Section 3: Herbicide resistance
Glyphosate resistant annual ryegrass in wheat
(Photo: A. Storrie)
Herbicide resistant weed populations are found throughout all cropping areas of Australia. At November 2016, there were at least 47 weed species in Australia with resistance to one or more herbicide modes-of-action (MOA). The number of weed species with resistance and the areas affected are increasing (follow this link for up to date details)
A herbicide resistance problem can develop through selection of naturally occurring resistant weeds or already resistant weeds can be imported through flood, animals, or practices such as the purchase of contaminated grain or use of contaminated machinery. Species shift to naturally tolerant species also occurs.
3.1 How weed populations become dominated by herbicide resistant plants
Naturally occurring resistant individuals exist in the weed population and survive the herbicide application because of their resistance. These survivors produce viable seed which enters the soil seedbank. A proportion of this seed germinates the following season and the resulting plants are also resistant to the particular herbicide(s) used. If the same herbicides continue to be used with no follow up control of survivors, over time the number of resistant individuals increases in the population.
- Resistance is the inherited ability of an individual plant to survive and reproduce following a herbicide application that would kill a 'wild type' individual of the same species
- Forty-seven weed species in Australia have populations that are resistant to at least one herbicide mode-of-action (MOA) as at November 2016
- As at November 2016, Australian weed populations have developed resistance to 16 distinct MOAs or their sub-groups (click here for up to date statistics)
- Herbicide resistant individuals are present at very low frequencies in weed populations before the herbicide is first applied
- The frequency of naturally resistant individuals within a population will vary greatly within and between weed species
- A weed population is defined as resistant when a herbicide at a label rate that once controlled the population is no longer effective (sometimes an arbitrary figure of 20% survival is used for defining resistance in testing)
- The proportion of herbicide resistant individuals will rise (due to selection pressure) in situations where the same herbicide MOA is applied repeatedly and the survivors are not subsequently controlled
- Herbicide resistance in weed populations is usually permanent as long as seed remains viable in the soil. Only weed density can be reduced, not the ratio of resistant-to-susceptible individuals. The exception is when the resistance gene(s) carry a fitness penalty so that resistant plants produce less seed than susceptible ones - but this is rare.
Figure 1. How a weed population becomes resistant to herbicides
Factors influencing the speed of development of herbicide resistance:
- Initial frequency of resistance gene(s) (and MOA of the applied herbicide) - The higher the frequency the fewer years of herbicide use (selection) are needed to lead to a resistant population.
- Size of the weed population - The more weeds - the higher the selection pressure.
- Proportion of the weed population treated - If most of the weed population is sprayed / 'selected' and there are few 'unselected' weed seeds in the soil seed bank when a herbicide is applied, selection for resistance will occur at a much faster rate than if a large number of weed seeds are viable and un-germinated in the soil when the herbicide is applied
- Herbicide efficacy - The more weeds killed, the higher the selection pressure. The exception is for genes that impart a small / incremental increase in tolerance to the herbicide. Lower doses of herbicide that allow a larger number of weed escapes are more likely to select for a number of 'lower order' resistance genes. Always use robust rates
- Weed biological factors - Weeds with high population numbers, a high level of genetic diversity and which open-pollinate are at higher risk
- Double knock - practices that stop weed survivors from setting seed, or stop that seed entering the seedbank, reduce selection pressure.
3.2 Herbicide resistance in Australia
2,4-D resistant radish, Wongan Hills
(Photo: A. Storrie) Throughout the world, herbicide resistance is an increasing problem. The International survey of herbicide resistant weeds provides details of worldwide and Australian herbicide-resistant weeds.
In Australia, a large and increasing number of grass and broad-leafed weeds are confirmed as herbicide resistant. Annual ryegrass and wild radish are resistant to the largest number of herbicide modes-of-action and also have the greatest number of resistant populations. A number of weeds such as awnless barnyard grass and windmill grass are resistant to glyphosate and pose a significant threat to Northern no-till farming systems. Resistance to glyphosate in annual ryegrass is now widespread as is wild radish resistant to phenoxies. There has been a rapid increase in weeds resistant to glyphosate since 2007.
The world’s first glyphosate resistance was first discovered at Echuca, Victoria in 1996. As at November 2016, populations from the following species were all confirmed as resistant to glyphosate, with several additional species undergoing process to confirm resistance:
- annual ryegrass
- awnless barnyard grass
- liverseed grass
- brome grass
- red brome grass
- windmill grass
- flaxleaf fleabane
- wild radish
- feathertop Rhodes grass
- sweet summer grass
- prickly lettuce
- coat buttons (Tridax)
The first case of paraquat resistance in Australia was northern barley grass in 1983. Paraquat resistance in barley grass is now widespread in lucerne hay and seed crops in south eastern Australia. Small square weed (Mitracarpus hirtus) was the first case of resistance to paraquat in Australia that developed outside of broadacre agriculture.
