop establishment stages. Any weed that germinates after, or survives this application, will set seed that will return to the seedbank, thus maintaining weed seedbank numbers and ensuring continuation of the weed problem.

To drive down the weed seedbank over time, it is important to use later season seedbank management tactics in association with early post-emergent tactics. Seedbank capture and management tactics work similarly to help drive the weed seedbank down.

Table 4. Effect of annual applications of different herbicide treatments on wild oat seedbank numbers after 5 years (Cook 1998)

Herbicide treatment

Percentage change in wild oat numbers over 5 years

Pre-emergent alone


Post-emergent alone


Post-emergent + selective spray topping


The effectiveness of selective post-emergent herbicides is influenced by a range of plant and environmental factors.

Inactivation of herbicides can occur due to:

  • leaf and cuticle structure
  • dust particles, or
  • washing product off the leaf due to rainfall or dew.
-- top--

2.3 Weed control in wide-row cropping

In northern New South Wales and Queensland, wide-row cropping (1 meter +) has been used for some years as a means to improve yield reliability in summer crop production. Wide rows are also used in wheat and chickpea cropping (50 cm +) in the northern grains region to improve stubble handling and moisture seeking abilities of sowing operations.

Wide-row cropping has also been used in Western Australia as a strategy to overcome herbicide resistant wild radish and, to a lesser extent, annual ryegrass.

Key weed management tactics in wide row crops include:

  • Inter-row tillage
  • Shielded spraying
  • Band spraying
Table 5. Pros and cons of weed control tactics in wide rows




Wider rows

Enables easier stubble flow at sowing and more stubble to be retained with benefits for erosion control, moisture capture and retention and soil health

Less crop competition with weeds.  Can lead to reduced yield if weeds are not controlled

More options for moisture seeking at sowing

Minimal yield downside in some pulse crops such as chickpeas


Yield loss in high yielding cereals and chickpeas – especially when wider than ~30 cm

Skip and double skip row configurations in rain fed summer crops like cotton and grain sorghum enable more reliable water supply during grain fill and thus yield when combined with sowing on a full profile of soil water in a soil with a high PAWC

Skip and double skip row configurations in summer crops like cotton and grain sorghum leave a biological desert in the wide inter-rows when there is a long fallow period into and out of a summer crop.  This can lead to problems with mycorrhizae inoculation and issues with P and Zn nutrition in the next crop.  Also the low level of stubble cover can lead to poor infiltration of rain and higher evaporation rates, with difficulties in getting soil water replenished for the next crop.  Short term (6-8 weeks) cover crops of millet have been successfully used to mitigate these issues.

Option to use a smaller tractor pulling fewer tynes per sowing width thus reducing costs


Better airflow in the canopy and reduced issues with foliar disease


Easier tramlining and controlled traffic farming adding benefits associated with reduced compaction and more accurate and timely application of inputs


More opportunity for precision fertiliser placement


Shielded sprayers

Enables use of a non-selective herbicide that could enable control of weeds that can't be adequately controlled with selective herbicides (yield benefit) or enable control at lower cost.

Lack of registrations. As bipyridyl herbicides are the only ones currently registered and only in some crops, timing on small weeds only and excellent application coverage are essential.


Risk of crop injury and yield loss.  Extreme care is needed when setting up and using shielded sprayers.


Technology and time investment


Misses weeds growing in the crop row so the weed seedbank is still replenished


Selects for resistance to the product used

Band spraying

Uses less herbicide thus limits resistance pressure as well as reducing cost

Potential for weed escapes and seed bank replenishment from weeds in the untreated zone.  Mitigate against this by using in association with inter-row tillage or shielded spraying (limited registrations).

Inter-row cultivation

Does not need herbicides so resistance is not an issue

Can only be used in a drying seedbed or escapes are likely


Crop yield loss due to: Compaction especially if soil is wet; moisture loss; weed escapes (transplants and weeds in the crop row); root pruning


Timing on small weeds and in a drying soil profile are critical to success.  Weed escapes are likely if weed densities are too high or the soil is too wet.


May not be possible if there is a high stubble load

Soil disturbance can reduce subsequent emergence of some surface germinating weeds such as fleabane or milk thistle.

Soil disturbance can stimulate some weeds to germinate, leading to a larger weed emergence on the next rain


Soil disturbance can bury weed seeds and cause increased seedbank dormancy in some weeds as well as reducing natural mortality rates compared to weed seeds left on the soil surface.

-- top--

2.4 Spot spraying, chipping, hand roguing and wiper technologies

Teams of weed chippers used to be a common sight in cotton and other crops.  When weed numbers are low or when still contained in patches, hand weeding, spot spraying and other methods including selective crop destruction can be used to stop weed seed set and seed bank replenishment.

