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Confirmed resistance to the double-knock tactic in tall fleabane  

The double-knock is a widely-utilised and highly effective weed management tool but without monitoring and removal of survivor weeds, the commonly used ‘glyphosate followed by paraquat’ system is still open to failure.

For weeds that have a natural tolerance to glyphosate, the double-knock has provided growers with an excellent tool to take two swipes at weeds like fleabane, sowthistle and feathertop Rhodes grass and achieve a greater level of control.

NSW Department of Primary Industries weeds researcher, Dr Md Asaduzzaman (Asad) has uncovered disturbing evidence of double-knock resistance in tall fleabane (Conyza sumatrensis) samples collected during weed surveys funded by the Cotton Research and Development Corporation (CRDC).

NSW Department of Primary Industries weeds researcher, Dr Md Asaduzzaman (Asad) has uncovered disturbing evidence of double-knock resistance in tall fleabane (Conyza sumatrensis) samples collected during weed surveys funded by the Cotton Research and Development Corporation (CRDC).

“Our surveys in 2016 and 2017 showed that cotton fields were generally weed-free but herbicide resistance is building in weeds along farm roadsides, drains and channels and around infrastructure,” he said. “We identified two tall fleabane biotypes that have resistance to glyphosate, paraquat and the double-knock tactic of an initial glyphosate application followed with an application of paraquat 7 days later.”

The rate response analysis showed that one of these biotypes is 4.9 times more resistant than the susceptible biotype, requiring 2.5 L/ha Paraquat-250 to kill 50 per cent of the plants from the resistant population compared to just 0.5 L/ha to achieve the same result in the susceptible population (see Table 1). While this level of resistance is generally considered ‘moderate’ it is clear that resistance is building and must be taken very seriously given the importance of the double-knock tactic in most cotton and grain production systems in Australia.

Table 1 Resistance levels of tall fleabane screened against paraquat, glyphosate and glyphosate + paraquat (R-resistant > 50% survival; DR-developing resistance < 50% and > 20% survival and S-susceptible < 20% survival)



Tall fleabane biotype



% Plant survived under

Paraquat-250 @ 2 L/ha Glyphosate-540 @ 1.2 L/ha Glyphosate followed by Paraquat (Double-knock)
TFB01 >75 100 75
TFB02 75 75 75
TFb-Suscept 0 0 0

These two populations, collected near Nandi, Queensland and Coleambally, NSW, are the first paraquat-resistant tall fleabane to be identified in Australia. Resistance to paraquat in this species has previously been recorded in Japan, Sri Lanka and Taiwan.

Location of confirmed cases of double knock resistance in tall fleabane.

“Although the tall fleabane plants from these two populations showed signs of herbicide damage, such as narrowing of leaves and slow growth, when the double-knock was applied, they were able to survive and produce seed,” said Dr Asad. “This species produces a large quantity of seed, germinates quickly and the seed can travel over 10 km in the wind so dispersal of paraquat / glyphosate resistance traits will be impossible to contain.”

Tall fleabane seedlings 28 days after the double knock (glyphosate + paraquat) tactic was applied.

This discovery makes tall fleabane the second species in Australia to have confirmed resistance to both glyphosate (Group M) and paraquat (Group L), the first being a population of annual ryegrass identified in Western Australia in 2013. Having demonstrated resistance to the dual application of these herbicides in the otherwise effective double-knock tactic is cause for great concern.

Weed populations take longer to evolve resistance to paraquat and glyphosate compared to some other modes of action, but it will happen after years of regular applications without survivor control.

Like other fleabane species, tall fleabane is susceptible to crop competition but flourishes in poorly competitive, wide-row crops such as dryland cotton. Combatting herbicide resistance and keeping weed numbers low will require the implementation of a wider range of weed control tactics rather than relying heavily on the double knock tactic.

“Growing more competitive crops and using a wider range of pre- and post-emergence herbicides and strategic tillage will help manage this weed,” said Dr Asad. “Above all is the need to monitor and remove any survivor weeds in line with the cotton industry’s weed control strategy of ‘2 + 2 and 0’ that recommends two non-glyphosate tactics in-crop plus two non-glyphosate tactics in the fallow and zero survivors.”

In other research Dr Asad is testing the opportunities for cotton growers to use cover crops to create additional opportunities for herbicide rotations, run down the seed bank and delay the adaption of weed populations by reducing the frequency of single modes of action herbicide use.

