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Implementing ‘mix and rotate’ strategy to combat herbicide resistance

with Tony Lockrey, consulting agronomist, AMPS Moree

Mixing and rotating herbicide modes of action is a key strategy in the WeedSmart Big 6 – but it’s a herbicide response to a herbicide problem. So, while it’s critical, it must be implemented within a diverse weed management program.

Tony Lockrey, senior agronomist with AMPS Agribusiness at Moree has seen herbicide resistance get out of control on some farms in northern NSW while other growers have responded early and managed to maintain a broader spectrum of effective herbicides in their program.

AMPS Moree consulting agronomist Tony Lockrey has seen good results when herbicides are rotated and mixed in each phase – the fallow, pre-sowing, in-crop and for desiccation.

“It has to start with herbicide resistance testing – specifically for susceptibility,” he says. “Knowing what does work is very important as you’ve probably already got a fair idea about what doesn’t.”

Once all the effective actives are ‘on the table’ it’s time to look at what crops can be grown to allow the use of the widest range of herbicide groups in the rotation, and where you might be able to find synergistic mixes that can further delay resistance and potentially allow the use of actives that are no longer effective on their own.

“When we sit down to plan out an integrated weed control program we want to make sure there is rotation and mixing going on in each phase – in the fallow, pre-sowing, in-crop and for desiccation, where required,” says Tony. “When this is done in conjunction with a determination to stop seed set and remove survivors then it is possible to keep weed numbers low.”

With an increasing number of proprietary herbicide mixes coming onto the market and the broad spectrum of synergistic and antagonistic interactions between potential mixing partners it pays to be well-informed and to seek advice.

If I already rotate modes of action why do I have to mix too?

Short answer: Rotation buys you time; mixing buys you shots. Mixing and rotating buys you time and shots.

Longer answer: Rotation of effective modes of action can significantly delay the onset of herbicide resistance and needs to be built into your crop rotation plan. Herbicides in Group A and Group B are particularly susceptible to multiple exposure resistance with as few as six exposures being enough to select for the resistant mutation.

By mixing MOA groups, either in the same tank mix or applied separately to the same population (like a double knock), those plants that survive one MOA are often killed by the second.

How does testing for susceptibility help when there’s a weed blow-out?

Short answer: Knowing what will work against a resistant population helps drive down the seed bank and helps you regain control.

Longer answer: One real-world example is a paddock near Moree where Group A resistant wild oats were discovered in 1998 following a history of repeated use of Topik® (Group A – fop), Verdict® (Group A – fop) and, later, Axial® (Group A – den). Testing of this population showed the wild oats was very susceptible to Group B sulfonylurea, so Atlantis was used to drive down the weed numbers. A new plan was then put in place with Groups B, A, C and M used across the winter cropping program, but there was still too much reliance on Group B. The current plan for the farm now includes pre-emergent herbicides from Groups K, J and D used individually and in mixes.

How do I integrate more mixes into my herbicide program?

Short answer: Look for opportunities for synergistic mixes throughout the fallow and cropping seasons. In many instances the most important mixing partner is more water.

Longer answer:

Many growers are looking for tank mixes to improve control of glyphosate-resistant seedlings. Knowing which mixtures are beneficial and which are antagonistic is important.

In the fallow, there are often opportunities to use the mix and rotate strategy to great effect in a double-knock application, such as:

  • Group M (glyphosate) + Group I (2,4-D or fluroxypyr or picloram) followed by Group L (paraquat)
  • Group M (glyphosate) followed by Group L (paraquat) + Group G (Sharpen® or flumioxazin)
  • Group M (glyphosate) followed by Group L (paraquat) + Group K (Dual® Gold)
  • Group A (Shogun®) followed by Group L (paraquat) + Group K (Dual® Gold)

The fleabane on the right was unresponsive to glyphosate on its own but mixing picloram with triclopyr or 2,4 D to the glyphosate application was effective (left). 

Pre-plant examples include paraquat plus a triazine herbicide (Group C) or paraquat plus an imidazalinone (Group B), which are commonly used to provide broad spectrum knockdown and residual control. Dual® Gold (Group K) is another common fallow residual option which is very compatible with glyphosate, triazines and paraquat.

An example of an in-crop mix is the addition of clethodim to haloxyfop (both Group A) to improve control of fop-resistant grasses in broadleaf crops where both are registered.

At the end of the season there is also some opportunity to mix desiccants for some crops.

None of these mixes are provided as recommendations – seek advice for your own situation and always read and follow the label.

What about application set up for mixtures?

Short answer: Some herbicides require better coverage. In many instances the most important mixing partner is more water.

Longer answer: Suitable product and water rates, droplet size and the right adjuvant, are critical for optimising herbicide efficacy.

For example, while a fallow mix such as glyphosate plus a Group A, or a Group G (depending on the target weed), is physically compatible, the components have different requirements for optimal performance. Seek advice about the best water rate to use, the potential impact of an oil-based adjuvant (required for most Group A and Group G herbicides) on glyphosate efficacy for some summer grass weeds, and other possible risks.

Factsheet – Mixing knockdown partners with Group G

How do I avoid generating multiple and cross-resistance?

Short answer: Implement as many different weed control strategies as possible. The WeedSmart Big 6 is a practical foundation for an integrated program of herbicide and non-herbicide tactics.

