Experimental trials are identifying viable solutions to combat newly emergent resistance to both glyphosate and paraquat in annual ryegrass. The work highlights how long-term, multi-pronged approaches are working to address emerging and existing issues

Growers, agronomists and researchers are working to test methods for controlling an annual ryegrass (Lolium rigidum) population that is resistant to both glyphosate and paraquat.

First detected in the summer of 2022-23, the dual resistance was present in a 20-kilometre area east of Mount Barker in the Albany Port Zone.

However, as practices being used in the Mount Barker area are also used in other areas, there is risk other populations will develop.

The need for more options to manage weeds is behind GRDC’s investment in a multi-pronged approach in weeds research (see 'Investments aim to keep weeds at bay' below).

Rare incidence

Both glyphosate and paraquat have supported the adoption of no-till grain production systems. This assists with economical and effective weed management in conservation agriculture.

At Albany, control options that can prevent this dual-resistant ryegrass from establishing are being worked on by growers, agronomists and researchers.

Among them is Elders Albany agronomist James Bee. He says that the incidence of dual resistance is rare, and it is essential to keep it that way.

“It is sublethal doses of paraquat applied at fencelines and firebreaks that caused the problem,” he says.

But that highlights to everybody the need to review and upgrade fenceline control strategies or run the risk of developing dual resistance.

The dual-resistant ryegrass inevitably made the leap from fencelines to the cropped areas.

Ground Zero

For the past two years, one of the affected paddocks has doubled as a trial site for researchers at the Australian Herbicide Resistance Initiative (AHRI) and the Western Australian Department of Primary Industries and Regional Development (DPIRD).

Principal among them is Dr Roberto Busi, who is now at DPIRD and an adjunct professor with the University of Western Australia.

By the end of the 2024 season, Dr Busi had identified a triple combination of crop-selective herbicides that show promise against the dual-resistant ryegrass.

In these trials, ryegrass control rates rose from a low of 20 per cent in 2023 (when nothing appeared to work sufficiently well against dual resistance) to 97 per cent in 2024 with the novel triple tank mixes.

“We saw growers turn to cultivation in a bid to staunch the spread, but it wasn’t working,” Dr Busi says.

“We, instead, turned to a chemically aggressive strategy since spraying allows growers to cover more area more easily. The regimen costs more – about $100 per hectare. But we identified something that works.”

By the end of the 2024 season, Dr Roberto Busi had identified a triple combination of crop-selective herbicides that show promise against the dual-resistant ryegrass. It will be tested within the new Weed Management Initiative. Photo: Evan Collis

The triple combination

Agronomists are expected to have input, especially regarding a carbetamide inclusion. This is a pre-emergent Group 23 (E) herbicide typically used in legume phases of crop rotations.

It brings a completely different mode of action into the triple mix. The affected farms in the Albany area do not typically grow legumes but alternate between barley/wheat and canola.

The fenceline where glyphosate and paraquat-resistant ryegrass has been detected. Photo: Roberto Busi

“Including a legume would be a big change to what people are growing,” Mr Bee says. “But not because people don’t want to grow them. Including a legume allows for a more robust rotation with better herbicide options.”

Dr Busi’s 2024 trial compared no herbicide treatment with glyphosate or paraquat alone and a fenceline-specific product (Bromacil). He then tested two new tank mixes using crop-selective herbicides and achieved 90 and 97 per cent rates of ryegrass control.

One mix, which deliberately omits paraquat altogether and replaces it with glufosinate, resulted in the best-performing treatment.

Paraquat use is under review by the Australian Pesticides and Veterinary Medicine Authority (APVMA). The regulator’s revised timeframe for its final regulatory decision is the fourth quarter of 2025.

“We kept rates as high as possible because we did not want another failure like 2023,” Dr Busi says. “Fortunately, the chemically aggressive approach worked. We are now tweaking and refining the approach in ongoing trials.”

Validation work

AHRI will undertake that validation work within the new Weed Management Initiative. That work will occur within the western node of this national initiative that runs through to 2030.

