Grains Research and Development

Section 2: Agronomy to enhance weed management

Section 2.1 - Crop choice and sequence Section 2.2 - Improving crop competition  Section 2.3 - Herbicide tolerant crops Section 2.4 - Improving pasture competition Section 2.5 - Fallow phase Section 2.6 - Controlled traffic or tramlining for optimal herbicide application Section 2.7 - Further information

Agronomy to enhance weed management

Successful weed management relies on implementation of the best available agronomic practices to optimise crop environment and growth. Basic agronomy and fine tuning of the cropping system are the first steps towards weed management. Using the best available agronomy will increase the benefit gained from the weed management tactics employed

2.1 Crop choice and sequence

Many agronomic and weed management issues arise from the sequence in which crops are sown.

  • Rotations provide options for different weed management tactics
  • Crop rotations can improve crop fertility and help manage disease and insects. Healthy crops are more competitive against weeds.
  • Many weeds are easier or more cost effective to control in specific crops, pastures or fallows.

Knowledge of relative competitiveness, sowing time, maturity, available herbicide options and difficult to control ('No Go') weeds is important.

The ability to compete with weeds varies between crop type and variety. In high weed pressure paddocks, a competitive crop will enhance the reduction in weed seed-set obtained through other weed management tactics. It will also reduce the impact that surviving weeds have on crop yield and the quantity of seed set by any surviving weeds.

Table A1. ‘No Go weeds’ and ‘key weeds to target’ for different types of canola

Canola type

Key No go weeds

Key weeds to target

IT (Imidazolinone tolerant)

Group B resistant Brassicas (eg wild radish, mustards, wild turnip)

Grass weeds – esp. Brome grass, Groups A and M resistant grass weeds, ‘imi’ susceptible broadleaved weeds


Group A resistant grasses and limited broadleaved weed control options,, brassicas, (eg wild radish, mustards, wild turnip), fumitory, black bindweed, vetch and limited broadleaved weed control options

Grass weeds

RR (Roundup® Ready)

Glyphosate resistant annual ryegrass, wild radish

Grass and broadleaf weeds

TT (Triazine tolerant

Triazine resistant brassicas

Grass weeds, triazine susceptible broadleaf weeds, fumitory

For a list of crop choice options to aid weed management please click here to go to the tables within the Integrated Weed Management Manual.

Difference in crop competition between high and low sowing rates
Red legged earth mite damaging canola  
(top) (Photo: D. Severton, DAFWA)  &
Red legged earth mite hosted on capeweed 
(bottom) (Photo: D. Cousins, DAFWA)

Some key issues:

Select crop sequences and varieties to deal with the significant pathogens and nematode issues for each paddock.

  • Weeds are alternate hosts to some crop pathogens. Effective weed management can reduce disease pressure.
  • Rhizoctonia can affect seedling crop growth, leaving the crop at greater threat from weed competition.  Removing weeds for a period prior to sowing can significantly reduce the level of Rhizoctonia inoculum
  • Weed growth in the fallow or in-crop can increase moisture use and exacerbate yield loss from diseases such as crown rot.
  • Residual herbicides used in the fallow or preceding crop may limit crop options.

Further information

Choosing rotation crops (north) (GRDC factsheet, 2011)

Choosing break crops (south) (GRDC factsheet, 2011)

Break crop benefits (west) (GRDC factsheet, 2011)

Crop sequencing - what are the key issues to making profitable and sustainable crop sequencing decisions? (GRDC Update paper, 2013)

2.2 Improving crop competition

The impact of weeds on crop yield can be reduced and the effectiveness of weed control tactics increased by crop competition. The rate and extent of crop canopy development are key factors influencing a crop’s competitive ability with weeds. A crop that rapidly establishes a vigorous canopy, intercepting maximum sunlight and shading the ground and inter-row area, will provide optimum levels of competition.

Leaf area index at the end of tillering in wheat, is highly correlated with the crops ability to compete with weeds.

Canopy development is influenced by:

Difference in crop competition between high and low sowing rates
The difference in crop competition between 
low (top) and high (bottom) seeding rates
(Photo: D. Minkey)
  • crop type and variety
  • row spacing, sowing rate and sowing depth
  • crop nutrition
  • foliar and root diseases
  • nematodes
  • levels of beneficial soil microbes such as mycorrhiza
  • environmental conditions including soil properties and rainfall
  • Light interception and crop row orientation

Each will in turn affect plant density, radiation adsorption, dry matter production and yield. Early canopy closure can be encouraged through good management addressing the above factors.