Paraquat-resistant annual ryegrass was confirmed in south eastern South Australia in 2010.
Table 1. Weeds resistant to paraquat in Australia
|Conyza bonariensis||Flaxleaf fleabane|
|Eleusine indica||Crowsfoot grass|
|Gamochaeta pensylvanica||Wooly cudweed|
|Hordeum glaucum||Northern barley grass|
|Hordeum leporinum||Barley grass|
|Lolium rigidum||Annual ryegrass|
|Mitracarpus hirtus||Small square weed|
|Solanum nigrum||Blackberry nightshade|
|Vulpia bromoides||Silver grass|
All cases of resistance to paraquat are in situations with long histories of use (>15 years).
In 2010 a random survey was conducted in Western Australia and ryegrass seed was collected from 362 of the nearly 470 fields visited (Owen et al in press). Almost 100% of annual ryegrass populations are resistant to Group A ‘fop’ herbicides.
Table 2. Herbicide resistance in annual ryegrass in Western Australia 2010 (Owen et al in press)
|Herbicide|| % of populations with|
|clethodim (250 mL/ha)||65|
|clethodim (500 mL/ha)||42|
Glyphosate resistant annual ryegrass in a vineyard in SA (Photo: A. Storrie). Other sources of up to date information on the herbicide resistance status of Australian weeds can be found here .
Up to date information on weeds confirmed resistant to glyphosate and or paraquat can be found at the Australian Glyphosate Sustainability Working Group Website.
The Australian Glyphosate Sustainability Working Group is a collaborative initiative involving research, industry and extension representatives with the purpose of promoting the sustainable use of glyphosate in Australian agriculture. Their website is a key source of information on glyphosate and paraquat resistance.
CropLife Australia Ltd Herbicide Resistance Management Committee - has developed a series of Resistance Management Strategies for herbicides for most MOA groups. The specific guidelines for the use of crop protection products are designed to reduce the selection pressure for resistance.
Weed species at risk of developing resistance
Suspected glyphosate resistant
barnyard grass, Bellata, NSW (Photo: T. Cook). Global examples of herbicide resistance are shown in the table linked here.
In Australian farming systems, the risk of herbicide resistance will be determined by the frequency of herbicide use without seedbank management of survivors and/or the level of competition provided by the crop. Fallow and non-crop areas are often at high risk.
The 20 highest risk weeds for developing glyphosate resistance in the northern grain region are shown in Table 3. The highest risk farming systems were summer fallow and both glyphosate-resistant and non glyphosate-resistant non-irrigated cotton. It is interesting to note that multiple species on the list have already developed glyphosate resistance in this region.
Non-irrigated cotton was generally at a higher risk of developing resistance compared to irrigated cotton due to the historically greater reliance on glyphosate for weed management.
Table 3. Top 20 species in the north eastern grain region at risk of developing glyphosate resistance (Werth et al. 2011)
(R = resistant populations exist, T = populations being tested for resistance)
|Brachiaria eruciformis||Sweet summer grass (R)|
|Conyza bonariensis||Flaxleaf fleabane (R)|
|Urochloa panicoides||Liverseed grass (R)|
|Chloris virgata||Feathertop Rhodes grass (R)|
|Sonchus oleraceus||Sowthistle/milk thistle (R)|
|Echinochloa colona||Awnless barnyard grass (R)|
|Eleusine indica||Crowsfoot grass|
|Phalaris paradoxa||Paradoxa grass/canary grass|
|Hordeum spp.||Barley grass|
|Lolium rigidum||Annual ryegrass (R)|
|Dactyloctenium radulans||Button grass|
|Digitaria ciliaris||Summer grass|
|Chloris truncata||Windmill grass (R)|
|Cirsium vulgare||Spear thistle|
|Silybum marianum||Variegated thistle|
|Sorghum halepense||Johnsons grass|
|Eragrostis cilianensis||Stink grass|
|Avena spp.||Wild oats/black oats|
|Lactuca serriola||Prickly lettuce (R)|
The risk for winter weeds is mainly expansion of current known problems and increasing development of what has been termed 'alphabet resistance', where weeds are resistant to multiple MOA's. Some key threats include:
- Glyphosate resistance in annual ryegrass, wild oats and a range of other summer and winter grass and broadleaved weeds
- Group B and I resistance in brassica and other weeds
- Group A and B resistance in wild oats
- Extensive use of trifluralin (Group D) over many years in southern Australia has led to widespread and rapidly increasing areas of resistance in annual ryegrass. Resistance has recently been noted in annual ryegrass to key pre-emergent herbicides used to replace trifluralin.
