Ask an Expert

Agronomists, researchers, growers answering important questions

Ask an Expert

When is clean, clean enough with Mechelle Owen

Retaining seed on-farm for planting next season makes good economic sense, unless that seed is contaminated with weed seed.

AHRI researcher, Mechelle Owen, says many growers are unaware of the level of weed seed contamination that can occur in crop seed retained for planting, even after cleaning.
AHRI senior research officer, Mechelle Owen, surveyed grower-retained seed to assess the level of weed seed contamination and the effect of grain cleaning. What she found alarmed the growers involved and pointed to the importance of sowing clean seed into clean paddocks.
“The first finding was that the vast majority of the surveyed growers retained seed for planting and were conscious of the need to remove weed seed contaminants,” she says. “Unfortunately, over 70 per cent of the seed samples we analysed were still contaminated with weed seed, even after cleaning.”
The remaining samples were weed-free, proving that attention to detail can produce a clean sample suitable for planting.
The other finding was that much of the weed seed collected at harvest time is herbicide resistant. “Growers would never deliberately plant herbicide resistant weed seed along with their crop,” says Ms Owen. “But that is what happens if the retained seed is not thoroughly cleaned. Many weeds present at harvest are there because they are herbicide resistant.”

Planting crop seed contaminated with potentially herbicide resistant weed seeds will increase the in-crop weed pressure.
How much contamination is too much?
Short answer: Every weed sown has the potential to return seed to the seed bank.
Longer answer: 100 weed seeds per kilo of cereal or pulse seed sampled equals around one weed per square metre when the crop is sown.
Does the type of cleaning system used make a difference?
Short answer: Yes, gravity tables are best.
Longer answer: The growers surveyed used a range of cleaning methods including gravity tables, sieves, combination and rotary screen cleaners. Combination and rotary screen cleaners produced a similar level of decontamination, sieves provided the poorest result and grading tables gave the best results. Crop type also influenced the level of weed seed contamination, with greater weed seed contamination in cereal samples compared to lupin samples. Any cleaning is beneficial but the more professional the better as independent contractors consistently achieved cleaner seed.
How do I measure the level of contamination?
Short answer: Check a 1 kg sample.
Longer answer: Collect a 1 kg sample of the retained seed and separate the crop seed from all other material.
What’s the best way to reduce or avoid weed seed contamination?
Short answer: Harvest seed from paddocks with minimal weeds.
Longer answer: Plan ahead. Clean grain harvesting and handling gear thoroughly between paddocks. Harvest seed from low-weed-burden paddocks and use a gravity table clean the seed. Check the level of contamination after grading.
How to ask a WeedSmart question
Ask your questions about planting clean seed to help manage herbicide resistant weeds, using Facebook, Twitter @WeedSmartAU or leave a comment below.
Listen to the podcast on GRDC Driving Agronomy 

Ask an Expert

Are your weeds fit or suffering

For some weeds, the ability to survive herbicide treatment comes with a ‘fitness penalty’ that makes them less able to thrive when faced with strong competition.

Glasshouse experiments show that some herbicide resistant weeds suffer a ‘fitness penalty’ that reduces their ability to compete with other plants when no herbicide is applied. Unfortunately not all herbicide resistance causes a fitness penalty.
AHRI research fellow, Dr Martin Vila-Aiub studies the ability of herbicide resistant weeds to survive when faced with other environmental pressures in the absence of herbicide application.
“What we have found is that some weeds that are selected for their ability to survive herbicide treatment are less able to grow well and reproduce—this is called a ‘fitness penalty’,” says Dr Vila-Aiub. “However, some herbicide resistant weeds do not suffer any ‘fitness penalty’ and these ‘super weeds’ can adapt well to every control tactic applied to them.”
For the weeds that do suffer a fitness penalty, Dr Vila-Aiub says there are opportunities for growers to exploit this new weakness and to greatly reduce seed production of these herbicide resistant weeds without using herbicides.
There are many variables involved in this field of study that make it difficult for researchers to issue general recommendations. Importantly, it is not possible to observe resistant weeds in the field and make assumptions about what might be causing their apparent lack of fitness or otherwise.