Wiper technologies are useful when there is a suitable height differential between the crop and weeds to allow a weed wiper to apply concentrated herbicide to the weed while avoiding contact with the crops plants.

Where new weed infestations occur in low numbers, eradication may be possible. In such situations more intensive tactics to remove weeds can be used in addition to ongoing management tactics which aim to minimise weed impact.

Some key points:

  • Stay vigilant and on the lookout for new / isolated weeds
  • Be prepared to get out of the ute and hand pull weeds
  • Keep a rubbish bag handy for weeds which already have seeds developed
  • Correctly identify new weeds and what control measures are appropriate
  • Manage and isolate outbreaks and hot spots
  • Stop weed seed set!
  • Plan follow-up observation and management
  • GPS mark isolated weed patches and diary to check for later germinations

For further information see tactic 2.4 of the Integrated Weed Management Manual on spot spraying, chipping, hand roguing and wiper technologies.

-- top--

2.5 Weed detector sprays

Weed detector sprayers are for the control of scattered weeds in crop fallows. Weed detector-activated sprayers use sensors to detect the presence of weeds. When a weed is detected, a solenoid turns on an individual nozzle and the weed is sprayed.

This technology is in widespread use in the Northern Grains Region, with reductions in the volume of herbicide used in fallow per hectare of 80 to 95% commonly reported.

This technology allows the use a range of herbicide modes-of-action and or higher than usual rates while remaining economical.

An APVMA minor use for the WeedSeeker® Permit (PER11163) allows several different mode-of action to be used in fallows (valid until February 2019). Go to the APVMA site and enter the permit number.

Some added benefits / issues of this technology include:

  • Lower drift risk as coarse droplets are used and only a low percent of the paddock is sprayed
  • Infrared signal enables night time use which is a dual edged sword. A significant downside of night spraying is a greatly elevated risk of inversion drift conditions. Inversions occur most nights and minimum wind speeds of 13 km/hr are required for night spraying.
  • Weeds in wheat stubble should be larger than ~ 5 cm for reliable detection.
  • Maintaining correct boom height, staying within design travel speeds and avoiding spraying in strong winds are essential for reliable performance.

Useful links:

-- top--

2.6 Killing weeds with tillage

Cultivation can kill many weeds including herbicide resistant and hard-to-kill populations. Cultivation is useful as a ‘one-off’ tactic in reduced tillage or no-till operations. Recent research in the Northern Grain Region has shown well-timed cultivation in a no-till system can give a range of benefits with manageable reduction on conservation farming goals. Planned cultivation can also be used as a non-herbicide component of a ‘double knock’ system (see Double-knock tactic in the Integrated Weed Management Manual).


1. Well-timed cultivation in a drying soil effectively kills weeds.

Cultivation destroys weeds in a number of ways, including:

  • seed burial (reducing the ability of some species to germinate if sufficiently deep)
  • severing roots
  • plant desiccation, where plants are left on the soil surface to die
  • breaking seed dormancy or seed being placed in a more favourable environment to encourage germination for subsequent control (as this stimulates weed emergence on the next rain event, management of this expected larger weed emergence must be factored in).

2. In preparing a seedbed, cultivation provides a weed-free environment for the emerging crop and can improve soil surface conditions (clods and stubble) for even application of pre-emergent herbicides.

3. Cultivation can control weeds in situations where herbicides are ineffective or not an option.

4. Pre-sowing cultivation or full disturbance cultivation at sowing reduces the reliance on knockdown herbicides and therefore the likelihood of weed populations developing herbicide resistance.

5. Shallow cultivation to incorporate pre-emergent herbicides reduces loss due to volatilisation and photodegradation.

Whole-farm benefits

Weed management can have an additional benefit when cultivation is used for:
  • incorporating soil ameliorants (e.g. lime or gypsum)
  • overcoming stratification of non-mobile nutrients such as phosphorus or redistribution of potassium that has been concentrated in surface zones after years of no-till
  • breaking up a hard pan or sub-soil restriction
  • pupae busting (i.e. breaking the life cycle of Helicoverpa spp. in cotton cropping systems).

Issues with tillage

The term strategic tillage has been widely quoted in recent times. In many instances when tillage is used to combat herbicide resistant weeds, the timing of tillage is often more likely to be driven by weed escapes than by good planning. A recent example was when a number of glyphosate resistant grass species became established prior to cereal harvest in NSW in wet finishes in the 2010, 2011 and 2016 seasons. At harvest these weeds were already too big for cost effective control using herbicides and tillage was rightly used in many situations. There is nothing strategic about using tillage at the start of a summer fallow as this degrades soil cover leaving the soil more exposed to wind and water erosion and evaporation over the summer period and importantly, reduces stubble cover and reduces infiltration rates.