Longer answer: Rotating and mixing herbicide groups can give you room to move in holding off resistance or getting more out of some marginally effective products.

The only way to stave off herbicide resistance completely is to have low weed numbers and to be vigilant about preventing survivors from setting seed. Have a diverse cropping program, use herbicides to provide early weed control, set your crops up to compete strongly and monitor and remove survivor weeds.

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What’s the benefit of a double paraquat knockdown?

The ‘double knock’ strategy has long been used and promoted as a valuable tool in the battle against glyphosate resistance in weeds, with paraquat typically applied to control weeds that survived the ‘first knock’ of glyphosate.  
James Jess, research and technical services manager, Western AG in Ballarat, says growers in his client group and beyond have used a double paraquat application to great effect this year, and avoided a very serious blow-out of glyphosate resistant annual ryegrass.
James Jess, research and technical services manager, Western AG in Ballarat. (Source: Syngenta)
“The 2020 season did not provide an opportunity for an effective pre-seeding knockdown and growers across the higher rainfall zones of Victoria found many large, well-tillered ryegrass plants flourishing in their crops,” he says. “We sent samples to Peter Boutsalis at Plant Science Consulting and the results of the Quick Test showed high levels of glyphosate resistance.”
The live plant samples sent to Plant Science Consulting were tested for their response to rates from 2 to 5 L/ha of 600 g active ingredient glyphosate, and many survived rates of 4 L/ha and above. Given the high level of resistance, Western AG put down a trial to compare a range of double knock options so they could give their clients more confidence going into the 2021 season.    
“We knew that the surviving plants growing in fields across the district would be setting a huge amount of seed and that growers would face a devastating situation unless we took decisive and effective action in 2021,” says James. “In the badly infested patches growers also took measures to reduce the amount of seed entering the seed bank at the end of the 2020 season using hay cutting or harvest weed seed control. In paddocks with mainly glyphosate susceptible ryegrass, desiccating feed barley is also a good way to drive down weed numbers.”
‘Double-knock to protect glyphosate’ is one of the WeedSmart Big 6 tactics, which will be the centre of discussion at WeedSmart Week in Esperance, WA in August this year. This flagship event always attracts growers from interstate keen to see how other farmers are keeping weed numbers low in different systems. Early bird registration is now open.
What double-knock options did you trial?
In brief: 1. Glyphosate followed by paraquat and 2. two sequential paraquat applications.
The details: A series of timings were also tested for both the double knock options. Each of the ‘second knocks’ were applied 3, 7 and 12 days after the first knock application.
The two paraquat applications either 3 or 7 days apart were clearly effective in the trial, with the first paraquat application providing 90 per cent control of the glyphosate resistant ryegrass. This means that in a year where it is not possible to implement a double knock, we know that a single application of paraquat at robust rates will still do a good job of reducing weed numbers.

Get more details from the Western AG trial site report.
What advice did you give your clients going in to the 2021 season?
In brief: Delay seeding and implement a double paraquat knockdown.
The details: It was essential to get on top of the glyphosate resistance in ryegrass and avoid a blow-out. Last season the resistance level was high but the plants were still mainly found in manageable patches. Given the amount of seed that was likely added to the seed bank and potentially spread during harvest, it was essential that extra emphasis was put on having a clean seedbed going in to the 2021 season.
Once an effective knockdown has been applied we then recommend growers use a pre-emergent herbicide to reduce weed emergence when the crop is young. Later germinations are then suppressed by the competitive crop. In our trial we used Sakura incorporated by sowing (IBS), which provided excellent early weed control.
Has the recommendation been adopted successfully?
In brief: Yes, rapid and widespread adoption.
The details: There was immediate adoption of this tactic in response to the situation that emerged last season in the high rainfall zone of Victoria. Over 70 per cent of the Western AG client base in the higher rainfall western districts of Victoria implemented a double-paraquat application pre-seeding to target glyphosate resistant ryegrass before seeding the 2021 winter crop. This high level of adoption was a result of the strategy being actively promoted to clients, with the trial results giving growers the confidence to implement the recommendation.
The Western AG double knock trial last year provided growers with the confidence they needed to take decisive action on glyphosate resistant ryegrass before seeding the 2021 crop.
The blow-out was a real eye-opener for growers about how important it is to keep weed numbers low and the resistance mechanisms in play for all agricultural chemicals – not just herbicides.
The double paraquat tactic is also a good knockdown prior to sowing Roundup Ready canola to meet the stewardship requirements for using the RR technology.
Although resistance to paraquat is currently quite rare in annual ryegrass, it has been found in situations where paraquat has been applied at sub-lethal rates over a long period of time. There have also been new cases of paraquat resistance confirmed, and identified as developing, in ryegrass populations in WA, SA, Victoria and NSW this year.
With this in mind, a simple switch to double paraquat as a pre-seeding knockdown is not recommended as a standard practice but rather as a strategic tactic to contain glyphosate resistance in ryegrass. Once that has been achieved, a set of diverse strategies, including herbicide mixes, must be implemented and any survivors must be removed before they set seed.
Resources

Western AG Double knock field demo results – 2020
Double paraquat and RR canola – Podcast with Mark Lawrence 

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News

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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