Dr Martin Vila-Aiub studies the ‘fitness penalty’ associated with herbicide resistance in some weeds to find ways to exploit this weakness using environmental factors.
What does it mean if weeds survive a herbicide treatment but look sick?
Short answer: There may be an opportunity to exploit this weakness.
Longer answer: Sometimes herbicide resistant weeds can survive a treatment but are very damaged by the herbicide. These plants are less likely to compete well against a vigorous crop or pasture. In a competitive environment, these plants are less likely to set seed and so the population size can be significantly reduced.
If I think my weeds might be herbicide resistant, what should I do?
Short answer: Get samples tested to determine if they are resistant and to identify any herbicides that are still effective.
Longer answer: Providing a competitive environment is usually a beneficial tactic in a weed management plan. Weed populations that suffer a fitness penalty associated with their herbicide resistance trait can be significantly reduced in size. Resistant weeds with no fitness penalty are likely to still grow well but may be inhibited by the rapid early growth of a suitable crop cultivar.
What other fitness penalties can be inferred with herbicide resistance?
Short answer: Increased susceptibility to insect and disease attack.
Longer answer: The herbicide resistant biotypes of some weed species become more attractive to pests and diseases compared to plants that are susceptible to herbicides. Some species use lowered photosynthesis activity as a mechanism to survive herbicide treatment. This has a side effect of reduced seed production, which reduces the resistant population’s ability to thrive and spread.
Do herbicide resistant weeds have other weaknesses that growers can exploit?
Short answer: Probably, but more research is needed.
Longer answer: Some herbicide resistant weeds may have a new ‘weak link’ in their ecology. For example, glyphosate resistant ryegrass and wild sorghum are susceptible to glyphosate applied at low temperatures. In another experiment, researchers showed that some herbicide resistant plants produce seed that does not germinate without light. Burying these seeds just a few millimetres was enough to prevent germination. These responses have been shown experimentally but can not be relied on as management tactics until the processes are better understood.
How to ask a WeedSmart question
Ask your questions about taking advantage of reduced fitness to help manage herbicide resistant weeds, using Twitter @WeedSmartAU or leave a comment below.

Ask an Expert

The pasture phase for weed control with Tim Condon

Strategic grazing, spray-topping and fodder conservation in a pasture phase are excellent tactics to reduce annual grass weed populations and manage herbicide resistance.

A mixed farming system with a pasture phase in the crop rotation has many benefits including another strategy to significantly reduce weed seed set using grazing, herbicide and fodder conservation (Photo: DAFWA).
Tim Condon says that livestock in the farming system add another suite of weed control options, especially now that herbicide resistance is becoming more common in major crop weeds such as annual ryegrass, wild oats, barley grass and brome grass.
“The key is to have a weed management strategy that spans the whole pasture phase, not just the year before returning to cropping,” he says.
Farmers can use livestock to significantly reduce the seed bank of these grass weeds through a coordinated strategy involving heavy grazing, herbicides and possibly haymaking or silage.
“Spray-topping alone generally reduces seed set of the targeted grass species by around 80 per cent,” says Mr Condon. “Once the withholding period is over a follow-up period of intensive grazing can further reduce the number of weed seeds returning to the soil.”
A planned program that includes spray-topping, winter cleaning and a spray fallow across three years before returning to cropping can have excellent results, greatly reducing weed pressure in the cropping phase. Strategic heavy grazing before and after herbicide treatments increases chemical efficacy and removes survivors before they set seed.
Spray-topped barley grass (Photo: DAFWA)
What herbicides are registered to spray-top pasture?
Short answer: Paraquat and glyphosate.
Longer answer: Both paraquat and glyphosate are registered to target grass weeds in pastures. A single application of glyphosate can cover a wider range of species flowering at different times. When targeting a single species such as annual ryegrass, paraquat may provide an alternative to glyphosate as it is possible to accurately target the correct growth stage. Weeds that produce seed heads over an extended period may require a second herbicide treatment or heavy grazing to remove late seed heads before they mature.
When is the best time to spray-top?
Short answer: Before the weed seed heads reach the milk-dough stage.
Longer answer: Paraquat has a narrow window of effectiveness—between flowering and milk-dough stage—but is fast acting. Having the majority of the weeds flowering together is very important to gain the full benefit of this herbicide. Glyphosate can be applied before or after flowering, but if left until milk-dough stage some seed heads can mature before the herbicide has its full effect because glyphosate is slower-acting than paraquat. Glyphosate may be a better option if there are several weed species present.
When should the pasture be heavily grazed?
Short answer: Before and after spray-topping.
Longer answer: Heavy grazing beforehand helps to synchronise flowering and makes it easier to time the spray-topping operation. After observing the relevant withholding period livestock can return to graze the pasture and will often preferentially graze the regrowth, including any new weed seed heads and any survivors. Remove the livestock while there is still plenty of ground cover.
What role can fodder conservation play?
Short answer: Removal of any seed heads present after the herbicide treatment.
Longer answer: Cutting pasture for silage or hay, followed by spray-topping, is a great double knock option for preventing weeds setting seed in the pasture phase. This is particularly useful if suitable numbers of stock are not available for intensive grazing after spray-topping. Slashing or mechanical topping annual grass weed seed heads emerge in the pasture is another option to stop seed set.
How to ask a WeedSmart question
Ask your questions about musing livestock and fodder conservation to help manage herbicide resistant weeds, using Twitter @WeedSmartAU or leave a comment below.

