Aggressive agronomy on weed seed blowouts

Take home messages

  • Be aggressive on weeds in high pressure paddocks — crop competition is your weapon.
  • Follow up crop topping in canola or pulses with narrow windrow burning (NWB) — the ‘canola combo’.
  • Stack the ‘big six‘ for six years — a combination of herbicides and non-herbicide tools for maximum effect.


2016 will long be remembered in southern and central NSW as ‘that really wet year’. Yields were generally above average on better drained soil types but crops on clay soils suffered substantial waterlogging. Constant rainfall throughout winter shortened the residual value of pre-emergent herbicides and wet conditions restricted trafficability for post emergent spraying. The effects of waterlogging also reduced crop competition, with the combination of all these factors allowing weeds to proliferate, even in paddocks that were previously considered ‘clean’. Careful planning of rotation and herbicide options is essential to manage weed numbers in 2017 and beyond.

Weed seed banks – strive for zero

AHRI research and paddock survey data has clearly shown the value of managing weed seed bank numbers. Peter Newman from AHRI in WA has annually recorded annual ryegrass populations in 25 focus paddocks for the past 16 years and has shown that for 12 paddocks, growers have achieved (and maintained) near zero weeds in continuous cropping systems. These growers simply don’t let weeds set seed, using tactics including crop topping, mixing and rotating herbicides and harvest weed seed control (HWSC) (mainly NWB). In the other 13 paddocks growers have successfully reduced their weed numbers to a low to moderate level (5-10 plants/m2) using a herbicide focus with little or no HWSC. 

Figure 1 shows the difference in ryegrass numbers between the groups, highlighting the impact HWSC has had on keeping the weed seed bank population near zero. Keeping in mind that ryegrass sets around 200 seeds per plant, the key message for growers in this region following the weed blow out in 2016 is that an aggressive approach is required to prevent seed numbers from continuing to build.

HWSC is obviously critical to achieving near zero weed status, but ultimately successful growers keep seed bank numbers low every year by using all the tools available. This stems from a commitment by the grower and their agronomist to plan ahead and target weeds at every opportunity using an integrated approach, not just with herbicides alone. 

Line graph showing focus paddocks and surviving ryegrass numbers in spring

Figure 1. Focus paddocks and surviving ryegrass numbers in spring (Source: Peter Newman, AHRI).

Recent HWSC research

The long term impact of HWSC on annual ryegrass is driving the introduction and development of HWSC options for Australian growers. The iHSD is now commercially available following several years of field stationary mill testing that have proven its efficacy and commercial capacity (Table 1). This system has the potential to deliver 99% control of the seed of major weed species during harvest. 

Chaff tramlining (chaff decks) is the most recently introduced system and is now the second most widely adopted system behind narrow windrow burning. This system is a perfect fit with growers as they move to tramline or controlled traffic systems and look to reduce residue burning. Both approaches are similarly highly effective weed control tools and can be expected to result in an annual reduction in annual ryegrass populations of approximately 60%.

Table 1. Efficacy of the iHSD mill on the survival of four weed species

Weed species Seed kill (%)  SE 
Annual ryegrass 93  0.7 
Wild radish 99.8  0.2 
Wild oats 99.9  0.1 
Broome grass 99  0.2 

Table 2. Impact of narrow windrow burning and chaff tramlining on the autumn emergence of annual ryegrass compared with conventional harvest (control) at two sites, North Parkes, NSW

Treatment  Site 1 Site 2 
Annual ryegrass (Plants m/2
Control 18 11 
Narrow windrow burn 6
Chaff tramlining 7
LSD (P=0.05) 8

Target weedy paddocks — let’s get competitive

For paddocks where large numbers of weeds have set seed in 2016, crop competition (along with a robust herbicide program and HWSC) will play an important role in getting them back on track. There are a few ways to ‘get competitive’:
  • Sow professionally graded seed. Start with a clean slate and eliminate any more weeds being added to the mix. Early crop vigour can also be increased by grading out smaller seeds. 
  • Sow competitive crop species or varieties. Hybrid canola reduced weed biomass at flowering by 50% in CSU trials when compared to open pollinated triazine tolerant (OP TT) varieties (CSIRO 2014). SARDI and CSU trials have shown Fathom barley and Condo wheat exhibit strong early vigour which translates into late season weed competition while maintaining high yields (GRDC - Using crop competition for weed control in barley and wheat.) 
  • Increase plant population in weedy paddocks, especially with cereals using higher sowing rates. This can be a challenge depending on your rainfall zone, subsoil moisture levels and variety, but NSW DPI research has proven targeting higher plant populations will reduce weed biomass (Lemerle et al., 2004).
  • Sow east-west to reduce ryegrass seed production. NSW DPI data has shown similar yields from east-west sowing with weed biomass reduced by 30% (GRDC - Non-herbicide tactics to help suppress weed growth). Orientation trials in WA saw yields increase by 24% in cereals with a 37 to 54% suppression in weed biomass (AHRI - Sow west young man).
  • Reduce row spacing. Narrow rows have been proven to increase crop competition against weeds but also increase yields due to reduced competition between crop plants. Glen Riethmuller from DAFWA has run a rotation trial at Merredin, WA for 29 years using 9cm, 18cm, 27cm and 36cm row spacings. At harvest 2013, there was only 1 ryegrass seed/m2 in the 18cm rows compared with 171 seeds/m2 in the 36cm rows. Reduced row spacing is a controversial topic as growers have invested heavily in no-till seeding systems on wider rows and are reluctant to change, yet hard data is proving narrow rows means less weeds, higher yields and more profit (AHRI - narrow row spacing).
  • Ameliorate soil with lime or gypsum to improve crop growth and subsequently weed competition. Tools such as normalised difference vegetation index (NDVI) imagery, pH mapping and electromagnetic (EM) surveys help identify soil types and crop areas where lime and/or gypsum is needed. This often correlates with areas of poor crop growth and where weeds such as wild radish or silvergrass flourish.
  • Consider precision seeding equipment to encourage rapid early crop development. Crops will generally establish quicker and compete better with weeds when sown with parallelogram tines or precision disc planters in clod free soils. Their value is particularly evident when dry sowing or when conditions are marginal. The cost benefit of investing in precision seeding gear is influenced by yield potential, soil type and scale but the benefits of weed competition also warrant consideration in the mix.
Any of these practices used in isolation will help reduce weed pressure when used with herbicides, but the biggest benefit comes with implementing a combination of tactics, for example, competitive varieties sown at higher plant populations on narrow rows into well structured soils.