Paraquat resistance has previously been confirmed in 10 species in Australia, including flaxleaf fleabane (Conyza bonariensis).

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What to expect at WeedSmart Week 2021

Big 6 at WeedSmart Week 2021 – Double knock to protect glyphosate
The WeedSmart Forum is set for Tuesday 17 August, 2021 at the Civic Centre in Esperance WA. The program features growers, agronomists and researchers discussing ways to use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
Greg Warren from Farm and General in Esperance will be sharing his thoughts on the control of weeds like summer-germinating ryegrass, marshmallow, fleabane and portulaca.
He says the growers around Esperance are tackling glyphosate resistance in annual ryegrass, along with brome and barley grass and other emerging weeds using a range of integrated control tactics. The double knock plays a key role in preserving glyphosate (and soil moisture) and providing a clean seed bed for planting crops.

 
Big 6 at WeedSmart Week 2021 – Increase crop competition
WeedSmart Week 2021 is set for Tuesday 17 to Thursday 19 August, 2021 in and around Esperance WA. The last two days feature local growers hosting visits to their farms and discussing how they use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
One of the farms hosting a visit during WeedSmart Week is Warrakirri’s 12,800 cropping operation at Condingup. Farm manager Con Murphy has implemented a variety of tactics to combat their main weeds – annual ryegrass and wild radish. Since 2015 the farm has undergone an intensive soil amelioration program to improve the drainage and ameliorate the sandy soils across the farm.
Con says the benefits have been seen in better germination and establishment that sets their cereal, pulse and canola crops up to compete strongly with weeds. There is also a benefit at the end of the season where rain in August or September enters the soil profile without causing waterlogging, and providing a better finish for their crops.
Since 2016-17 about 80% of the farm has been ripped and a portion has been ripped 2 or 3 times because the sandy soils tend to slump after substantial rainfall events, recreating the hardpan.
Con will be showing the WeedSmart tour group how their ripping, drainage, liming and spading program has helped grow more crop and less weeds!
Listen to the podcast with Warrakirri’s Con Murphy talking about the impact of improved drainage on crop competition

Big 6 at WeedSmart Week 2021 – Implement harvest weed seed control
The WeedSmart Week machinery display is set for Wednesday 18 August, 2021 at Dave Campbell’s shed near Esperance WA. The 3-day WeedSmart Week program features growers, agronomists and researchers discussing ways to use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
We’ve saved the harvest weed seed control discussion for the machinery session on Wednesday 18 August. Ben White from Kondinin Group will host the machinery session with spray and harvesting gear on display including impact mills from Seed Terminator, Redekop and iHSD (both hydraulic and belt-driven), Emar chaff deck, and spray technologies including Goldacres’ G6 Crop Cruiser series 2, and weed detection technologies using drones and weed identifying cameras (green on green).
Ben White, Kondinin Group (Photo: Melissa Powell, courtesy of GRDC)
Growers doing the WeedSmart Big 6
WeedSmart Week 2021 is set for Tuesday 17 to Thursday 19 August, 2021 in and around Esperance WA. The last two days feature local growers hosting visits to their farms and discussing how they use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
One of the growers who will open up their farm for a visit is Adrian Perks who farms at Condingup, 70 km north-east of Esperance. Adrian runs a continuous cropping program on his 4300 ha property, growing canola, wheat, barley, faba beans and lupins. This diverse rotation allows him to mix and rotate both chemical and non-chemical weed control tactics. Over half of Adrian’s farm is sandplain, on which he has implemented a soil amelioration program to address non-wetting to increase the competitiveness of his crops. He currently uses chaff decks for harvest weed seed control and is introducing an impact mill this season. Adrian monitors the tramtracks for weed growth and if he feels the weed pressure is too high, he uses a shielded sprayer to reduce seed set. The bus tour will include four farm visits and a machinery display.
Listen to Adrian on the Regional Update podcast.
Adrian Perkins, Condingup WA
 

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WeedSmart agronomist set to tackle high rainfall zone weeds