Ask an Expert

Keeping fence lines clean with Sally Peltzer

Dr Sally Peltzer said that approximately 25% of glyphosate-resistant populations within broadacre cropping situations across Australia come from fence lines and other non-cropping areas of the farm.

“These non-crop areas are not subject to the pressures of crop competition and are usually given lower priority than the paddocks when it comes to weed management,” explained Dr Peltzer. “The over-reliance on glyphosate along borders with no control of survivors is high risk and herbicide resistance under these circumstances is inevitable.”
“Also, weeds on paddock borders are typically not sprayed until later in the season when many of the plants are large and much less susceptible to herbicide,” she said. “It is also difficult to get good spray coverage on these large plants.”
Once herbicide resistant weeds establish on paddock borders the seed can easily spread into the paddock with wind, water or machinery.
Dr Peltzer is encouraging growers to treat weeds growing along paddock borders as a high priority job that requires good timing and a variety of management tactics, including non-herbicide options. “New populations of herbicide resistant weeds growing along paddock borders are being identified all the time,” she said. “The problem is already widespread and demands a serious re-think of how growers manage these areas around their farms.”

What is the first step if I think some of my paddock border weeds are not dying after being treated with herbicide?
Short answer: Get samples tested.
Longer answer: There are two tests available, a ‘quick test’ conducted on plant samples and a traditional test conducted using seed samples. Testing is needed to establish the level of resistance and to identify herbicide groups that are still effective on the sampled plants.
Are there new treatment strategies specifically for weeds growing along paddock borders?
Short answer: Yes.
Longer answer: Over the last few years field trials have helped identify some tactics that have the potential to improve weed control in non-crop areas. Spraying weeds on paddock borders in July and August is probably too late. Spraying in May when the weeds are smaller is more effective and a second treatment later in the season is commonly required to treat late germinations. The later treatment will also have a beneficial effect on managing summer weeds.
Other than glyphosate, what chemical options are available for treating weeds along paddock borders?
Short answer: A residual herbicide followed by a knockdown.
Longer answer: Weeds growing along paddock borders require targeted management tactics aimed at preventing seed set. Field trials in Western Australia have shown that an early treatment with a residual herbicide such as triazine or bromacil and a knockdown such as paraquat has a beneficial effect on seed set.
Other options for fence line control, including non-herbicide options, are available here.
How to ask a WeedSmart question
Ask your questions about managing cross-resistance, or testing for herbicide resistance, using Twitter @WeedSmartAU or leave a comment below.

