Glyphosate Resistant Weeds – the Implications for Summer Crops
Steve Walker12, David Thornby1, Jeff Werth1 and Michael Widderick1 1
Agri-Science Queensland, Department of Employment, Economic Development and Innovation (DEEDI), Leslie Research Centre, PO Box 2282, Toowoomba, Queensland 4350 2 Corresponding author:
[email protected]
ABSTRACT Glyphosate has played a pivotal role in allowing the widespread adoption of conservation cropping in the last two decades, as this herbicide provides reliable and broad-spectrum weed control in fallows and crop establishment. However, repetitive and exclusive use has resulted in the evolution of resistance in some weed populations in the northern region of Australia: annual ryegrass, barnyard grass, liverseed grass and fleabane. Modelling and paddock history have shown that glyphosate resistance becomes a noticeable problem after 15 to 20 years of repeated glyphosate use in zero-tilled paddocks where few or no alternatives to glyphosate are used and seed-set on sprayed survivors is not prevented. Subsequently, paddocks infested with glyphosate resistant weeds are more difficult and expensive to control, as most alternatives to glyphosate are less robust, generally cost more, and can be more toxic than glyphosate. Poorer fallow weed control results in reduced stored soil moisture and increased weed seed-banks, resulting in lower summer crop yields. Once developed, glyphosate resistance will last for many years, as long as resistant seeds remain viable in the soil. However, research on new control options, weed ecology and modelling has resulted in strategies to help avoid this major problem, as well as options to manage resistant populations. Regular paddock monitoring and testing of glyphosate sprayed survivors for resistance are strongly recommended. As well, growers can now estimate the risk level for their current practices and crop sequences with the newly available Risk Assessment Tool on the DEEDI website.
(c) The State of Queensland (Department of Employment, Economic Development and Innovation) 2010.
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BACKGROUND Glyphosate is the world’s most widely applied non-selective herbicide, and has been used to control a broad spectrum of weeds in perennial crops, urban and industrial areas, prior to sowing annual crops, and post-emergent in transgenic annual crops since 1974 (Powles 2008). As well, glyphosate is used extensively for fallow weed control in grain and cotton crop rotations in north-east Australia, allowing the move towards conservation cropping systems with reduced or zero tillage practices. Evolution of glyphosate resistance in weeds was thought to be highly unlikely (Bradshaw, Padgette et al. 1997), until the world’s first case of glyphosate resistance was detected in annual ryegrass (Lolium rigidum Gaudin) in Australia (Pratley, Baines et al. 1996).
Herbicide resistance is an evolutionary event resulting from intense herbicide selection over genetically diverse weed populations (McGillion and Storrie 2006). It is defined as the inherited ability of an individual plant to survive an herbicide application that would kill a normal population of the same species. Herbicide resistance does not equate to poor performance of herbicides, and resistant weeds can often survive application of herbicide at rates that are much greater than the recommended rate. Glyphosate resistance is naturally present in weed populations but at extremely low frequencies. However, intensive selection pressure with regular glyphosate spraying causes the susceptible individuals to be killed and the rare resistant individuals to survive, produce large amounts of seed and over time gradually dominate the population.
This paper outlines the current situation of glyphosate resistance in north-east Australia, future risks, impacts on summer grain cropping, and strategies to avoid resistance.
GLYPHOSATE RESISTANCE CURRENTLY IN AUSTRALIA AND FUTURE RISKS At present, glyphosate resistance has been confirmed in 98 annual ryegrass populations in a variety of cropping, horticulture and non-agricultural situations, as well as five barnyard grass (Echinochloa colona L. Link) and two liverseed grass (Urochloa panicoides P. Beauv) populations in Australia (Table 1). All of these glyphosate resistant populations have occurred in situations where there has
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been intensive use of glyphosate, often over 15 years or more, with few or no other effective herbicides used, and few or no tillage operations. As well, numerous populations of flaxleaf fleabane (Conyza bonariensis L. Cronquist) were recently determined to be glyphosate resistant (Walker, Wu et al. 2010). A large portion of the resistant fleabane weeds were found in chemical fallows of northern NSW and southern Queensland. The summer grasses were found in paddocks with a long history of winter crop / summer fallow rotation, whereas the winter species were found in paddocks with a long history of summer crop / winter fallow rotation. In addition, one glyphosate resistant population was found in a summer fallow / transgenic cotton rotation.