Key issues:

  • Good agronomy generally means a competitive crop
  • A competitive crop greatly improves weed control by reducing weed biomass and seed-set
  • Different crops and varieties compete with and suppress weeds differently eg Hybrid varieties of canola provide better competition than triazine-tolerant varieties
  • High crop sowing rates reduce weed biomass and weed seed production and may improve crop yield and grain quality. Optimising for yield and quality is advised
  • Take care to sow seed at optimum depth
  • Fertiliser placement can improve crop growth, yield and competitive ability
  • Many studies show a reduction in weeds with increased sowing rate and narrower rows
  • Furrow sowing or moisture seeking techniques at sowing can help establish the crop before the weeds
  • Sowing at the recommended time for the crop type and variety maximises crop competitive ability which will reduce weed biomass and seed-set
  • When delaying sowing to allow for control of the first germination of weeds, choose the crop type and variety most suited to later sowing to minimise yield loss
  • Sow problem weedy paddocks last to allow a good weed germination and subsequent kill prior to sowing

A summary table of some of the key research in Australia to assess the effect of increasing crop sowing rate in the presence of weeds can be found in the IWM manual part 3: Agronomy

Further information

Using crop competition for weed control in barley and wheat (GRDC Update paper, 2015)

WeedSmart Youtube video: Crop competition with Peter Newman

WeedSmart bulletin with links advice and links on crop competition

Difference in crop competition between high and low sowing rates
Common sowthistle growing in fallow (no competition)
vs growing in crop (wheat and barley). There was no
in-crop herbicide applied to control the weed. The
lack of sowthistle in-crop is entirely due to crop
competition. The 2001 Condamine (Qld) season
had a relatively dry start so the crop established before
the weeds. (Photo: M. Widderick)

2.2.1 Crop type

Crop type has a large bearing on the impact of crop competition with weeds, and weed competition with crops.  Barley can more than double the level of crop competition from wheat.  Early sown canola can also be highly competitive with weeds, but late sown canola can be very slow to close its canopy and only poorly compete with weeds.

2.2.2 Sowing rate

A general rule of thumb is to optimise the sowing rate for good crop agronomy.  If weed issues dominate, increasing sowing rate in some crop types can lead to incremental increases in a crops ability to out compete weeds.

As sowing rate in wheat increases to around 150 plants per metre – so does the crops ability to compete with weeds.  It is rare to see significant increases in screenings at sowing rates of ~ 150 plants / metre, even in seasons where there is a tight finish, although this can and does occur.  A key reason for this is that most screenings come from secondary tillers and at higher seeding rates, a higher percent of the crop is produced on the primary tiller. 

GRDC Update papers:
Non-herbicide tactics to help suppress weed growth
Aim for the narrowest possible row spacing WA

2.2.3 Row spacing

The more evenly distributed the crop plants are, the better they will capture sunlight and block out light available for weeds.  In wheat, yield losses are likely at row spacings above ~ 30cm in a weed free environment.  Where weeds are present and there are limited or poor herbicide options to assist, narrow rows and higher seeding rates can assist greatly with weed management.

Further information

Aim for the narrowest possible row spacing WA (GRDC Update paper, 2014) 

Crop placement and row spacing (north) (GRDC factsheet, 2011)

Crop placement and row spacing (south) (GRDC factsheet, 2011)

Crop placement and row spacing (west) (GRDC factsheet, 2011)

Narrow row spacing: is it worth going back? (2015)

Between the rows – a study of crop row spacing in wheat and canola rotations of the Riverine Plains region (2015)

2.2.4 Sowing depth

Getting an even crop to emerge ahead of the weeds can provide a big competitive advantage.  A few days head start can achieve a lot.  In Winter, even and relatively shallow seeding depths can mean a difference of a week or more in crop emergence, as well as greater vigour compared to deeper sown crops that have had to struggle up from depth.

2.2.5 Sowing time

Barley remains an adaptable crop often over a relatively long sowing window.  Canola has a more limited sowing window, which if exceeded will mean slow growth and poorer competitive ability in colder conditions.  Often there is a trade off between sowing early and obtaining good competition, or sowing later and allowing more time for depletion of the weed seedbank before sowing.  The expected weed seedbank dormancy will have a bearing on which strategy is likely to be the more successful.

Further information

Time of sowing factsheet (north) (2011)

Time of sowing factsheet (south) (2011)

2.2.6 Fertiliser use and placement

Placing fertiliser under the crop row where the crop can access it and not broadcasting it, makes it less available to weeds in the inter-row area, thus improving the crops ability to out-compete the weeds.