Every locality has its own spectrum of weeds, and growers face different opportunities and challenges regarding the control tactics they can employ.
The WeedSmart Big 6 approach is a practical way to ensure that an integrated weed management program is put in place that disrupts weed seed production and the evolution of herbicide resistance.
Commencing in January 2021, Jana Dixon has joined the WeedSmart team of extension agronomists, with a focus on applying the Big 6 to manage weeds in the high rainfall cropping systems of southern Australia – from Esperance in WA to south-eastern SA, Tasmania and south-western Victoria.
Jana will add to the dedicated and experienced extension agronomists on the WeedSmart team with Peter Newman in the Western region, Chris Davey in the South, Greg and Kirrily Condon in the East and Paul McIntosh in the North.
Jana Dixon has joined the WeedSmart team of extension agronomists, with a focus on applying the Big 6 to manage weeds in the high rainfall cropping systems of southern Australia – from Esperance in WA to south-eastern SA, Tasmania and south-western Victoria.
Jana hails from the Mid North of SA, and began working at Pinion Advisory (previously Rural Directions) while she was studying agriculture at the University of Adelaide. She has been employed full-time at Pinion Advisory since January 2019 as an agribusiness consultant, based in Clare, and spends most of her time delivering agronomy and farm business advice to clients from a wide range of cropping regions in South Australia.
Pinion Advisory is a foundation WeedSmart sponsor and Jana has been involved in two WeedSmart Week events already – the first as a participant and grower group organiser at the Horsham event in 2019 and then as the local organiser for WeedSmart Week 2020 in Clare.
In welcoming her to the WeedSmart team, program manager Lisa Mayer says Jana brings energy, commitment and insight to deliver communications focussed on the southern region’s high rainfall regions.
“Growers in the southern high rainfall zones are facing some serious issues with herbicide resistance influencing their farming decisions,” says Ms Mayer. “Jana will be engaging with agronomists, growers and researchers in each of the distinct high rainfall zones to understand the complexities and look for practical ways to apply the WeedSmart Big 6 in various cropping scenarios.”
“We plan to deliver WeedSmart Week in Esperance, part of Western Australia’s high rainfall cropping zone, in August 2021 and Jana will play a key role in the planning and delivering of our annual 3-day flagship event.”
Jana says her experience with the WeedSmart program has been very positive and she has been particularly impressed with the support the program has from all sectors of the grains industry.
Newly appointed WeedSmart extension agronomist, Jana Dixon (green cap) leading discussions with farm visit host, Ben Marshman, Owen SA, and growers and agronomists attending WeedSmart Week 2020 in Clare.
“I have spoken to many growers and agronomists who have found real value in the information that the WeedSmart program delivers,” she says. “For many it is as much about considering another operator’s philosophy on dealing with weeds, and taking a fresh look at their own systems, rather than just learning about a new tactic or the traits of a new herbicide in isolation from the big picture.”
She says the high calibre of industry people who contribute their time and expertise to the program is testament to the value WeedSmart has to agribusiness, growers, agronomists and researchers alike.
In taking on the responsibility for delivering information tailored for the high rainfall zones Jana says she is pleased to have an extensive network of contacts through Pinion Advisory, with offices in a number of high rainfall areas to provide easy access to local agronomists and growers. She is also aware that there are major differences in weed spectrums and farming systems in each high rainfall zone and plans to take full advantage of the opportunity this role presents to expand her understanding of different approaches to weed management.
“The long and favourable growing season and the associated prolonged periods of weed germination, is a key factor that I see potentially impacting on a grower’s weed management strategies in these regions,” she says. “On the other hand, access to highly diverse rotations and a focus on crop competition are two strategies that can play an important role in achieving excellent weed management in these regions.”
“I am keen to engage with anyone working and farming in the high rainfall zones to build my knowledge and understanding,” she says. “And to create opportunities to develop and extend the WeedSmart Big 6 strategies, both herbicide and non-herbicide, that work in each area and in different situations.”
WeedSmart is the industry voice delivering science-backed weed control solutions with support from the Grains Research and Development Corporation (GRDC), major herbicide, machinery and seed companies, and university and government research partners, all of whom have a stake in sustainable farming systems.
You an follow Jana on Twitter and keep up to date with the HRZ here.

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Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations.
Full rates when mixing herbicides too!
When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality.
Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL.
Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.

 
Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice.
Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.
 

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This webinar was hosted by Jana Dixon, WeedSmart’s High Rainfall Zone extension agronomist.

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Join Chris Davey, YPAG and WeedSmart Extension Agronomist and Mark Congreve, ICAN Senior Agronomist as they assess the sowing issues in South Australia and discuss strategies on using pre-emergents.
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Soil type/texture
Rainfall forecast (after application)
Characteristics of the herbicide
Characteristics of the crop
Sowing time (tine vs disc) – sowing depth, speed, soil throw, stubble
Resistance status

GRDC Spray Application Manual
This Spray Application GrowNotes™ manual provides information on how various spraying systems and components work, along with those factors that the operator should consider to ensure the sprayer is operating to its full potential.
This manual focuses on issues that will assist in maintaining the accuracy of the sprayer output while improving the efficiency and safety of spraying operations. It contains many useful tips for your spray operations.