  • in wet soil conditions, the percent weed kill delivered by tillage is often poor due to re-planting of weeds back into moist soil.
  • compaction can occur - particularly in wet soils
  • tillage speeds breakdown of stubble and reduces protection from water and wind erosion
  • in the weeks prior to sowing, it can lead to a loss of soil water needed for crop establishment
  • in cracking clay soils, tillage can close surface cracks and reduce the soils ability to accept high intensity summer storm rainfall, with ensuing runoff and soil loss
  • tillage will bury weed seeds which may prolong seedbank dormancy in many weed species and can lead to less effective results from some pre-emergent herbicides used at sowing
  • tillage often costs more, requires higher capital investment, more labour and is slower than spraying.
  • tillage works best in dry or drying soil environments. Weeds are easier to kill when small with smaller root systems. Larger plants may need a more aggressive implement and/or multiple passes.
GRDC factsheet on strategic tillage (2014)
-- top--

2.7 Biological control

Biological control for the management of weeds uses the weed's natural enemies (biological control agents). These natural enemies include herbivores such as insects, but also includes sheep, where there is direct consumption of the weed. They also include microorganisms such as bacteria, fungi and viruses which can cause disease, change weed vigor and competitiveness relative to the crop and decay the weed seed in the seedbank. Other plants can also be included here, where they release substances that suppress weed growth.  This is known as ‘allelopathy’.

Further reading:

The following website has information regarding biological control programs:

Further information: see the section of the Integrated Weed Management Manual on biological control.

-- top--

2.8 Further information

Chapter on managing weed seedlings in the Integrated Weed Management Manual

GRDC fact sheets and other publications

Update papers

-- top--

GRDC video links on managing weeds

Group A herbicides in fallow (2017)
GRDC YouTube Know More video- presenter: Mark Congreve, ICAN. Mark Congreve discusses the use of Group A herbicides to control grass weeds in fallow.

Pre-emergent herbicides - part 1 - Solubility and binding (2015)
This 2 part video series has been produced for GRDC's manual on Pre-Emergent Herbicides.
Part 1 looks at the key factors affecting the performance of pre-emergent herbicides.

Pre-emergent herbicides - part 2 - incorporation by sowing (2015)
This 2 part video series has been produced for GRDC's manual on Pre-Emergent Herbicides.
Part 2 looks at applying herbicides using the IBS or Incorporation by Sowing technique.

GRDC webinar - Managing sorghum injury when using metolachlor herbicides. Grass weeds are rapidly adapting to zero till farming systems and are a major challenge to growing sorghum, especially as there are no effective in-crop herbicide options for selective grass weed control.

Weed Seed Bank Destruction - Weed Seeker
Jamie Grant, (Qld grower) Weed Seeker technology, spray rig setup and customisation with on-board weather station for effective summer fallow weed management.

Crop Competition for Weed Management - Narrow Row Spacings
Paul Castor, (MCA Goondiwindi) highlights the benefits and potential issues for growing summer crops on narrow rows for greater crop competition with weeds.

Double Knock Applications - a Grower's Experience
Jamie Grant, (Qld grower) on using the 'double knock' tactic for effective weed control in summer fallow.

Spray Application of Herbicides - Application volume in stubble
Bill Gordon, (Spray Application Consultant) on how different stubble loads influence carrier volumes for contact and systemic products.

Spray Application of Herbicides - Applying Group A (ACCase Inhibitor) Herbicides
Bill Gordon, (Spray Application Consultant) on improving application of Group A herbicides alone or as part of a double knock, using adjuvants, nozzle selection, boom pressure, boom speed.

Spray Application of Herbicides - Nozzle Selection
Bill Gordon, (Spray Application Consultant) on what nozzles should be used when applying contact and translocated products. How variations in boom speed, application volume, nozzle selection will influence efficacy of the double knock.

Spray Application of Herbicides - Spray Deposition
Bill Gordon, (Spray Application Consultant) compares deposition of translocated and contact products. Bill demonstrates methods for testing deposition in stubble and between the rows. Using the test results to guide improvements to spray deposition for different products.

Spray Application of Herbicides - Travel Speed
Bill Gordon, (Spray Application Consultant) on boom speed influences on herbicide deposition on weeds. Improving coverage at higher speeds through nozzle selection, boom direction and water carrier volumes.

Double knock applications - Northern Region
Michael Widderick and Steve Walker, (DAF Qld & QAAFI Northern Weeds Specialists) and Paul Castor, (Consultant MCA Goondiwindi) explain the 'double knock' strategy for managing glyphosate resistant and tolerant weeds in summer fallow. Timing of double knock, ecology of Liverseed, Feathertop Rhodes, Flaxleaf fleabane, Barnyard grass and other tactics for controlling problem weeds.

Double knock applications - Double knock strategies for resistant weed populations
Steve Walker, QAAFI University of Qld, explains the 'double knock' tactic for controlling glyphosate resistant weeds. Double knock options, timing and monitoring for effective control of glyphosate resistant weeds.