2.2.7 Disease and insect pest management

A healthy crop grown with good agronomy that is free from pests and diseases is better able to compete with weeds.

Further information on crop competition

Utilising crop competition for weed management in the wheatbelt of Western Australia (WA Crop Update paper 2016)

2.3 Herbicide tolerant crops

Herbicide tolerance traits are introduced into crops either by conventional breeding methods or by genetic modification (GM), where genes are introduced from another organism

Difference in crop competition between high and low sowing rates
Triazine Tolerant canola in Western
(Photo: A. Storrie)

Crops with herbicide tolerance traits bred using conventional methods have been used in Australia for many years and include triazine tolerant (TT) canola introduced in 1994 and imidazolinone tolerant (IT) wheat, introduced in 2001. Genetically modified (GM) herbicide tolerant (HT) cotton has been commercially grown in Australia since 2000, while Roundup Ready® (RR) canola was first commercialised in some states in 2008.

HT crops are tolerant to a herbicide that normally would cause severe damage.  Thus HT crops offer the option of weed control tactics from different herbicide Mode of Action (MOA) Groups, that would normally not be able to be used in these crops. 

With the ease and high levels of weed kill often experienced with glyphosate use in RR crops, the frequency of use of other control tactics has declined.  Diversity in weed management tactics has decreased and selection pressure for the development of resistance to glyphosate has increased.  In an attempt to offset this, many of the stewardship packages associated with HT technologies, require the use of alternate technologies in situations where weed density or the evaluated risk of resistance to that herbicide is high.

Specific HT crop technology stewardship programs are a source of more detailed information. Examples include:

Herbicide tolerant crops:

  • Provide additional crop choice, enabling use of alternative weed management tactics
  • Can sometimes enable a crop type to be grown where herbicide residues may be present in the soil from a previous crop
  • Can reduce the total amount of herbicide used and/or weed control costs
  • Provide another option to use some herbicides. This should always be used in an IWM program and within the guidelines for the relevant Stewardship program for that technology
  • Herbicide resistance management guidelines for Australia for MOA groups can be downloaded from the CropLife Australia Limited website (

Some requirements of technology stewardship packages include:

  • Use technologies and weed management strategies that are appropriate to the weed spectrum and pressure
  • Adhere to all herbicide label directions
  • Maintain good paddock management records
  • Use agronomic practices to minimise out-crossing with other crops

Further information

Herbicide tolerance of PBA Jurien a new narrow leafed lupin variety (WA Crop Update paper 2016)

Tolerance of Hyola 525RT canola to glyphosate mixes (WA Crop Update paper 2016)

2.4 Improving pasture competition

Pastures represent an important component of many rotations and provide a valuable opportunity to manage weed problems using tactics not able to be used in cropping situations. These include grazing, mechanical manipulation and herbicides. Dense stands of well-adapted pasture species compete against weeds, reducing weed numbers and weed seed-set. Competitive pastures greatly improve the effectiveness of other tactics to manage weeds in the pasture phase.

Some weeds such as fleabane, have few viable management options in pastures and this is where blowouts often occur.

Identification and management notes on a large range of weeds of pasture are available at

2.5 Fallow phase

Fallows are defined as the period between two crops, or between a crop and a defined pasture phase. Often the objective of a fallow is to store and conserve soil moisture and nitrogen for the next crop, reduce the weed seed bank and stop weed growth that could impede the sowing operation.


  • A fallow period on its own, or in sequence with a number of crops, can be highly effective in reducing the weed seed bank.
  • A fallow period can incorporate several tactics to reduce weed seed banks.
  • A double knock of glyphosate followed with paraquat can gives high levels of weed control and can assist control of some hard-to-kill or glyphosate resistant survivors.
  • If planned, it is sometimes possible to use other herbicide MOA groups with residual activity (Groups C, B, I or K) in fallow.
  • In a fallow, it is easier to spot escapes and take action to stop seed set, than in a crop.

Key factors for success:

  • Control weeds of fallows when they are small
  • Try to include a range of tactics that include different MOA groups, paraquat and residual herbicides to avoid over reliance on glyphosate alone.  Occasional tillage should also be considered when there is a drying seedbed.