3.3 Testing for resistance
Testing the herbicide resistance status of weeds provides information on which herbicides still work on the target weeds in that paddock. This essential information on future weed management options potentially saves wasteful use of ineffective herbicides and enables products to be selected to protect crop yield.
Testing can be conducted either by using test strips in the paddock or by sending plants or seeds to a commercial testing service. Paddock tests can provide useful information, but can be difficult to interpret due to variable paddock conditions and the often increased size of weeds when test strips are applied after a spray failure.
Commercial testing services grow and test weeds under glasshouse conditions - thus removing climatic or paddock variability that may affect the results. They are able to easily test a number of different herbicides at several rates and compare the results to standard susceptible and resistant biotypes sprayed at the same time.
Types of test
There are two types of commercial resistance tests:
1. Seed test - seed is collected and sent to the testing service. Results may take up to four to five months. Suitable for pre-emergent and post-emergent herbicides.
2.Quick-test® - live plant seedlings are collected and sent to the testing services. Results within six weeks. Unsuitable for pre-emergent herbicides.
Which paddocks to test
Test any paddock where herbicide resistance is suspected as the cause of a spray failure.
High risk paddocks are those where there is a long history of herbicide use and no conscious management to prevent herbicide survivors from setting seed.
The table below gives a rule-of-thumb for the number of years before a problem is likely to develop for the major herbicide groups. These do not need to be consecutive applications.
Table 4. Number of years using a particular herbicide mode-of-action before herbicide resistance is likely to be a problem
|Herbicide group||Years of application (typical)|
When can plants be sampled?
While plants can be sampled before a herbicide is applied (e.g. break of season), the ability to detect resistance is much better when testing occurs after herbicides have been applied and poor control is noticed. Sample patches of weed escapes from across the paddock as this is normally where resistance appears in the early stages and provides information as to where the rest of the paddock is likely to be heading. Collecting seed randomly from across the paddock will under-estimate the level of resistance present as resistant patches could easily be missed.
How to sample for testing
Consult the testing service for more details on seed collection for herbicide resistance testing.
Seed test - Approximately 3,000 seeds of each weed (for annual ryegrass an A4-sized envelope full of good seed heads) is required to test multiple herbicides modes-of-action. This equates to about one cup of annual ryegrass seed or six cups of wild radish pods.
Quick-test® - For each herbicide to be tested, 50 plants are required if small with few tillers, or 20 larger later stage tillering plants.
For more information on testing and sampling
Peter Boutsalis, Plant Science Consulting - Quick-test® and seed test
John Broster, Charles Sturt University: Ph: 02 6933 4001 Fx: 02 6933 2924 Email: email@example.com - seed test only
A number of different surveys are available on the extent of herbicide resistance.
Australian glyphosate sustainability working group - their website has a database of confirmed resistance cases in Australia for glyphosate, paraquat and phenoxy herbicides
International survey of herbicide resistant weeds - international database, searchable by species, country and mode of action
Australian Herbicide Resistance Initiative (AHRI) - page dedicated to surveys in Western Australian cropping regions
Charles Sturt University, Graham Centre Weed Research Group - CSU conducts screening of weed populations for resistance. Annual reports on results are available on their site.
3.5 Further information
GRDC fact sheets and other publications
How widespread is resistance (northern) (2013)
Developing a field assay kit to detect glyphosate resistance (northern) (2013)
GRDC video links on herbicide resistance
Groundcover TV: Herbicide resistance fence-line trials
Trials to evaluate the effectiveness of current and new herbicides for the control of resistant weeds along fence-lines is delivering positive results.
IWM: Resistance Testing - Quick Test Sample Collection
Chris Preston University of Adelaide weeds researcher explains how to collect live weed samples for a herbicide resistance quick test and where to send for testing.
Glyphosate Resistant Summer Grasses - Emergence Patterns & Control
Michael Widderick, DAF Qld Weed Specialist on understanding weed emergence patterns when managing glyphosate resistant summer weeds such as Liverseed, Barnyard and Windmill grass.