Ask an Expert

Getting started with harvest weed seed control with Maurie Street

In Western Australia farmers have been forced into harvest weed seed control tactics and have overcome most of the practical barriers to adoption. In the eastern states pressure is mounting as herbicide resistance is becoming more widespread.
Dubbo farmer, and researcher with the Grain Orana Alliance, Maurie Street, reckons that on farms where herbicides are still effective, harvest weed seed control tactics could delay the further development and spread of herbicide resistant weeds on their properties. Unfortunately, on many farms harvest weed seed control is now essential as high levels of resistance are already widespread.
Narrow windrow burning is an effective harvest weed seed control method that growers can implement with minimal up-front costs.“A major advantage of early adoption is the preservation of the herbicide chemistry that is still effective at the moment,” he said.
“This window of opportunity is closing fast though and growers who want to still have effective herbicide options will need to act immediately—this season if possible.”
Whatever the resistance status, Mr Street believes the experience of Western Australian growers should be enough to encourage other growers to make the change sooner rather than later. He said the changes were relatively easy to make and were proven to effectively reduce weed seed bank numbers within a few seasons.
“If you want to be growing crops without uncontrollable weeds robbing yield in five years time, you need to be implementing some form of harvest weed seed control as soon as possible,” he said.
Canola, pulses and low yielding cereals are the best crops to trial narrow windrow burning. With these crops the risk of fire escapes is minimal and there is little ground cover penalty.
What harvest weed seed control method is the easiest to trial?
Short answer: Narrow windrow burning.
Longer answer: All harvest weed seed control methods are proven to have similar effects on reducing the number of seeds returning to the soil. Narrow windrow burning is often recommended as a good way to start because there are minimal start-up costs involved. Growers can see the benefits of harvest weed seed control for their environment and farming system before making a larger investment in chaff carts or a Harrington Seed Destructor.
If I decide to trial narrow windrow burning, what do I need to do first?
Short answer: Attach a windrow chute to the header and cut the crop low.
Longer answer: A simple addition of a chute on the rear of the header will direct the straw and chaff into a narrow windrow. Plans for the required modification are readily available on the GRDC and other websites. Once the machine is modified the other essential change is to the harvesting height. For harvest weed seed control to be effective the weed seeds must enter the harvester. To be sure of collecting almost 100 per cent of the weed seed at harvest the crop must be cut low—around beer can height. Any higher and the effort is wasted.
Which crops should I burn?
Short answer: Pulses, canola and low yielding cereals.
Longer answer: Fire escapes are a risk with narrow windrow burning. In pulses, canola and low yielding cereals the risk of fire escapes is reduced and there is minimal impact on ground cover since these crops produce low levels of biomass. It can be done in cereals over 3 t/ha but requires skill and experience in choosing safe times to burn.
How to ask a WeedSmart question
Ask your questions about managing cross-resistance, or testing for herbicide resistance, using Twitter @WeedSmartAU or leave a comment below.

Ask an Expert

‘Weeds on my farm are resistant to herbicides I have not used?’ with Roberto Busi

Weeds with resistance to multiple herbicide groups are becoming increasingly common. The most alarming cases are where plants are resistant to herbicide groups that they have never been exposed to.
Dr Roberto Busi is interested in understanding the rapid evolution of mechanisms that plants use to survive a herbicide application.

The most alarming herbicide resistance cases are where plants are resistant (right) to herbicide groups that they have never been exposed to.
 “Each herbicide group interferes with different physiological mechanisms, such as photosynthesis or protein and lipid synthesis, within the target plant,” he said. “There are two main ways that plants can be resistant to a herbicide group.”
The first is when the plant possesses a mutant version of the target protein. When the herbicide reaches the site of action, it cannot bind to the target protein and so the physiology of the plant is unaffected. This is known as ‘target site resistance’ and has been the most common mechanism in herbicide resistance.
The second way occurs when the herbicide is prevented from reaching the target site. In this scenario the target site remains susceptible but the plant is able to de-activate or de-toxify the herbicide before it has its intended effect. This is known as ‘non-target site resistance’ and is the main mechanism involved when plants are resistant to more than one herbicide mode of action (MOA).
“Cross-resistance is becoming more common and in glasshouse experiments we have proven that it can occur within just a few generations,” said Dr Busi.

AHRI researcher Dr Roberto Busi is interested in understanding the rapid evolution of mechanisms that plants use to survive a herbicide application.
If I am careful to rotate herbicides with different modes of action will I avoid cross-resistance?
Short answer: Not necessarily.
Longer answer: In some crops there are only a few registered herbicides for the control of weeds. Even if these chemicals are rotated they are all still being regularly applied and it is possible for survivors to be resistant to both modes of action. For example, annual ryegrass plants have been found to be resistant to wheat-selective herbicides from Groups A, B, D and, most recently, J and K.
Is it possible for a weed on my farm to be resistant to a herbicide MOA that I have not used?
Short answer: Yes.
Longer answer: If a plant possesses genes that give it ‘non-target site resistance’ to one MOA group, it may also be automatically resistant to another MOA group. This can occur even if these chemicals have not been applied to this plant. Importantly, annual ryegrass plants can stack target-site and non-target-site resistances.
If I have cross-resistant weeds do I still have weed management options?
Short answer: Yes.
Longer answer: Preventing seed set on herbicide resistant plants is absolutely critical. It is necessary to look for survivors after a spray application and to eliminate them. Sending plant samples away for testing against a range of herbicides will tell you if there are herbicide options available to kill the resistant plants. Once the immediate threat has been dealt with it is important to develop a weed management plan than includes several non-herbicide tactics to reduce the frequency of herbicide use.
How to ask a WeedSmart question
Ask your questions about managing cross-resistance, or testing for herbicide resistance, using Twitter @WeedSmartAU or leave a comment below.