Professor Ken Flower leads the team at AHRI. He aims to deliver efficient weed control solutions for Australian growers. Photo: Evan Collis

Glufosinate use

Glufosinate, a fast-acting, non-selective foliar herbicide that kills weeds by direct contact, is included in one of the novel triple mixes being tested.

It inhibits a nitrogen-processing enzyme (glutamine synthetase) and kills by disrupting the plant’s metabolism.

It can control a wide variety of weeds. Importantly, glufosinate requires warm, humid conditions for best results, requiring a mixing partner to deliver good weed control.

But, like all herbicides, its sustainable use depends on rotating and restricting its use in the same paddock to no more than one application in four years.

Mind the fencelines

Mr Bee says that spraying along fencelines and in-crop can look very different. It is not unusual to delay fenceline work until after the paddocks are sown.

Sprayers can also prove unwieldly at the paddock’s edges. That can result in fenceline spraying in winter from tanks mounted on the back of a ute.

All it takes is a muddy patch of ground that requires the ute to speed up to avoid becoming bogged for a patch of fenceline to receive a sublethal dose of herbicide.

“The resistance can then encroach from the fenceline,” Mr Bee says.

The first run with a harvester performed without a seed destructor can then spread the resistant ryegrass seed into the cropped zone. It is a persistent problem that requires an integrated and consistent solution.

More information: Roberto Busi, roberto.busi@uwa.edu.au
James Bee, james.bee@elders.com.au

Investments aim to keep weeds at bay

From sprayer set-up to technological investments in AI, researchers are working to address weeds and resistance

GRDC’s ongoing investment in weed research, development and extension (RD&E) is steadily increasing to address both emerging and existing issues.

The importance of investment in weed management is highlighted by the continual emergence of resistant weed populations. This includes the detection of paraquat and glyphosate-resistant annual ryegrass in the Albany Port Zone of Western Australia.

Strategic investment

A new $47 million Weed Management Initiative (WMI) launched earlier this year aims to provide sustainable and effective weed management strategies through four research nodes across Australia.

The 5.5-year initiative brings Australia’s researchers and communications and extension specialists together to work on weed management RD&E.

GRDC will invest $25 million, with growers benefitting from another $22 million in co-contributions from the University of Western Australia, the University of Adelaide, Charles Sturt University and the University of Queensland.

Four regional nodes are being established across the country, with more than 20 weed research scientists and 12 new PhD student scholarships will be awarded to build critical capacity.Research will focus on the on evolutionary dynamics of weed adaptation and herbicide resistance; the biochemical, molecular and genetic basis of novel herbicide resistance; mitigation of resistance to chemical and non-chemical management; and novel approaches to weed management and integrated weed management.

Research and extension in action

Growers and advisers are also identifying knowledge gaps through GRDC’s National Grower Network (NGN).

This enables GRDC to respond with investments like the NGN sprayer calibration and application efficacy workshops to address needs.

Innovation in chemistry and technology

GRDC has committed to early stage herbicide research, particularly through its partnership with the Herbicide Innovation Partnership with Bayer.

By investing at the discovery stage, GRDC ensures that Australian data and conditions are considered early in the development process. This increases the likelihood that new herbicides will be available to Australian growers sooner. It gives Australian researchers and growers a seat at the table, influencing which molecules are pursued and how they are assessed.

Technological investments include green-on-green and green-on-brown weed research and, more broadly, using AI and sensors to help manage resistance, further improving profitability and sustainability.

GRDC is investing in cutting-edge tools and knowledge to help growers and agronomists make informed decisions about integrated weed management.

Herbicide resistance census

Another investment aims to determine the incidence of herbicide resistance in Australia. The aim is to understand the incidence and extent of herbicide-resistant weeds.

Data from this project will inform investment decisions to develop strategies to manage and prevent resistance and support optimal weed management decisions.

Resources:

• GroundCover^TM story: Why it is important for GRDC to invest in discovery
• GroundCover^TM story: Machinery innovations to revolutionise weed control
• GRDC Update Paper: How we best manage glyphosate and paraquat resistance in annual ryegrass
• GRDC Update Paper: Weed control – where are we heading and what are the alternatives?