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Insecticide shown to reverse metabolic herbicide resistance

Herbicide resistance occurs at a genetic, molecular and cellular level in ways that challenge some of the most agile scientific minds. In following interesting lines of enquiry, scientists working to understand the mechanisms that drive metabolic resistance sometimes come across some unexpected findings.

One such finding is the discovery that an insecticide can reverse metabolic resistance to a herbicide, making the resistant population susceptible to the herbicide once more.

Left: Trifluralin applied to a pot containing trifluralin-resistant ryegrass seed. Centre: Trifluralin applied immediately after applying phorate insecticide granules to pots with trifluralin-resistant ryegrass seed. Right: Trifluralin applied to a pot containing trifluralin-susceptible ryegrass seed.

Left: Trifluralin applied to a pot containing trifluralin-resistant ryegrass seed. Centre: Trifluralin applied immediately after applying phorate insecticide granules to pots with trifluralin-resistant ryegrass seed. Right: Trifluralin applied to a pot containing trifluralin-susceptible ryegrass seed.

Metabolic resistance is the lesser known cousin to target site resistance in the world of herbicide resistance mechanisms. Target site resistance is comparatively easy to identify and study, being a more direct ‘cause and effect’ type mechanism that usually confers quite high levels of resistance.

Metabolic resistance however is more complex and more difficult to study due to many internal mechanisms involving secondary enzyme production and activity. This type of resistance is often moderate however it is also frequently effective across multiple herbicide mode of action groups. It is not uncommon for plants with metabolic resistance to be resistant to herbicides that they have never been exposed to. This has a dramatic and limiting effect on herbicide choice and makes herbicide rotation a much less powerful control tactic.

In simple terms, metabolic resistance occurs when the plant uses its metabolic pathways to produce enzymes that ‘protect’ target sites from the applied herbicide molecules. If the herbicide molecule never reaches the target site then the plant survives. The same enzyme or multiple enzymes can ‘protect’ multiple target sites, resulting in cross-resistant plants.

Weed surveys in Western Australia have revealed a high rate of multiple-resistance in annual ryegrass populations with 70 per cent of populations possessing both metabolic and target site resistance to herbicides.

Research into gaining a better understanding of one group of enzymes, known as P450s, has uncovered an unlikely synergism between an insecticide and current pre-emergent herbicides to control ryegrass. Australian Herbicide Resistance Initiative (AHRI) researcher, Roberto Busi, has shown that it is possible to reverse metabolic resistance to trifluralin in annual ryegrass using an organo-phosphate insecticide.

In conjunction with Colorado State University researcher, Todd Gaines, Dr Busi is working to better understand the genetic basis of metabolic resistance and how this knowledge can be used to control metabolic resistant weeds.

“There are just five types of pre-emergent herbicides, utilising only two modes of action, with no new modes of action in the pipeline,” says Dr Busi. “The most recent pre-emergent product, pyroxasulfone (Sakura), was commercialised in 2012 yet even before it was brought to market, research had shown its mode of action can be ‘broken’ within just three generations using low application rates to result in 10-fold resistance.”

This means that it is very important to find ways to keep current herbicides effective rather than just looking for new modes of action. In the case of trifluralin-resistant annual ryegrass, Dr Busi’s research demonstrated that inhibiting the production of P450 enzymes was the key to reversing resistance to this useful pre-emergent herbicide.

“Inhibiting the production of P450 enzymes requires the suppression of different genes in the plant that are responsible for regulating production of the enzymes at different stages of the plant’s development,” he says. “In ryegrass there are probably several different P450 enzymes that are active during the plant’s development that are offering protection against the herbicide, so there is a high level of complexity involved in trying to manipulate the genes responsible for herbicide resistance.”

“Using the insecticide phorate, applied in granular form to the soil immediately before spraying with trifluralin, we were able to prevent establishment of plants with known resistance to trifluralin,” he says, “But the effect was not as clear for plants that were resistant to Sakura. Phorate is not the solution to metabolic resistance but this proof-of-concept research confirms that it is possible to manipulate and even reverse metabolic resistance with the use of existing pesticides.”

Phorate is not currently registered for use in any crop except cotton and the described use is not permitted in the field. Phorate is highly toxic and it was used under carefully controlled laboratory conditions for these experiments.

It is not desirable to turn off P450 production in a crop so chemicals that inhibit P450 production are best suited to use with pre-emergent herbicides. Current research is investigating ways to design better P450 inhibitor mechanisms using gene technology and to use these mechanisms in future crop breeding programs to confer crop tolerance to certain herbicides.

Further experiments showed that ryegrass plants with metabolic resistance to Sakura use another metabolic pathway involving enzymes known as GST. In a similar manner, these mechanisms can probably confer cross-resistance for pyroxasulfone (Sakura), prosulfocarb (Boxer Gold) and triallate. To date, the fifth pre-emergent active ingredient, propyzamide, does not seem prone to metabolic resistance.

“For now, our best advice to growers and agronomists is to rotate between these three groups of pre-emergent herbicides – 1. trifluralin, 2. Sakura, Boxer Gold and triallate and 3. propyzamide – and we are researching the potential benefits of mixing herbicides from these three groups as a means of delaying metabolic resistance,” says Dr Busi. “As always, full label rates must be applied.”

rotate-pre-ems

For more information, check out our webinar recording with Roberto and Todd discussing metabolic resistance to herbicides.

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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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