Weed researchers fight back against resistance

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Photo of a wild radish pod

The Global Herbicide Resistance Challenge Conference in Fremantle in February brought together the world’s leading weed researchers to pool their knowledge and experience. Nicole Baxter sought out some of the research highlights

While long-term weed management is regarded by many as requiring a commitment by growers to sustain integrated weed management practices, science is nonetheless delivering new insights into how herbicide resistance occurs and how it might be avoided.

Presentations at the Global Herbicide Resistance Challenge Conference in Fremantle, Western Australia, revealed an increasing push, worldwide, into the biochemical and molecular basis of herbicide resistance in weeds to find and disarm the biological mechanisms that create or allow resistance.

Dr Todd Gaines, a postdoctoral research associate at the Perth-based Australian Herbicide Resistance Initiative (AHRI), but who is working at Bayer CropScience in Germany, said the conference was a timely opportunity to bring global weed researchers together.

Key points

  • International weed researchers gathered in Fremantle, WA, recently to discuss and share research on herbicide resistance
  • One highlight was new evidence presented by a Japanese researcher that demonstrated the role of two genes in metabolising Group B herbicides
  • Researchers say it is vital for growers to use integrated weed management tools to drive down weed seedbanks

He said it gave researchers a chance to learn from the experiences and work of others and to connect research that might otherwise have continued in isolation. For him, one of the most interesting pieces of new information was hearing about a double mutation in a gene – the EPSPS gene – that allows plants to rapidly become highly resistant to glyphosate.

When glyphosate is applied to a susceptible weed it stops activity within the plant of an enzyme – known as 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) – without which the weed dies. In some glyphosate-resistant plants, more copies of the EPSPS gene are present and more EPSPS enzyme is produced than normal, allowing the plant to survive glyphosate.

Dr Gaines said in other glyphosate-resistant plants, a single mutation in the EPSPS gene provides low-level resistance.

“A presentation by Adam Jalaludin of the University of Western Australia revealed that a second mutation could subsequently occur in the EPSPS gene, and the two in combination produced high glyphosate resistance levels,” he said. “This finding is important for the paddock because it shows that responding to poor control and low-level resistance by simply increasing glyphosate rates can result in even higher levels of resistance evolving.”

He was also drawn to a presentation by Satoshi Iwakami, of Kyoto University in Japan, on two genes important for the metabolism of acetolactate synthase (ALS) inhibitors (Group B herbicides).

Mr Iwakami’s research involved Group B herbicides commonly used in rice, such as bentsulfuron-methyl. Many ryegrass populations are also resistant through increased metabolism to Group B herbicides such as chlorsulfuron.

“Mr Iwakami presented groundbreaking results that conclusively demonstrated the role of two genes in metabolising Group B herbicides,” Dr Gaines said.

“Plants typically contain over 300 versions of this type of gene, known as a cytochrome P450, and until now very little evidence was available to know which specific cytochrome P450 genes were responsible for metabolising herbicides.”

The conference showed the value of multiple approaches to the same broad field of research coming together at a single forum. Dr Gaines pointed to work on EPSPS gene amplification with reports of this resistance mechanism from the study of four different weed species. “It’s starting to look like this resistance mechanism is far more common than we first thought,” he noted.

Several papers were on transcriptomics (the study of how messenger ribonucleic acid is expressed in any given weed population), which Dr Gaines sees as a useful way to help understand non-target-site-based resistance.

Target-site-resistance mechanisms reduce a herbicide’s ability to block the activity of the target enzyme in the plant.  

Non-target-site-resistance mechanisms reduce the concentration of herbicide reaching the target enzyme and can involve many pathways, including breaking down the herbicide (metabolism) and reducing how much herbicide moves through the leaves of the plant.

There was much discussion about the different mechanisms of herbicide resistance. Dr Gaines said what he took away from these talks was that for certain herbicides it is possible to find some weeds with target-site resistance only, some weeds with non-target-site resistance only and some weeds with both types of resistance.

For the acetyl-CoA carboxylase (ACCase) inhibitors (Group A herbicides), ALS inhibitors (Group B herbicides) and glyphosate (Group M), Dr Gaines said this means there is likely to be continued problems with unpredictable cross-resistance or multiple resistance.

“For all herbicides, we have suggestions that non-target-site resistance is involved. Moving ahead it is important to remember that for any case of herbicide resistance, non-target-site resistance is likely to be a part of the picture.”

An unknown mechanism of resistance to glyphosate was reported in giant ragweed, common in the US and Canada, where the plant amputates the glyphosate-treated area and regrows. Work is underway in Canada and the US to better understand this mechanism.

Photo of a wild radish pod being measured.

Wild radish pods measured as part of herbicide resistance research at AHRI in Perth.

PHOTOS: Evan Collis

Weeds management

The importance of using the registered label rate of herbicide (rather than lower doses) was raised by both local and international researchers, emphasising the critical role that growers will continue to play over and above any scientific advances.

For example, Dr Hugh Beckie of Agriculture and Agri-Food Canada said that while future crops with stacked traits of resistance would be a great benefit, growers would still need to integrate the technology with diverse, astute weed-management systems.

“The benefit of these technologies is that they can diversify herbicide use from a single mode of action to multiple modes of action,” he said.

Dr Beckie said he was worried by reports in the US that some growers were already abusing glufosinate-resistant (Liberty Link®) crops, indicating past lessons had still not been learnt.

He said US studies had shown quite clearly that a regime of alternative weed-management practices could reduce glyphosate resistance and maintain or improve growers’ returns.

Several conference speakers also noted that growers with glyphosate-resistant weeds were returning to tillage as a management tool, highlighting the need to resolve the resistance issue before all the agronomic gains made through no-till are undone.

Dr Beckie echoed the words of Dr Chris Preston, of the University of Adelaide, who said plainly: “Attempting to manage herbicide resistance solely with herbicides is doomed to failure.”

Dr Beckie, along with many speakers, said that glufosinate-resistant crops could only be sustained if growers adopted integrated weed management. “When I think back 20 years, integrated weed management was absent from the discussion. So we have come a long way.”

Another highlight for Dr Beckie was work by Dr Catherine Borger, of the Department of Agriculture and Food, WA (DAFWA), showing that orientating crops in an east–west direction and increasing the seeding rate to increase crop density successfully lowered the weed seedbank.

Dr Beckie said 10 years ago there was little discussion about the weed seedbank, but more focus on this important area was now evident. He noted the increasing importance of decision-support tools such as the Weed Seed Wizard and RIM (Ryegrass Integrated Management), which help growers understand the weed seedbank and offer strategies for reducing it over time.

Dr Beckie felt the “game changer” in Australia was going to be harvest weed-seed management, which had already lowered the weed seedbank to almost zero in some districts. He also expected that this work, led by former DAFWA and now AHRI researcher Peter Newman, would be picked up by other grain-growing countries.

Going forward, Dr Beckie sees extension as the key: “We’ve done a lot of good research that hasn’t been taken up by growers, so that is the challenge, and where resources need to be put.”

More information:

Dr Todd Gaines,

Dr Hugh Beckie,


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