Double knock applications - Extent of herbicide resistance in the north
Paul Castor, Consultant MCA Goondiwindi, outlines the problems growers are facing with herbicide resistant grass weeds in the northern region. Paul gives suggestions for managing these issues through 'double knock', pre-emergent herbicides and WeedSeeker technology.

Double knock applications - target weed species and application strategy
Michael Widderick, DAF Qld Weed Specialist on the principle behind the 'double knock' for controlling summer weeds such as Feathertop Rhodes grass, Barnyard grass and Flaxleaf fleabane.

GRDC webinar: Maximising the performance of paraquat based herbicides in northern fallow (2015) 
With the continual rise of glyphosate resistance, growers are increasingly relying on fallow weed control strategies incorporating the use of bipyridyl (Group L) herbicides. This webinar focused on Group L herbicide strategies and how to achieving the maximum performance from these contact herbicides.

GRDC webinar: Grass weed control with Group A herbicides (2015)
This webinar highlights situations where Group A herbicides have shown to be an effective integrated weed management tactic, how to maximise performance in the field, and considerations to delay resistance to this mode of action.

GRDC webinar on Advances in weed management: Spray application in summer fallows (2013)
This webinar investigates the balance between minimising spray drift and ensuring reliable efficacy via efficient spray deposition on the target when making spray applications during the warmer summer months. Presenter - Bill Gordon

GRDC webinar on Advances in weed management: Stubble and soil binding of pre-emergent herbicides for annual ryegrass control in winter crops (2013)
Pre-emergent herbicides are one of the key management tactics for managing annual ryegrass in winter crop production systems in Australia. Efficient utilization of these tools requires an effective understanding of how these herbicides interact with soil and stubble and, following on from this, their requirements for incorporation. This webinar investigates the properties of key pre-emergent herbicides pyroxasulfone (Sakura®), prosulfocarb/s-metolachlor (Boxer® Gold), dimethenamid-P (Outlook®) and trifluralin (Treflan®) and how the herbicide properties affect stubble binding and soil mobility. Presenter - Dr Dale Shaner (USDA-ARS retired).

GCTV10: Double Knock Timing
Refining timeframes to get the best from follow-up herbicide applications.

GCTV11: Strategic Tillage, Does No-Till mean Never Till?
In modern farming systems where minimum or no tillage is the norm is there a place for an occasional or strategic till for the control of weeds and soil borne disease?

-- top--

State reference pages

A number of state departments have regional weed control references. 
A list of publications or weeds pages by state is shown below.

New South Wales



Western Australia

-- top--

Other information

Control summer weeds for yield and profit (WeedSmart YouTube video 2016 - part of the 'Nuke weeds before seeding' series found here.)

Increase pre-em efficiency through mix and rotate strategy (WeedSmart YouTube video 2016 - part of the 'Nuke weeds before seeding' series found here.)

Understanding pre-emergent herbicides (WeedSmart Webinar YouTube video 2015)

When is it worth rotating from Select to Factor? (WeedSmart YouTube video 2015)

Spray small radish twice (WeedSmart YouTube video 2015)

Maximise Clethodim Performance: Impact of Frost, The University of Adelaide and The Grains Research Development Corporation (2015)

Disc Seeding Systems & Pre-emergent Herbicides, The University of Adelaide, University of South Australia, AgGrow and The Grains Research Development Corporation (2015)

Inter- and intra-row herbicide application strategies for the control of annual ryegrass (Lolium rigidum) in wide-row faba beans (Vicia faba) - 16th Australasian Weeds Conference (2008)

Pyroxsulam for broad-spectrum weed control in wheat - 16th Australasian Weeds Conference (2008)

Summer weed control - SANTFA YouTube clip - SA farmers speak about the importance of summer weed control.

Modelling to estimate glyphosate resistance risk in barnyard grass in the northern Australian grain regionProceedings of the 16th Australian Weeds Conference

Sabeeney, J. (2006). Syngenta Crop Protection Trials 1998–2002. Syngenta Spray.Seed® brochure.

Thornby, D.T., Walker, S.R. and Whish, J.P.M. (2008). Modelling to estimate glyphosate resistance risk in barnyard grass in the northern Australian grain regionProceedings of the 16th Australian Weeds Conference, Cairns, Queensland, pp. 513–515. (Eds R. van Klinken, V. Osten and D. Panetta.) Weeds Society of Queensland Inc.

Somervaille, A., Betts, G., Gordon, B., Green, V., Burgis, M. and Henderson, R. (2012). Adjuvants – Oils, surfactants and other additives for farm chemicals – Revised 2012 edition. Grains Research and Development Corporation, Kingston, Australian Capital Territory.

Advice at hand for fallow grass-weed control - Mark Congreve (2014)

-- top --