For southern and western regions, further information can be found in the Summer Fallow Weed Management Manual


Group A Herbicides in Fallow (GRDC factsheet, 2014)

Summer fallow spraying factsheet (GRDC factsheet, 2012)

Summer Fallow Weed Management Manual (GRDC publication, 2014)

2.6 Controlled traffic or tramlining for optimal herbicide application

Difference in crop competition between high and low sowing rates
Wheat sown using controlled traffic on wide 
rows on the Darling Downs, Qld. Controlled
traffic cropping allows more options for weed
control and management.
(Photo: W. Holding)

Controlled traffic’ or tramlining refers to a cropping system designed to limit soil damage by confining all wheel traffic to permanent lanes for all field operations, including seeding, harvesting and all spraying activities.

Some form of traffic lane will reduce compaction between the tramlines resulting in increased health of the crop due to improved soil characteristics, thus improving the competitive ability of the crop. This form of precision agriculture results in;

  • the more efficient use of pesticide application due to reduced overlaps,
  • the ability to treat weeds in the inter-row more easily, and
  • allows for easier management of weed seeds at harvest.


  • Accurately spaced tramlines provide guidance and a firmer pathway for more timely and accurate application of herbicide, which in-turn improves weed control and reduces input costs by between 5 and 10%.
  • In wide-row controlled traffic systems inter-row shielded and band spraying as well as inter-row tillage might be options.  Precision guidance technology makes such practices potentially more practical, but there are very few registrations allowing use of herbicides in this manner.

Further information

Weed control in wheel tracks (2013)

2.7 Further information

Integrated Weed Management Manual Part 3: Agronomy

GRDC fact sheets and other publications

Summer Fallow Weed Management Manual (2014)

Group A Herbicides in Fallow (2014)

Weed control in wheel tracks (2013)

Summer fallow spraying fact sheet (2012)

Time of sowing fact sheet (north) (2011)

Time of sowing fact sheet (south) (2011)

Choosing rotation crops (north) (2011)

Choosing break crops (south) (2011)

Break crop benefits (west) (2011)

Crop placement and row spacing (north) (2011)

Crop placement and row spacing (south) (2011)

Crop placement and row spacing (west) (2011)

Update papers

Weeds and crop stubble as a pathogen ‘host’ or reservoir. What species are involved with what crop impact ‐ implications for management (2016)

Mechanisms of weed suppression a new weapon in the making (2016)

Mechanisms of weed suppression by wheat genotypes (2016)

Seeding systems and pre-emergence herbicides (2015)

Using crop competition for weed control in barley and wheat (2015)

Summer weeds reduce moisture and nitrogen (2015)

Impact of crop residues on summer fallow weeds (2015)

Biology and management of summer weeds  (2015)

Non-herbicide tactics to help suppress weed growth (2014)

Aim for the narrowest possible row spacing WA (2014)

Crop sequencing - what are the key issues to making profitable and sustainable crop sequencing decisions? (2013)

Landscape and weed influence on pest and beneficial insect populations (2013)

Compete with weeds - give your crop heaven and your weeds hell (2013)

Progress in developing weed competitive wheat (2010)

Other information


'Ask an expert' post: Can planting a tight crop improve weed control? (2015) and Youtube video (2015)

Diverting weed seed onto permanent tramlines (WeedSmart YouTube video 2014)

Use your crop as a weapon (2014)

'Up the competition’ with Professor of Agricultural Innovation, Deirdre Lemerle (2014)

Youtube video: Learn to think outside the drum (2014)

Higher seeding rates lower weeds (2013)

Narrow row spacing: is it worth going back? (2015)

AHRI Insight

Heal thy soil, heal thy crops, kill thy weeds (2015)

Left jab, right hook (2014)

Sow west young man (2014)


Canola competition for weed suppression - Deirdre Lemerle, Peter Lockley, David Luckett and Hanwen Wu pp. 60-62, 17th Australasian Weeds Conference (2010)

Competitive effects of sorghum cultivars and densities on weed suppression - Hanwen Wu, Steven R. Walker, Vikki A. Osten and Geoff Robinson pp. 483-486, 16th Australasian Weeds Conference (2008)

Weed suppression by crop competition in three crop species in Western Australia - Abul Hashem, Catherine P.D. Borger and Glen Riethmuller pp. 63-66, 17th Australasian Weeds Conference (2010)

Cover cropping as a weed management tool in southern Australian farming systems- Hanwen Wu, Eric Koetz, De Li Liu, Min An, Deirdre Lemerle and Jim Pratley pp. 504-505, 16th Australasian Weeds Conference (2008)

Managing herbicide resistance in Southern Australian farming systems using Roundup Ready® canola - James E. Neilsen and Ian N. Taylor pp. 254-257, 17th Australasian Weeds Conference (2010)