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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 in Ground Cover Magazine sought out some of the research highlights:
Weed researchers fight back against resistance GC

The risks of glyphosate resistance vary so widely on Australian cotton farms that growers need to closely assess their management practices and the weed species present on each farm, according to weed researcher, Dr Jeff Werth.
Dr Werth, Department of Agriculture, Fisheries and Forestry Queensland (DAFFQ) says a new risk assessment framework enables growers to tailor their weed management to focus on those species that are at a high risk of evolving resistance.
“Growers will need to use effective alternatives to glyphosate which, when targeted at their at-risk weed species, will help to ensure glyphosate’s long term sustainability,” he said.
“Glyphosate resistance will have a major impact on current cropping practices in glyphosate-resistant cotton systems.”
He says the risk assessment framework will aid decision-making for resistance management.
“We developed this framework and then assessed the biological characteristics of 65 species and management practices from 50 cotton growers,” Dr Werth said.
“This enabled us to predict the species most likely to evolve resistance, and the situations in which resistance is most likely to occur.”
Dr Werth says species with the highest resistance risk were sweet summer grass (Brachiaria eruciformis), flaxleaf fleabane (Conyza bonariensis), liverseed grass (Urochloa panicoides), feathertop Rhodes grass (Chloris virgata), sowthistle (Sonchus oleraceus) and awnless barnyard grass (Echinochloa colona).
“The summer fallow and non-irrigated glyphosate-resistant cotton were the highest risk phases in the cropping system,” he said.
“When weed species and management practices were combined, flaxleaf fleabane in summer fallow and other winter crops were at very high risk.
“Sowthistle had very high risk in summer and winter fallow, as did feathertop Rhodes grass and awnless barnyard grass in summer fallow.”
He says the assessment confirmed previous perceived risks and demonstrates its usefulness but the overall risk is highly dependent on the weed species present in the different phases of the rotation.
“The average risk for ‘other’ winter crops was approximately half of that for summer fallow and non-irrigated glyphosate-resistant cotton.
“However, when flaxleaf fleabane was present in the ‘other’ winter crops phase, individual risks were the same as for the summer fallow, and higher than non-irrigated glyphosate-resistant cotton.”
Eleven of the 50 surveyed growers indicated that they grew non-irrigated glyphosate-resistant cotton. In this phase, the risks for flaxleaf fleabane, liverseed grass and awnless barnyard grass were high.
“Currently there is only one confirmed resistant awnless barnyard grass population in a non-irrigated glyphosate-resistant cotton system, but the number of cases is likely to increase.
“It is concerning that there were growers, who indicated they did not control survivors of glyphosate application, despite the requirements to do so.”
Dr Werth says these growers are likely to have thought that glyphosate provided sufficient control negating the need for further action.
The individual responses of non-irrigated glyphosate-resistant cotton growers did, however, indicate that they all used an alternative to glyphosate at some stage, and so no grower in this survey relied on glyphosate only for weed control, he says.
“The risk scores for the summer fallow were consistently high. Glyphosate has been relied upon for weed control in the summer fallow phase for several years.”
He says research is now concentrating on strategic use of residual herbicides and tillage to reduce the reliance on glyphosate. In fallow situations, Group A herbicides (ACCase inhibitors) are starting to be used.
The WeedSmart campaign brings together industry organisations including the Grains Research and Development Corporation (GRDC), research providers and major crop input firms to deliver the message that herbicide resistance is a difficult but not insurmountable problem – but changes need to occur on-farm.
Funding for the risk assessment and online toolkit was provided by the Cotton Research and Development Corporation (CRDC).
For more information on herbicide sustainability practices, visit www.grdc.com.au. 