Ask an Expert

‘Look for escapes—all year’ with Michael Widderick

Just as an integrated weed management program is planned across the year and even across a full crop rotation cycle, so too is it necessary to monitor every weed management tactic to check for escapes.
Dr Michael Widderick says even a well-planned approach to weed management is unlikely to be bullet-proof. “The reason we use a variety of tactics in weed management is because we can’t rely on any one tactic to be 100 per cent effective, every time,” he says.
Agri-Science Queensland Principal research scientist, Dr Widderick encourages growers to observe what happens after each weed management event and respond to what they see.
“Each tactic is chosen to achieve a specific purpose and growers need to know the outcome of that tactic to help plan the next step.”
“Unfortunately, there is often not much time between a few suspect plants and paddock-wide herbicide resistance,” he says. “It is very important to monitor weeds and act early if you are suspicious.”
Dr Widderick says the practice of weed management is as much about observing what happens and responding to it as it is about having a number of tactics in place to keep driving down the weed seed bank.
Intensive management of a patch of survivors like this can stop the spread of herbicide resistance.
What should I be looking for after a fallow weed control tactic using herbicide?
Short answer: Patches of survivors.
Longer answer: Herbicide resistance is much easier to see in a fallow paddock. If you have applied a herbicide treatment, go back a week or two later and look for individual plants or small patches that have survived the treatment. Often herbicide resistance will be first noticed when other weed species in a paddock are still controlled but one species is relatively unaffected. This usually starts in patches but can quickly spread across the paddock if the patches go undetected and set seed.
What should I do if I notice weed escapes in-crop?
Short answer: Test seed or plant samples.
Longer answer: Managing a weed blow-out in-crop is very difficult. Weeds that have ‘escaped’ the controls implemented at sowing, or that have germinated after the pre-emergent herbicide has degraded, will often not be seen until they have grown above the crop and set seed. Taking samples for testing against the herbicides you used will provide important information for the next step in your weed management plan. Such an event may trigger the use of a harvest weed seed tactic such as narrow windrow burning or, in extreme cases, green or brown manuring.
What should I do with weeds in non-crop areas that survive a herbicide treatment?
Short answer: Try another tactic.
Longer answer: Herbicide resistant weeds in non-cropping areas do occur and they are a source of seed that can lead to an infestation in cropping paddocks. Many non-crop areas such as roadsides, fencelines and around buildings are routinely treated with glyphosate. Any over-reliance on a single tactic is likely to end with herbicide resistance in weeds and can easily spread across a farm.

Listen to Michael Widderick’s 3 minute interview with Chris Brown (GRDC Radio, Southern Update)
How to ask a WeedSmart question
Ask your questions about identifying herbicide resistance, or any herbicide resistance management strategy, using Twitter @WeedSmartAU.

