Australian Herbicide Resistance Initiative (AHRI)

Rapid On-Farm Testing of Resistance in Lolium rigidum to Key Pre- and Post-Emergence Herbicides

Publication years Archives: 2021

Research publication Martina Badano

Overreliance on herbicides for weed control is conducive to the evolution of herbicide resistance.

Lolium rigidum (annual ryegrass) is a species that is prone to evolve resistance to a wide range of herbicide modes of action. Rapid detection of herbicide-resistant weed populations in the field can aid farmers to optimize the use of effective herbicides for their control.

The feasibility and utility of a rapid 7-d agar-based assay to reliably detect L. rigidum resistant to key pre- and post-emergence herbicides including clethodim, glyphosate, pyroxasulfone and trifluralin were investigated in three phases: correlation with traditional pot-based dose-response assays, effect of seed dormancy, and stability of herbicides in agar. Easy-to-interpret results were obtained using non-dormant seeds from susceptible and resistant populations, and resistance was detected similarly as pot-based assays. However, the test is not suitable for trifluralin because of instability in agar as measured over a 10-d period, as well as freshly-harvested seeds due to primary dormancy.

This study demonstrates the utility of a portable and rapid assay that allows for on-farm testing of clethodim, glyphosate, and pyroxasulfone resistance in L. rigidum, thereby aiding the identification and implementation of effective herbicide control options.

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The herbicide pyroxasulfone was widely introduced in 2012, and cases of evolved resistance in weeds such as annual ryegrass (Lolium rigidum Gaud.) and tall waterhemp [Amaranthus tuberculatus (Moq.) Sauer] have started to emerge.

Pyroxasulfone is detoxified by tolerant crops, and by annual ryegrass that has been recurrently selected with pyroxasulfone, in a pathway that is hypothesized to involve glutathione conjugation. In the current study, it was confirmed that pyroxasulfone is conjugated to glutathione in vitro by glutathione transferases (GSTs) purified from susceptible and resistant annual ryegrass populations and from a tolerant crop species, wheat. The extent of conjugation corresponded to the pyroxasulfone resistance level.

Pyroxasulfone conjugating activity was higher in radicles, roots, and seeds compared to coleoptiles or expanded leaves. Among the GSTs purified from annual ryegrass radicles and seeds, an orthologue of Brachypodium distachyon GSTF13 was >20-fold more abundant in the pyroxasulfone-resistant population, suggesting that this protein could be responsible for pyroxasulfone conjugation.

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BACKGROUND: Lolium rigidum is the weed of greatest economic impact in Australia due to its formidable capacity to evolve herbicide
resistance. In this study, 579 field-sampled L. rigidum populations were tested for resistance to 21 herbicides applied at
the recommended rate. Nine herbicide treatments were binary mixtures.

RESULTS: A total of 15 876 individual resistance tests were conducted by screening two million seeds at the recommended label
rate. The overall frequency of resistant populations was 31%, 14%, 71%, 6% and 0% in response to the post-emergence herbicide
treatments clethodim, clethodim + butroxydim, imazamox + imazapyr, glyphosate and paraquat, respectively. The resistance
frequency to stand-alone pre-emergence wheat-selective herbicides ranged from 10% to 34%. Conversely, the levels of
resistance to pre-emergence mixtures or stand-alone propyzamide were significantly lower, ranging from 6% to 0%. In winter,
the responses to glyphosate, paraquat, cinmethylin, prosulfocarb, pyroxasulfone and trifluralin were reassessed, with 7%, 0%,
0%, 21%, 21% and 28% as the respective resistance frequencies. South Australia and Victoria are identified as epicenters for
L. rigidum population resistance to pyroxasulfone, whereas populations in New South Wales have the greatest resistance to
glyphosate and in Western Australia to clethodim.

CONCLUSIONS: For the first time, resistance levels to stand-alone herbicides and binary mixtures are geographically ranked
across the Australian continent by benchmark statistical analysis of resistance frequencies and distribution. The extension of
these results will raise awareness of rapidly emerging patterns of herbicide resistance, encouraging the adoption of
cost-effective modes of action and integration of diverse strategies for weed resistance management.

© 2021 Society of Chemical Industry.

Authors: Roberto Busi, Hugh J Beckie, Andy Bates, Tim Boyes, Chris Davey, Barry Haskins, Simon Mock, Peter Newman, Aimone Porri, Andrea Onofri

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Herbicides are the largest category of pesticides used in global agriculture, which is reflected in the rate of increase in the number of unique cases of herbicide-resistant weed biotypes since the late 1950s.

Recommended herbicide resistance management strategies and tactics have evolved over the past 50 years through cumulative research and experience, and have been regularly reviewed. Nevertheless, new perspectives may be gained by viewing current recommended strategies through the lens of insecticide, fungicide, and antibiotic resistance management.

What commonalities exist and what is the basis for disparate strategies? Find out by reading the paper in full.

The Grains Research and Development Corporation (GRDC) invests $1.5 million in AHRI each year to ensure Australian grain growers have access to world class research in strategies to mitigate weeds and control herbicide resistance.

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Specialists treating cancer patients have long known that some cancers can resist certain anti-cancer drugs by rapidly pumping drugs out of tumour cancer cells.  The anti-cancer drugs are pumped out before they have their toxic effect on cancer cells.  These cellular pumps are coded for by ABC transporter genes that are present in animals and plants.  The essential roles of these ABC transporter genes in humans are not fully understood and it is unfortunate that some of them can endow resistance to certain anti-cancer drugs, complicating cancer drug therapy.