Table 1 Confirmed glyphosate resistant weeds in Australia (Preston 2009, Walker and Robinson 2008) Weed
Situation
Annual ryegrass
Broadacre cropping
Horticulture
Other
Number of sites
States
Chemical fallow
27
NSW
No-till winter grains
18
NSW, Vic, SA, WA
Tree crops
4
NSW
Vine crops
15
SA, WA
Driveway
1
NSW
Fence line
22
NSW, SA, Vic, WA
Firebreak
2
SA, NSW
Irrigation channel
7
NSW
Airstrip
1
SA
Railway
1
WA
Barnyard grass
Broadacre cropping
Chemical fallow
5
QLD, NSW
Liverseed grass
Broadacre cropping
Chemical fallow
2
NSW
Flaxleaf fleabane
Broadacre cropping
Chemical fallow
27
QLD, NSW
Whilst the number of confirmed glyphosate resistant populations of summer grasses is currently low, considerably more populations could be identified in the near future. Modelling has shown that paddocks with 15+ years of zero-tilled, glyphosate-based summer fallows are at high risk of having st
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a glyphosate resistant summer grass problem (Figure 1). As these practices have been extensively used in the northern grain region of Australia in the last two decades, large areas are thought to be at risk.
A recent risk assessment showed that other species are also at risk of developing glyphosate resistance, including wild oat (Avena spp.), common sowthistle (Sonchus oleraceus L.) and sweet summer grass (Brachiaria eruciformis Sm. Griseb).
1
Proportion resistant
0.8
0.6
0.4
0.2
0 0
5
10
15
20
25
30
Time since first use of glyphosate (years)
Figure 1 Simulation of glyphosate resistance evolution in a barnyard grass population, confirmed as resistant to glyphosate in northern NSW, using historical data (Thornby and Walker 2009). Dots are results for 20 individual stochastic simulations; dotted line with error bars indicates the mean of 100 simulations.
IMPLICATIONS FOR SUMMER CROPS The obvious impact of glyphosate resistance is that this valuable herbicide is no longer effective in controlling these important weeds (Figure 2), either in the fallows or as a pre-plant knockdown, thus making these weeds more difficult to control. The options for control of these resistant weeds include alternative non-selective knockdowns such as paraquat, double knock (sequential
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application of glyphosate followed by another herbicide), residuals such as atrazine, diuron, metolachlor and imazapic, and tillage (Widderick, Osten et al. 2010). These alternatives have disadvantages associated with increased cost (up to an additional $100/ha/year), less robustness, restrictions with plant-banks and loss of the advantages associated with conservation cropping. Poorer weed control in the preceding fallows and prior to sowing will result in reduced stored soil moisture, which could then reduce summer crop yields. In addition, more surviving weeds will greatly increase the weed seed-bank, and thus create more problems in the following summer crops, and again potentially impacting on yield. Grasses can produce 2,000-12,000 seeds per plant (depending on the species and growing conditions) and fleabane up to 100,000 seeds per plant. As well, glyphosate resistance also puts the new technology of herbicide tolerant crops, such as Roundup Ready cotton and canola, in jeopardy.
Glyphosate resistance is permanent in weed populations for as long as the resistant seed remains viable in the soil seed-bank (McGillion and Storrie 2006). After cessation of glyphosate use, the ratio of resistant to susceptible individuals will remain the same – only the total number of weeds present can be reduced. Seed persistence of grasses and fleabane, whether susceptible or resistant, is relatively short in zero-tilled systems. However, approximately 5% of grass seed remain viable for longer than 2 years in the top 5cm of soil, and this increases to 20-25% remaining viable for over 2 years if seeds are buried to 10cm (Walker, Wu et al. 2010). If the seed-bank is reduced to very low numbers, the return to exclusive use of glyphosate without follow-up actions on survivors will result in a major resistance problem within a very short period. Thus, summer cropping will be adversely affected by glyphosate resistance for many years.