Glyphosate Resistant Summer Grasses - Management Strategies
Paul Castor, Consultant MCA Goondiwindi, discusses management strategies for glyphosate resistant summer weeds in broadleaf crops. Double knock timing, pre emergent herbicides and crop choice.
Seed Test - What's involved
Peter Boutsalis, (Plant Science Consulting) explains how to prepare a weed seed sample for resistance testing and how the seed test process works
Weed management-it's a numbers game (WeedSmart YouTube clip 2013)
Australian glyphosate sustainability working group
Information on minimising the risk and impact of glyphosate resistant weeds.
Documents on the website include:
- Managing glyphosate resistance (March 2013) - A 73 page document outlining management options for glyphosate resistance
- How widespread is resistance? - Paper from the northern GRDC Grains Research Updates (March 2013)
- Risk of paraquat resistance in pasture seed industry
- Management of glyphosate resistance in non-crop areas
- Managing glyphosate resistant annual ryegrass in a mixed farm in WA (case study) (March 2013)
- Managing glyphosate resistant liverseed grass in northern NSW (case study) (March 2013)
- Managing glyphosate resistant awnless barnyard grass and annual ryegrass in northern NSW (case study) (March 2013)
Australian Herbicide Resistance Initiative (AHRI)
AHRI conducts scientific and applied research as well as development and extension on many aspects of herbicide resistance and its management in Australian cropping systems. See the links below for more information and further links.
- Herbicide technology
- Resistance Mechanisms
- Agronomy and weed control
- Ryegrass Integrated Management (RIM)
- Resistance surveys
Preserving glyphosate through diversity (AHRI Youtube clip 2012)
WeedSmart in conjunction with AHRI offer a 30 day online course on herbicide resistance called Diversity Era. The course is free, however those interested must register to participate before each live round begins. Visit the Diversity Era website here.
Australian Herbicide Resistance Initiative (AHRI) Publications
Working in a number of research areas, AHRI has produced a large number of publications which are available to download. Topics include:
- Rapid evolution of herbicide resistance by low herbicide dosages Weed Science 2011 Sudheesh Manalil, Roberto Busi, Michael Renton, and Stephen B. Powles
- Reduced sensitivity to paraquat evolves under low glyphosate doses in Lolium rigidum. Agronomy Sustainable Busi, R. and Powles, S.B. (2011)
- Genetic control of cytochrome P450 metabolism based herbicide resistance mechanism in Lolium rigidum Roberto Busi, Martin Vila-Aiub and Stephen B. Powles
- Towards large-scale prediction of Lolium rigidum emergence (I). Can climate be used to predict dormancy parameters? Weed Research, DOI: 10.1111/j.1365-3180.2010.00832.x Owen, M.J., Michael, P.J., Renton, M., Steadman, K.J. and Powles, S.B. (2011)
- Towards large-scale prediction of Lolium rigidum emergence (II). correlation between dormancy and herbicide resistance levels suggest and impact of cropping systems. Weed Research, DOI: 10.1111/j.1365.2010.00835.x. Owen, M.J., Michael, P.J., Renton, M., Steadman, K.J. and Powles, S.B. (2011)
- The potential for Pyroxasulfone to selectivity control resistant and susceptible Rigid Ryegrass (Lolium rigidum) biotypes in Australian grain crop production systems. Weed Technology, 25, 30-37. Walsh, M.J., Fowler, T.M., Crowe, B., Ambe, T., and Powles, S.B. (2011)
- Glyphosate-resistant rigid ryegrass (Lolium rigidum) populations in the Western Australian grain belt. Weed Technology, 24, 44 - 49. Owen, M.J. and Powles, S.B. (2010)
- Herbicide-resistant weed seeds contaminate grain sown in the Western Australian grain belt. Weed Science, 58, 466-472. Michael, P.J., Owen, M.J. and Powles S.B. (2010)
- Evolution in action: plants resistant to herbicides. Annual Review Plant Biology, 61, 317-347. Powles, S.B. and Yu, Q. (2010)
- Glyphosate resistance in S. halepense and L. rigidum is reduced at suboptimal growing temperatures. By: M Vila-Aiub et al, Pest Management Science, 2013, 69, 228-232.
An international survey of herbicide resistant weeds
Preventing herbicide resistance in 'at risk' weeds (Qld DAF website)
Testing for glyphosate resistance (Qld DAF factsheet)
Risks and RATs: assessing glyphosate resistance risk in paddocks in north-eastern Australia (CAWS paper)
Stopping herbicide resistance in Queensland (Qld DAF factsheet)
Effectiveness of herbicide resistance management strategies (Qld DAF website)
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