ABC transporter genes are present in plants and now a specific ABC transporter has been shown to endow resistance to the herbicide glyphosate, in a similar way to the way these ABC transporters can give resistance to anti-cancer drugs in humans.   A nasty weed species, Echinochloa, growing in the Kununurra Ord River region of WA developed resistance to the well-known herbicide glyphosate, after several years persistent use of glyphosate.  In a world-first, this weed plant has been shown to resist glyphosate due to an ABC transporter that pumps glyphosate out of cells before it can have its toxic effect.

This is the first time that a plant ABC transporter has been found responsible for herbicide resistance.  This plant ABC transporter giving the ability to resist glyphosate by pumping glyphosate out of cells is very similar to human ABC transporters pumping anti-cancer drugs out of cancerous cells.

The international team comprising researchers from the Australian Herbicide Resistance Initiative at the University of Western Australia, from Hunan Academy of Agricultural Science and Kiev, Ukraine definitively established that the ABC transporter gene endowed glyphosate resistance by expressing this gene in other plants.

They inserted the ABC transporter gene into rice, corn and soybean, which made these crop species able to pump glyphosate out of their cells and they became resistant to glyphosate.  Control plants without addition of this ABC transporter gene were killed by glyphosate.

This intriguing similarity of ABC transporter genes conferring human cancer resistance to anti-cancer drugs and conferring plants with herbicide resistance adds to the need for both new, smarter chemicals and for alternative technologies if we are to continue to successfully combat scourges such as cancers, weed and other pest infestations that humans battle against.

This research was made possible through GRDC investment.

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Conventional methods of uniformly spraying fields to combat weeds requires large herbicide inputs at significant cost with impacts on the environment. More focused weed control methods such as site-specific weed management (SSWM) have become popular but require methods to identify weed locations. Advances in technology allows the potential for automated methods such as drone, but also ground-based sensors for detecting and mapping weeds.

In this study, the capability of Light Detection and Ranging (LiDAR) sensors were assessed to detect and locate weeds. For this purpose, two trials were performed using artificial targets (representing weeds) at different heights and diameter to understand the detection limits of a LiDAR.

The results showed the detectability of the target at different scanning distances from the LiDAR was directly influenced by the size of the target and its orientation toward the LiDAR. A third trial was performed in a wheat plot where the LiDAR was used to scan different weed species at various heights above the crop canopy, to verify the capacity of the stationary LiDAR to detect weeds in a field situation. The results showed that 100% of weeds in the wheat plot were detected by the LiDAR, based on their height differences with the crop canopy.

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A significant number of research and development personnel in crop protection companies are taking a leading role in both fundamental and applied herbicide resistance RDE nationally and globally. In this editorial AHRI Director Hugh Beckie explores this topic, with responses from some key crop protection companies including Syngenta, Corteva Agriscience, FMC and Valent.

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The combination of herbicides with different modes of action has been adopted not only to improve weed control but also to increase the environmental sustainability of plant-protection products.

In this study, researchers showed a synergistic effect of the auxin herbicide 2,4-D amine with the PSII-inhibiting herbicide metribuzin to control the global grass weed wild oat (Avena sterilis) population and investigated the underlying mechanisms.

Pretreatment with 2,4-D amine did not change the foliar absorption of metribuzin but did increase metribuzin translocation to the roots and new leaves, although enhancement of the metribuzin metabolism was also observed. Considering that the expression level of the target site psbA gene is significantly higher in leaves than in roots, increased metribuzin translocation to new leaves is likely the major cause of the observed synergism, even though enhanced metribuzin metabolism may offset the metribuzin efficacy. This is the first report on the synergistic mechanism between 2,4-D amine and metribuzin in weed control.

Authors: Heping Han, Gilmar Jos. Picoli Jr., Haibin Guo, Qin Yu, and Stephen B Powles

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This paper demonstrates that an early flowering time adaptation does not come at a growth cost when in competition with wheat (unfortunately seed production could not be assessed due to field netting keeping bees out). Flowering time however did result in an increasing number of flowers being located below the harvest cutting height. As the early flowering wild radish does not come at a competitive cost, it needs to be managed through diversity in both management tactic and timing.

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Glyphosate is routinely used in Australia to control the Arctotheca species Arctotheca calendula (L.) Levyns  (capeweed). This study identifies the first global case of field-evolved glyphosate-resistant capeweed collected from the grainbelt of Western Australia.


In 2020, a capeweed biotype that was collected from Borden in the southern Western Australian grainbelt was confirmed to be glyphosate-resistant (referred hereinafter as Spence population). When compared to the pooled mortality of six field-collected, glyphosate susceptible capeweed populations (S1, S2, S3, S4, S5 and S6), the Spence population was found > 11-fold more resistant to glyphosate than the pooled results of the susceptible populations (S1–S6) at the lethal dose of 50% (LD50) level. The growth of the Spence population was also less affected, requiring > 13-fold more glyphosate to reduce growth than the pooled susceptible populations at the growth reduction of 50% (GR50) level. Sequencing of the plastidic 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene indicated no known single gene mutation imparting glyphosate resistance.

This study, however, did not investigate any other known mechanisms that impart glyphosate resistance. When screened at the field-applied rate, this Spence population was also found to survive an inhibitor of acetolactate synthase (ALS) (metosulam) and an inhibitor of phytoene desaturase (diflufenican).


This is the first confirmation of glyphosate resistance evolution in a capeweed population globally. With capeweed resistance already confirmed to photosystem-I inhibiting herbicides (paraquat and diquat), this study emphasizes the importance of using integrated measures that do not depend only on the use of non-selective herbicides for controlling herbicide resistance-prone capeweed populations.


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