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100
QBG1
80
Survivors (%)
QBG4
60
40
20
0 0
100
200
300
400
500
Glyphosate dose (g/ha)
Figure 2 Survival of susceptible (QBG1) and glyphosate resistant (QBG4) barnyard grass seedlings in the glasshouse following treatment with range of glyphosate doses
AVOIDING GLYPHOSATE RESISTANCE Weed management can be markedly improved and the risk of herbicide resistance reduced if the principles of good crop agronomy and integrated weed management are adopted. This involves developing and implementing a strategy that uses a variety of chemical and nonchemical tactics that attack all parts of the weeds’ lifecycles – depleting the seed-bank, effectively controlling seedlings, stopping seed-set on sprayed survivors and avoiding introduction of new weed seeds (Widderick, Osten et al. 2010).
To maintain glyphosate as a highly effective and reliable fallow weed control tactic, the selection pressure for resistance needs to be reduced. The extent of preventive actions is determined by the level of risk. Growers can determine the risk level for their weeds and current practices using a simple on-line Risk Assessment Tool, which is now available on the DEEDI website.
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Growers should consider having their weeds tested for glyphosate resistance if (a) their paddocks are assessed at moderate or high risk for glyphosate resistance, and/or (b) there are survivors of a glyphosate application. A fact sheet on testing for glyphosate resistance is also available on the DEEDI website.
The early signs of glyphosate resistance appear as a few isolated survivors. These maybe healthy or recovering from initial glyphosate symptoms and surrounded by dead plants of the same weed species. In the following season, patches of survivors develop from these isolated plants if they are not prevented from setting seed, and within a short time may infest the whole paddock. Thus, diligent monitoring of paddocks after treatment with glyphosate is essential, and together with implementing an IWM strategy, glyphosate resistance can be significantly delayed or prevented. However, failure to take preventive actions now is likely to markedly reduce the viability of summer cropping the northern region in the near future.
ACKNOWLEDGEMENTS The authors would like to acknowledge the Grains Research and Development Corporation and the Queensland Government for supporting this research.
REFERENCES Bradshaw LD, Padgette SR, Kimball SL and Wells BH (1997) Perspectives on glyphosate resistance. Weed Technology 11, 189-198.
McGillion T and Storrie A (eds) (2006) Integrated weed management in Australian cropping systems – a training manual for farm advisors. CRC for Australian Weed Management, Adelaide, Australia.
Powles S (2008) Evolved glyphosate resistant weeds around the world: lessons to be learnt. Pest Management Science 64, 360-365.
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Pratley J, Baines P, Eberbach P, Incerti M and Broster J (1996) Glyphosate resistance in annual ryegrass. In ‘Proceedings 1996 Annual Conference of the Grasslands Society of New South Wales’, (eds J Virgona, D Michalk). Wagga Wagga, Australia, 126.
Preston C (2009) Australian Glyphosate Resistance Register. Australian Glyphosate Sustainability Working Group. Published online www.glyphosateresistance.org.au, Adelaide Australia.
Thornby D and Walker S (2009) Simulating the evolution of glyphosate resistance in grains farming in northern Australia. Annals of Botany 104, 747-756.
Walker S and Robinson G (2008) Flaxleaf fleabane – the next glyphosate resistant weed? In ‘Proceedings 16th Australian Weeds Conference’, (eds RD van Klinken, VA Osten, FD Panetta, JC Scanlan). Cairns, Australia, 88-90.
Walker SR, Wu H and Bell K (2010) Emergence and seed persistence of Echinochloa colona, Urochloa panicoides and Hibiscus trionum in the sub-tropical environment of north-eastern Australia. Plant Protection Quarterly (in press).
Widderick M, Osten V and Walker S (2010) Managing difficult to control weeds in summer crops. In ‘1st Australian Summer Grains Conference’. Gold Coast, Australia. (Eds B George-Jaeggli, DJ Jordan). (Grains Research and Development Corporation)
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