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Increase pre-em efficacy through a mix and rotate strategy

Part 1: Control summer weeds for yield and profit
Every $1 spent on summer weed control can potentially return up to $8/ha through moisture and nitrogen conservation. The impact on grain yield as a result of various summer weed control treatments is what Colin McMaster (NSW DPI R&D) refers to as “buying a spring”.  Listen to Colin and Pete Newman (AHRI) as they investigate the $$ benefits of controlling summer weeds.
Resources:

Ask an Expert column with Colin McMaster
GRDC Summer Fallow Weed Management Manual

 
Part 2: Increase pre-em efficacy through a mix and rotate strategy
We’ve done a good job of promoting herbicide rotation over the years. And whilst this advice still stands, recent research shows the benefits of mixing herbicides as well.
As American weeds researcher, Pat Tranel, puts it, “rotating buys you time, mixing buys you shots (of herbicide)”. Listen to Pat and Pete as they explore the benefits of the mix and rotate strategy.

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WeedSmart Shorts: What considerations should be made for glyphosate tolerant canola?

 
Jump to a section

Simon’s client experience with RR Canola and decision-making process 00:27
What’s the decision-making process when it comes to growing a glyphosate tolerant variety over conventional/TT canola? 01:36
Herbicide strategy & resistance testing: 02:44
Timing of glyphosate sprays 05:02
Varietal performance 07:06
Broadleaf weeds 09:33
Dealing with survivors, WeedSmart Big 6 11:12
Desiccation & windrowing 13:48
GM canola marketing 14:46
On-farm storage 15:14

In this new segment, WeedSmart Shorts, our expert agronomists around the country interview experts on topics in a ‘Question and Answer’ video format.
Simon was kind enough to be our first guest and Jana covers lots of important points with him throughout the interview. Above, you can find what questions are covered if you’d like to jump to one of the specific topics highlighted.
Simon says there has been uptake by his clients of glyphosate tolerant canola in South Australia, but there has been a varied response.

“We are finding that some are wanting to try a paddock, some are wanting to sit back and see how it goes. So, it is probably very similar to what happened in Victoria when it rolled out there as an option,” he says.
South Australian farmers in the past have been able to control the weeds with the existing canola options they have, says Simon, but what they are finding now is there is an increase in clethodim resistance, particularly at higher rates and so that is what is likely to be driving the decision-making process around what other canola options are available, such as RR canola.
While it is exciting to have another option for SA growers, Simon says it is critical that growers know their ryegrass resistance status before committing to planting glyphosate tolerant canola.
“Testing is the backbone behind the decision making around canola options and so once we’re aware of what herbicides still work effectively, that’s where we’re able to make a good, informed decision.”
Simon said it was important to know what works on ryegrass across the whole spectrum of herbicide groups.
“An example is, I did some testing for a new client recently and one of his populations came back as 80% resistant to glyphosate. Now, had we not done that test and put glyphosate resistant canola into that paddock, we would’ve been facing a disaster, but because we had that information on hand, we knew what our options were and what they weren’t,” Simon says.
 
 

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In this webinar, we focus on what factors influence the residual control of weeds by pre-emergent herbicides with a focus on Trifluralin, Sakura and new-to-market herbicides.
Join Chris Davey, YPAG and WeedSmart Extension Agronomist and Mark Congreve, ICAN Senior Agronomist as they assess the sowing issues in South Australia and discuss strategies on using pre-emergents.
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Characteristics of the herbicide
Characteristics of the crop
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Factsheets

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Fact Sheet

Farm Business Management Factsheet

Key points

Effective decision-making is at the core of successful farm business management.
Making informed, logical and timely business decisions is crucial to achieving business objectivess.
Understand the different elements that influence how decisions are made and the possible outcomes.
Consider who is responsible for the final decisions in the different areas of your farm business.
Ensure the decision is finalised and implemented in a timely manner.

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Stewardship First SprayBest Guide

The application of herbicides late in the season to prevent weeds setting seed or to desiccate crops must be carried out with caution and in line with herbicide label recommendations.
It is essential to check if these practices are acceptable to buyers, as in some situations markets have extremely low or even zero tolerance to some pesticide and herbicide residues.

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