The old adage that ‘when you buy quality, it only hurts once’ rings true in the case of the new herbicide Sakura® (pyroxasulfone) to combat annual ryegrass.
Many cheap products can appear to be great value at first, but when they break down soon after purchase they are often deemed to have been a waste of money. The same is true for herbicides.
When you buy seemingly expensive new herbicides and apply them at the full label rate, it hurts the hip pocket.
Cutting the rate saves money – making your budget look good – and appears to work well at first.
Until, a few years down the track, herbicide resistance emerges as a result of using below recommended label rates.
And, not only has the weed population on your farm developed resistance to this herbicide, it can also develop cross resistance to herbicides that you have not even used yet.
That is the alarming finding of GRDC-supported UWA-based Australian Herbicide Resistance Initiative (AHRI) researchers.
AHRI postdoctoral fellow Dr Roberto Busi warns that Sakura® (Group K) is a great pre-emergence herbicide to control ryegrass, but using below-label rates poses a risk of rapid evolution of ryegrass resistance and should be avoided.
The pattern of rapid resistance evolution of Sakura® is similar to previous research into the use of low rates of diclofop-methyl (Group B) on ryegrass.
It is recommended Sakura® be used at full label rates and in rotation with other pre-emergence herbicides as part of an integrated weed management strategy that also incorporates non-herbicide weed control methods like harvest weed seed control and growing a competitive crop.
Key factors to consider when using Sakura® in 2013 for best results include:

Use at full label rate of 118g/ha
Rotate if used last year
Best herbicide rotation is Sakura® (Group K) or Boxer Gold® (Group J & K) year one – trifluralin (Group D) year two (where trifluralin is still effective) – alternate Sakura® or Boxer Gold® year three
Pre-seeding knock-down with glyphosate (Group M) and/or paraquat (Group L) soon after the break
 Harvest weed seed control is a critical component of an integrated weed management strategy

To develop a cost efficient herbicide budget, AHRI suggests growers consider using the more expensive herbicides at full rates over smaller areas and concentrating on doing a good job – rather than cutting rates over bigger areas.
Incorporating non-herbicide weed control tools makes the farming system more sustainable and paves the way for higher levels of cropping intensity.
Using a combination of herbicide and non-herbicide weed control practices will help to ensure a long term cropping future.
More information:
AHRI researchers presented the latest Sakura® trial results at the Global Herbicide Resistance Challenge conference and the 2013  Agribusiness Crop Updates in Perth in February. Reports are available at: www.grdc.com.au/UpdatePapers.
Further information about this research project is also available at www.ahri.uwa.edu.au, and in the March/April edition of Ground Cover, see article: Protect new herbicide resource.

Making Herbicides Last
Featured Articles:

Herbicide Resistance – Global View: Glyphosate lost to the US
Herbicide Resistance – Australian View: Resistance rising across Australia
Managing Herbicide Resistance: Lift sowing to outcompete weeds

To read this supplement, choose from the following options:

Click here to download the supplement as a high resolution PDF
Click here to download the supplement as a low resolution PDF
Click here to read it as an online magazine

As weed seed management gains traction as a key strategy for managing herbicide resistance, northern region grain advisers have a new tool for assessing the extent of the challenge.
The Weed Seed Wizard was developed in WA and is being brought to Queensland and NSW with support from the Grains Research and Development Corporation (GRDC).
Workshops to help advisers get to know the computer simulation tool and design weed management strategies for clients that can improve yields are being held at three locations, including:

Moree, NSW: June 20, 10 am, contact Rob Long on robert.long@elders.com.au or 0428 971 751.
Toowoomba, Queensland: June 21, 9 am, contact Mark Congreve on mark@icanrural.com.au or 0427 209 234.
Emerald, Queensland: June 24, 9 am, contact Vikki Osten on vikki@osten.id.au or 0428 824 390.

Mr Congreve says Weed Seed Wizard can be fine-tuned to individual farms using local weather data, soil type and management options.
It can also be used to manage a range of weed species, such as barnyard grass, sweet summer grass, wild oats and annual ryegrass, depending on locality, he says.
“The Weed Seed Wizard can help advisers understand and predict the impact of weed management decisions on crop yield and weed seedbanks.”
While there is no cost to participate, numbers are strictly limited and it is advisable to book early to secure a spot.
Participants are required to bring a laptop (Macs and mobile devices won’t run the software as yet).
For more information on GRDC-supported weed management research, visit www.grdc.com.au/weedlinks.

