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Protecting knock-down herbicide options

Most cropping systems rely fairly heavily on a small group of non-selective or ‘knockdown’ herbicides. Since the widespread adoption of zero and minimum tillage, these herbicides have provided effective control of many grass and broadleaf weeds – but these useful herbicides could be lost to the industry if steps are not taken to increase the diversity of weed management tactics used.

Mark Congreve, ICAN senior consultant, says that the highly effective double knock tactic, which combines an application of glyphosate followed by paraquat, is at risk if growers don’t remain vigilant and ensure removal of any surviving plants.

The recent discovery of flaxleaf fleabane resistance to paraquat is a clear warning to grain producers that there is no room for complacency with double knock operations.

The recent discovery of flaxleaf fleabane resistance to paraquat is a clear warning to grain producers that there is no room for complacency with double knock operations.

“The double knock strategy of glyphosate, plus a Group I herbicide for weeds such as flaxleaf fleabane, followed by paraquat has provided excellent control of weeds that are difficult to kill with glyphosate alone,” he says. “Recent confirmation of a fleabane population that is resistant to paraquat, found in a New South Wales vineyard, is a clear warning to grain producers that there is no room for complacency following a double knock operation. In addition to this recent discovery, an annual ryegrass population from a West Australian vineyard was confirmed in 2013 to have resistance to both glyphosate and paraquat. This shows that a single plant can develop resistance to both of the main non-selective knockdown herbicides used in Australian grain production.”

Paraquat is a widely-used herbicide, being an active ingredient in over 100 herbicide products registered for use in broadacre cropping. It is a group L herbicide and as such is considered a ‘moderate risk’ for herbicide resistance. Having a moderate risk rating means that resistance generally takes longer to occur, not that it won’t occur.

“Paraquat resistance typically takes over 15 years of consistent use before resistant weeds are noticeable in the field,” he says. “This critical period has now elapsed on many farms where paraquat is used in cereals and broadleaf crops, and for general weed control around the farm.”

Paraquat resistance has been present and widespread in barley grass in lucerne production systems for many years in southern NSW and Victoria. While paraquat resistance is still relatively rare outside of lucerne systems, very high level resistance to paraquat was confirmed in three weed species (crowsfoot grass, blackberry nightshade and cudweed) taken from sugarcane and tomato blocks around Bundaberg in 2015.

Mark Congreve, ICAN senior consultant says growers need to be looking for survivor weeds after every herbicide application and responding to ‘rate creep’ by changing how they use herbicides across their cropping system.

Mark Congreve, ICAN senior consultant says growers need to be looking for survivor weeds after every herbicide application and responding to ‘rate creep’ by changing how they use herbicides across their cropping system.

In the event of widespread resistance to paraquat, Mark is concerned that there are no new modes of action likely to be commercialised within the next 10 years or more, so we need to protect what we have.

“It is essential that farmers do everything in their power to preserve the effectiveness of the herbicide groups currently available,” he said. “The key is to take a diverse approach to weed management and, most importantly, remove weeds that survive herbicide applications. This is the best way to keep weed numbers low and when numbers are low, resistant weeds can be controlled more effectively. It’s a numbers game!”

Mark suggests that growers check the results of every spray application, looking for individual plants ‘surviving’ or ‘re-growing’ after a spray application that has killed adjacent weeds. This may be a sign that the surviving plants carry the genetic mutation that ‘protects’ them from the herbicide’s mode of action.

“If this is observed, the first step is to remove those individual plants before they shed seed,” he said. “It is recommended to have the plants, or their seed, tested to confirm resistance and determine what herbicides those individuals are still susceptible to.”

A second warning sign is when a higher rate of a herbicide is needed to have the same effect as achieved on the target weed in previous years. Mark called this ‘rate, or dose, creep’ and said that it is the most common sign of resistance to herbicides like paraquat. “Paraquat resistance primarily occurs as a result of a plant having the ability to re-direct the herbicide molecules away from the chloroplasts in the cell and into the cell vacuole, where the herbicide has no effect,” he said. “If you are finding that you now need to use a higher rate of a herbicide such as paraquat, it is time to change how you manage those weeds.”

Non-crop areas around farms are often treated with paraquat annually. This can be a high risk practice unless survivors are removed after every spray application as there is no crop competition to restrict weed growth, resulting in production of large volumes of seed.

Herbicide resistance frequently occurs first along fencelines, roadways and irrigation channels where herbicide use tends to be the same year in year out and less attention is paid to survivor weeds or poor herbicide efficacy.

Currently there are 10 weed species with confirmed resistance to paraquat (Group L) and 13 species resistant to glyphosate (Group M) in Australia.



Table: Confirmed paraquat resistance in Australia (Source: Australian Glyphosate Sustainability Working Group Paraquat resistance factsheet)

Species Common name Year confirmed State Crop
Hordeum glaucum Northern barley grass 1983 Victoria lucerne
Arctotheca calendula Capeweed 1984 Victoria lucerne
Hordeum leporinum Barley grass 1988 Victoria lucerne
Vulpia bromoides Silver grass 1990 Victoria lucerne
Mitracarpus hirtus Small square weed 2007 Queensland mangoes
Lolium rigidum Annual ryegrass 2010 South Australia pasture seed
Gamochaeta pensylvanica Cudweed 2015 Queensland tomatoes, sugar cane
Solanum nigrum Blackberry nightshade 2015 Queensland tomatoes, sugar cane
Eleusine indica Crowsfoot grass 2015 Queensland tomatoes, sugar cane
Conyza bonariensis Flaxleaf fleabane 2016 NSW grape vines





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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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