Ask an Expert

Full label rates, always

Delta Agribusiness senior agronomist, Tim Condon, said any saving in
chemical costs is significantly outweighed by the risk of the low dose causing faster
herbicide resistance evolution.
“Scientific studies have demonstrated that resistance can rapidly evolve in weeds
subjected to low doses of herbicide,” he said. “Some weeds can develop resistance within
a few generations.”
Low dose applications of some herbicides have been proven to cause rapid development of herbicide resistance in some weeds. Photo: Andrew Storrie.
When used at low rates on Lolium rigidum (annual ryegrass) Hoegrass®, glyphosate and Sakura® have been found to result in the rapid evolution of resistance to both the herbicide selected as well as herbicides that have yet to come into contact with the targeted population.
“When mixing herbicides it is important that each product is still applied at the full label rate
to ensure high mortality,” said Mr Condon.
Mr Condon said applying different chemicals in one mix can provide an additive
advantage. “It is important to understand the mode of action of each herbicide on the plant
when preparing a herbicide mix,” he said. “This is just as important for pre-emergent grass
weed mixes as it is for post-emergent mixes aimed at broadleaf weed control.”
Having dual mode of action does not negate the need to change herbicide products and rotate modes of action.
Can mixing herbicides help slow the rate of herbicide resistance?
Short answer: Yes.
Longer answer: Surrounding weed seeds with a combination of pre-emergent herbicides
with different modes of action can give a high level of control and help extend the useful
life of all the chemicals used. The high level of control must be supported with additional
control measures for all survivors. All products with different modes of action must be
applied at full label rates for this to be an effective strategy.
Can I reduce the rates if the herbicides in the mix have the same mode of action?
Short answer: No.
Longer answer: Mixing two chemicals with the same mode of action can achieve some
additional efficacy, however the mix should deliver the combined full rate to ensure a lethal
dose. The amount of stubble present and crop safety are all important considerations
when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice.
If a herbicide product has more than one mode of action can I cut the rate or not rotate chemicals?
Short answer: No.
Longer answer: Many herbicides on the market are a combination of two or more modes of
action within the one product. These products must be applied at the full label rate to be
effective. Having dual action does not negate the need to change herbicide products and
rotate modes of action. Repeated use of any single strategy will reduce the effectiveness
of that strategy over time.

Ask an Expert

‘Why change if a herbicide is working well’ with Peter Newman

Recently, the Kondinin Group conducted a survey of 200 farmers and found that the overwhelming majority were practising herbicide rotation in their cropping systems.
Peter Newman, Communication Leader with the Australian Herbicide Resistance Initiative says this confirms that most growers are implementing practices aimed at reducing the risk of herbicide resistance on their farms. “Rotating herbicide chemistry is essential,” he says. “But it is not enough on its own. A diverse weed management program must include more tactics to help protect the chemistry that we have available to us for weed control.”
“Glyphosate is by far the world’s best herbicide,” he says. “It is highly effective in so many situations and its widespread use is justified. What we need to start implementing are tactics that will protect the usefulness of glyphosate, and other herbicides, well into the future.”
“We know that rotating chemical modes of action is useful as a broad tactic but when it comes to particular chemicals, like glyphosate, that are so widely used it is important that seeds from any weeds that survive a spray application do not enter the seed bank.”
“So a trusted herbicide can be used, and used often, provided there is a strategy in place to remove all survivors, including any seeds,” says Mr Newman. “If possible this strategy should include non-chemical tactics to provide the double-knock effect.”

Can I rely on herbicide rotation to avoid herbicide resistance?
Short answer: No.
Longer answer: There are documented cases of herbicide resistance occurring on farms where herbicide rotation was conscientiously implemented. We even have cases where ryegrass has developed resistance to glyphosate and paraquat, despite rotation between these two herbicides.
Is it true that weeds can be resistant to herbicides I haven’t ever used?
Short answer: Yes.
Longer answer: Metabolic resistance is an alarming phenomenon that we are gaining a greater understanding of . In a laboratory experiment annual ryegrass plants were treated with low doses of Sakura® to see if they would become resistant to this herbicide. They did, and they also became resistant to Avadex® and Boxer Gold®, which had not been applied during the experiment. This meant that some individual plants had the ability to stop the action of, or metabolise, these herbicides with different modes of action, before they reached the various target sites.
Do non-herbicide weed controls really help avoid herbicide resistance?
Short answer: Yes.
Longer answer: Managing herbicide resistant weeds is all about managing the weed seed bank. Farms where non-herbicide tactics are part of the system have less weeds and lower levels of herbicide resistance. The ideal second knock to follow a herbicide is a mechanical, non-herbicide option such as haymaking, cultivation or grazing. Effective weed management requires the implementation of as many tactics from the WeedSmart 10 Point Plan as possible. Using one or two of these tactics is not enough.
How to ask a WeedSmart question
Ask your questions about the spread of herbicide resistance, or any herbicide resistance management strategy, using this blog or using Twitter @WeedSmartAU.