Repeated application of herbicides with the same mode of action (MOA) is the single greatest risk factor for the evolution of herbicide resistance.
That’s the message two of Australia’s leading herbicide resistance researchers, Dr Chris Preston and Dr Stephen Powles, have joined forces to drive home to primary producers and advisers this winter cropping season.
The researchers agree the key to herbicide sustainability is to rotate MOAs and implement diverse chemical and non-chemical tactics, including harvest weed seed control practices to prevent survivors putting seed into the seedbank.
Dr Preston, University of Adelaide, Associate Professor – weed management says
rotation of herbicides is important but alone will not stop resistance, just delay it.
“Rotation of chemicals and MOAs needs to be used in conjunction with practices that reduce weed populations, such as seed set management practices,” he said.
“But it is vital for growers to protect the existing herbicide resource as the discovery of new, effective herbicides is rare.
“There is no quick chemical fix on the horizon so we advise growers to use breakcrops where suitable to manage disease and weed burdens.”
Dr Preston says despite the hundreds of individual products and the large number of active ingredients available for use, most herbicides can be grouped by chemical similarity into relatively few chemical groups.
“Often several herbicide chemistries will have similar biochemical effects on the plant and when these actions of herbicides are considered an even fewer number of modes of action occur.”
He says growers should become familiar with these herbicide modes of action and rotate them whenever practical.
“A good example is the new pre-emergent herbicides Sakura® and Boxer Gold®.
“Sakura® can only be used in wheat (not durum) and triticale; Boxer Gold® can be used in wheat and barley, so they should be used in those crops with trifluralin in other crops to form the basis of a MOA rotation.”
Dr Powles, Australian Herbicide Resistance Initiative (AHRI) Director and University of Western Australia Winthrop Professor hosted Australian and international scientists at the recent Global Herbicide Resistance Challenge.
He says rotation of chemicals and groups is important for protecting existing herbicides and urges landholders to adopt this practice along with the all-important harvest weed seed control techniques promoted by AHRI.
Dr Powles says there is a common misconception amongst producers that a quick chemical fix is on the horizon.
“There are many herbicide product retail names on the market but growers can always know and record the MOA because it is mandatory in Australia that herbicides display the MOA by alphabetical symbol on the herbicide container,” he said.
The recommendation to rotate chemicals and groups is an important part of the WeedSmart campaign which aims to galvanise the Australian cropping industry against herbicide resistance.
WeedSmart brings together industry organisations including the Grains Research and Development Corporation (GRDC), research providers and major crop input firms to deliver the message that herbicide resistance is a difficult but not insurmountable problem – but changes need to occur on-farm.
For more information on herbicide sustainability practices, visit www.grdc.com.au.

Controlling weed seeds at harvest is emerging as the key to managing the rising herbicide resistance levels putting Australia’s no till farming system at risk.
Peter Newman, Australian Herbicide Resistance Initiative (AHRI) communications leader told the recent Grains Research and Development Corporation (GRDC) Update at Narromine, NSW that herbicides are not the answer to herbicide resistance.
“Removing weed seeds at harvest is currently our greatest non-herbicide weed control tool in Australian grain cropping,” Mr Newman said.
“This practice is now widely adopted in the form of narrow windrow burning, chaff cart, bale direct, diverting weed seeds onto permanent tramlines, and now the Harrington Seed Destructor (HSD) developed with assistance from the GRDC.
“All of these tools are equally effective at removing weed seeds, averaging 55 per cent removal of annual ryegrass seeds according to GRDC-funded AHRI research.
“They all differ in their cost and the amount of residue that they remove from the paddock.”
Mr Newman told central west NSW growers and advisers data from a selection of 24 focus paddocks where the growers are cropping dominant with no livestock in the farming system showed positive trends over 12 years.
“Growers in the focus paddocks using harvest weed seed control reduced ryegrass levels from 183 ryegrass plants per square metre in 2001 to close to zero in 2008 and they have maintained these levels ever since, demonstrating the benefits of removing weed seeds at harvest.
“It is quite remarkable that growers have been so successful at eroding annual ryegrass seed banks of paddocks, while maintaining a cropping intensity of 88pc.”
Mr Newman says many of the original messages about managing herbicide resistance in 1990s were built around the concept of phase farming, however research shows it is possible to crop at high intensity while eroding the weed seed bank despite high levels of herbicide resistance.
“Growers who have had the most success at managing ryegrass populations are those who have practiced harvest weed seed control in the form of narrow windrow burning or by towing a chaff cart,” he said.
“Harvest weed seed control does not fix a system that is broken but can be the key to making a system work.”
To view a video or listen to audio of Peter Newman discussing harvest weed seed control strategies, visit www.grdc.com.au/MR-HarvestWeedSeedControl.
For more information on GRDC-funded research, visit www.grdc.com.au.