‘Canola combo’ — upsize your weed control

Crop topping canola proved very effective last spring as growers took the opportunity to target weeds prior to maturity, particularly high ryegrass populations that had survived an application of clethodim in winter. Weedmaster®DST® is a glyphosate herbicide registered for canola crop topping, either over the top from 20% seed colour change or under the cutter bar at windrowing. 

To upsize their weed control and kill any survivors, many growers in the region undertook the ‘canola combo’ and followed up crop topping with NWB. NWB is where chutes are attached to the back of the header to divert straw and chaff into condensed rows; the rows are then burnt prior to sowing to destroy any remaining weed seeds. Although NWB remains the dominant HWSC tool in the eastern states, chaff lining and chaff decks were used by an increasing number of growers in 2016 as part of a ‘canola combo’. The interest in these chaff collection tools (along with the integrated Harrington Seed Destructor (iHSD)) over NWB is driven primarily by time savings and not having to burn. While research is continuing to quantify their longer term weed control benefits, growers are getting on with the job and adapting machinery to suit their integrated weed management (IWM) systems.

Stack the ‘big six’

We’ve already discussed HWSC, crop competition and crop topping, but there are three other tools that can be employed to maintain a low weed seed bank over the longer term. Known as the ‘big six’, stacking these tools together over a minimum six year period has the greatest impact on weed seed populations. The ‘big six’ include:
  1. Stopping seed set. Use crop topping or hay cutting to eliminate seed set. Take no prisoners.
  2. HWSC. Huge range of options based on the capture of weeds seeds at harvest then bale, burn, destruct or rot. Header set up is critical to ensure weed seeds can be separated from the straw and stay in the chaff fraction. Windrowing prior to harvest is useful for success with HWSC, as weed seeds are captured in a row prior to shedding. Options ranked in order of cost from lowest to highest: NWB, chaff lining, chaff tramlining (chaff decks), chaff carts, bale direct, iHSD. There are more innovations emerging as growers look at all sorts of options to destroy weed seeds including microwaves, LPG gas burners and grazing chaff dumps with sheep.
  3. Double knock. Protect glyphosate, our most important herbicide, by double knocking with paraquat within three days following glyphosate. Aim to double knock every year during seeding and on summer fallows. Optical spot spray technology using the WEEDit or WeedSeeker® units allow effective double knocks using a wide range of alternative herbicides at high rates which are only applied to a small percentage of the paddock. Firebreaks are also an area where glyphosate is over used and double knocking with paraquat, cultivation or hay cutting will protect its longevity.
  4. Mix and rotate herbicides and crops. By mixing herbicides at full rates we can buy ourselves extra shots. Diverse crop rotations allow the use of diverse chemistry, for example, double or triple break rotations with pulse/canola/wheat/barley or hay/pulse/canola/wheat/wheat.
  5. Crop competition. Adopt smart agronomy to ensure the crop has the upper hand on weeds at all times from emergence through to maturity.
  6. Pasture phase. Mixed farming is very profitable and the pasture phase has long proven its value with alternative weed management tools such as hay or silage, spray topping or even heavy grazing at key times. Cropping paddocks with high grass weed burdens can be rotated to a lucerne/clover mix to utilise ryegrass for grazing and provide nitrogen before being cleaned up for a return to crop. 


Weed seed blowouts from the wet year in 2016 were common but it’s the ideal opportunity to get aggressive and adopt the alternative weed control options that you’ve been hearing about lately. Start with the double knock, then get competitive, upsize with the canola combo (or pulse combo), and ‘slam it home’ with HWSC. And do it all again next year. The rewards achieved will be worth getting out of your comfort zone.

Useful resources and references

CSIRO PUBLISHING Crop & Pasture Science, 2014, 65, 1300–1310

GRDC - Using crop competition for weed control in barley and wheat

Australasian Weeds Conference: Field evaluation of Australian wheat genotypes for competitive traits and weed suppression

Lemerle D, et al (2004) Reliability of higher seeding rates of wheat for increased competitiveness with weeds in low rainfall environments. Journal of Agricultural Science 142.

GRDC - Non-herbicide tactics to help suppress weed growth

AHRI - Sow west young man 

AHRI - Narrow row spacing - more crop, fewer weeds

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Funding for AHRI is provided through GRDC. The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

Contact details 

Greg and Kirrily Condon
PO Box 73 Junee NSW 2663
0428 477 348 (Greg) 

Michael Walsh