Landholders are urged to monitor the effectiveness of herbicide sprays and to act quickly to test surviving patches of weeds for resistance.
Dr Michael Widderick, Department of Agriculture, Fisheries and Forestry Queensland (DAFFQ) principal research scientist says knowing the status of weed populations may surprise some farmers but allows them to devise a better weed management strategy.
“Herbicide resistance testing is very important; you need to know what herbicides are still effective on the weeds you are trying to control,” Dr Widderick said.
“For example, if you’ve been relying on glyphosate for a number of years to control awnless barnyard grass and you find it’s not working as well as it used to and leaving patches of survivors, testing samples can give you a better picture of what herbicides are and aren’t working.
“From there you can devise a strategy for stopping seed set but if you apply herbicides that are not effective you might as well pour your money down the drain.”
There are two testing services available in Australia: Charles Sturt University (CSU), Wagga Wagga, NSW predominately tests weed seed samples; and Dr Peter Boutsalis’s Plant Science Consulting in South Australia tests both seed and plant samples.
Dr Widderick advises farmers who are concerned or already experiencing herbicide failures in-paddock to collect seed from the suspected resistant patch or across the paddock in order to evaluate a range of different herbicides on their efficacy in controlling the weed species.
“If you’ve had a herbicide failure you’ll often see patches of weeds that survive so collect seeds from several plants,” he said.
“For a species such as barnyard grass, for example, you’ll need to collect about two tablespoons of viable seed. You’ll know they are if viable if they fall off the barnyard plant quite easily.
“Collect the seeds and place them in an envelope or paper bag, but not in something that will sweat and keep the seed moist, and send for testing.”
The Quick Test is a plant test predominately used for grass weed species such as awnless barnyard grass.
“Collect plants that are between the early to late tillering stage. About 50 to 100 plants will need to be pulled up, quite a large number,” Dr Widderick said.
“Wash off the roots and wrap in a moist but not drenched paper towel, put those samples in a plastic bag to retain the moisture and send it to the testing service.”
DAFF weed researchers have teamed with the Northern Grower Alliance (NGA), funded by the Grains Research and Development Corporation (GRDC), NSW Department of Primary Industries, Tamworth to launch a survey of awnless barnyard grass glyphosate resistance levels.
An alarming 50 per cent of populations tested in the pre-glyphosate application random sample were resistant to glyphosate, Dr Widderick said.
A second survey was done following the treatment of glyphosate and survivors were gathered.
“From that survey we found that 90pc of the populations came back as having glyphosate resistant plants,” he said.
The next step is to explore further the level of resistance within these populations and extend the survey to cropping areas in western NSW and Queensland and into Central Queensland.
“We always suspected there were barnyard grass populations with high levels of glyphosate resistance but to get 50pc from that first survey was quite alarming because we didn’t expect it to be that high,” Dr Widderick said.
The recommendation to test for resistance is an important part of the WeedSmart campaign which aims to galvanise the Australian cropping industry against herbicide resistance.
WeedSmart brings together industry organisations including GRDC, research providers and major crop input firms to deliver the message that herbicide resistance is a difficult but not insurmountable problem – but changes need to occur on-farm.
For more information on testing services, visit www.plantscienceconsulting.com or www.csu.edu.au.
For more information on herbicide sustainability practices, visit www.grdc.com.au.

By Martin M Vila-Aiub, Pedro E Gundel, Qin Yu and Stephen B Powles
Just published AHRI research paper entitled “Glyphosate resistance in S halepense and L rigidum is reduced at suboptimal growing temperatures“.
This research was conducted with glyphosate resistant biotypes of tropical S halepense and temperate L rigidum in which we had established that the reduced glyphosate translocation resistance mechanism is present. These biotypes do not appear to have any target site EPSPS gene mutations.
Good work from the Sammons laboratory at Monsanto has demonstrated with glyphosate resistant Conyza biotypes with the reduced glyphosate translocation resistance mechanism (increased vacuolar sequestration) that the level of glyphosate resistance is considerably lower at low temperatures.
Here we examined the temperature dependence of glyphosate resistance in tropical warm season S halepense versus temperate cool season L rigidum biotypes with the reduced glyphosate translocation resistance mechanism. With these contrasting warm season versus cool season species the results are very clear that glyphosate resistance level is temperature dependent in that the reduced glyphosate translocation resistance mechanism is inefficient at low temperatures.   This is good confirmation of the results of Sammons et al and extends the observations to the warm season S halepense. Indeed, in some cases, it may be possible to achieve control of glyphosate resistant biotypes with this reduced translocation resistance mechanism if plants can be treated at times of low temperature.
To read more click here.

Growers can take steps in coming weeks to help keep the precious knockdown herbicide glyphosate working on their farms, according to the Australian Glyphosate Sustainability Working Group (AGSWG).
The AGSWG – a collaborative initiative supported by the Grains Research and Development Corporation (GRDC) – says reducing the number of weeds that growers have to deal with in-crop will help keep glyphosate an effective herbicide.
“To achieve this growers can use tactics such as the double knockdown – two applications of knockdown herbicides used within 10 days of each other – such as glyphosate followed by paraquat,” AGSWG executive officer Andrew Storrie said.
“The aim of the second herbicide is to kill any survivors of the first application. These weeds might be resistant to glyphosate.
“Using measures such as the double knockdown will help local growers avoid the scenario facing their counterparts in the United States and Canada, where an over-reliance on glyphosate has led to big problems with herbicide resistance, with 49 per cent of grain growers surveyed believing they have glyphosate resistant weeds.”
Mr Storrie said more than 20 cases of glyphosate resistance had been confirmed to date in Western Australia’s winter grains systems, and believed many more infestations were going unreported.
He said glyphosate should be applied at the full label rate to help reduce the risk of any weeds surviving it, and sprayed at the right time – in the case of annual ryegrass, when it had reached the two to three leaf stage.
“Annual ryegrass plants that survive being sprayed with glyphosate should be collected and laboratory tested for herbicide resistance, so growers can confirm which herbicides remain effective on them,” Mr Storrie said.
“Herbicide resistance testing can be arranged through your agronomist or local farm supply agent.
“The second knockdown herbicide application should be a robust rate of Spray.Seed® or paraquat but can include the pre-emergent herbicides trifluralin or Boxer Gold®.
“The price of paraquat has dropped significantly, making its use in a double knock strategy much more cost effective.
“Annual ryegrass plants which survive being sprayed with paraquat should be killed using whatever means is possible before seeding the crop.”
Mr Storrie said that if small weeds had not been controlled by an early application of a knockdown herbicide, they would be harder to kill.
“Growers in this situation may have to consider delaying seeding in these paddocks so they can apply a second knockdown herbicide,” he said.
For more information on managing glyphosate resistance visit the AGSWG website www.glyphosateresistance.org.au
You can download the GRDC Glyphosate Resistance Fact Sheet at www.grdc.com.au/GRDC-FS-GlyphosateResistance, while the GRDC Ground Cover supplement Making Herbicides Last will be included in the May-June edition of Ground Cover. Visit www.grdc.com.au/groundcover to sign up for or download Ground Cover.