Ask an Expert

‘Up the competition’ with Deirdre Lemerle

Excellent establishment of all crops and the strategic use of highly competitive cultivars are powerful management practices that can limit the opportunities for herbicide resistant weeds to take hold.
Lead scientist and director of the Graham Centre for Agricultural Innovation at Charles Sturt University, Professor Deirdre Lemerle said the genetic control for competitiveness involves multiple genes and is too complex to easily select for in most crops.
“Breeding for competitive advantage has not generally been considered a key aspect of plant breeding programs in the past,” she said. “We now know that it is an important agronomic trait and it is being considered in more breeding programs.”
“When it comes to crop establishment, our studies have shown there is no yield penalty for planting at higher sowing rates, even in low rainfall environments.”
The allelopathic effect of crops such as canola on weeds is a new area of research at the Graham Centre.
“The most competitive crops are still unlikely to prevent the spread of herbicide resistant weeds,” said Professor Lemerle. “Well established, competitive crops are known to restrict weed growth and so these strategies are included in the WeedSmart 10 Point Plan.”
“If the crop is competitive, the herbicide applied to the crop is likely to be more reliable—managing to minimise herbicide resistance must be seen as a package because none of the weed control options are effective on their own.” 
What are the most competitive crops?
Short answer: Triticale, barley and rye.
Longer answer: The competitive capacity of crops can be measured. Triticale, barley and rye are more competitive than wheat and some wheats are more competitive than others. Pulses are less competitive than cereals and the competitiveness of brassicas is very dependent on sowing time. Choose varieties that have rapid early growth and early canopy closure.
What is the best way to improve crop establishment?
Short answer: Aim for optimal crop density.
Longer answer: If weed pressure is a concern, choose a seeding rate at the higher end of the recommended range for your crop. Calibrate your equipment and use only clean, fresh, viable seed. In wheat, higher density planting causes fewer tillers per plant but larger grain, giving no yield penalty. Even though wider row spacing may be beneficial in conservation cropping systems for ease of planting and where stubble can reduce weed emergence, crops at wider rows are less competitive.
How can allelopathy help?
Short answer: It can inhibit the emergence and growth of some weeds.
Longer answer: Current research is investigating the potential for crops like canola to inhibit the growth of annual ryegrass. There is a large difference in the allelopathic effect of different canola cultivars. A cultivar may be chosen specifically to inhibit the growth of a target weed as part of a three to five year weed management strategy.
How to ask a WeedSmart question
Ask your questions about the spread of herbicide resistance, or any herbicide resistance management strategy, using this blog or using Twitter @WeedSmartAU.

Ask an Expert

Testing for herbicide resistance

“Testing takes the guesswork out of the equation and gives farmers baseline information that they can use to monitor changes in the weeds on their farms,” he said.
“If low level resistance is identified early there are many more management options available compared to situations where full blown resistance has taken hold.”
Dr Boutsalis said the over use and over reliance on particular herbicides will unavoidably lead to herbicide resistance developing. “We often hear of farmers applying herbicide even though they are not sure if it will work,” he said.
The $300 to $400 cost of testing is insignificant compared to the cost of wasted herbicide, lost production and the costs of driving down a large seed bank of resistant weeds.

What herbicide resistance tests are available to farmers in Australia?
Short answer: The ‘quick’ test using the whole plant and the ‘seed’ test.
Longer answer: The ‘quick’ test uses plant samples collected on farm and sent to the laboratory. The plants are revived and planted into pots then tested against the required herbicides. The ‘seed’ test requires the collection of ripe seed, which is planted out at the laboratory. After dormancy has been broken and the seedlings have started to grow they are tested for their response to herbicides. Both tests are equally accurate. The ‘quick’ test can not test for resistance to some pre-emergent herbicides, such as trifluralin.
Which is the most common test that farmers use?
Short answer: The seed test.
Longer answer: Collecting seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis.

Where is the best place to collect samples?
Short answer: From suspicious or high risk areas.
Longer answer: Herbicide resistance can develop in high risk areas like fencelines or at random through a paddock. Visual observations and changes on the yield monitor in the header can indicate good places to collect seed. If collecting plant samples, look for weeds at the early tillering stage that appear to have ‘escaped’ previous herbicide treatment. Collect 50 to 100 small plants or fewer larger plants. Shake off the soil from the roots, place in a plastic bag and send to the laboratory.
What’s involved in sending samples?
Short answer: Pick, pack, register and ship.
Longer answer: Each sample needs to arrive at the laboratory with suitable identification and instructions. Register the samples online to get a unique sample number and to provide the information required, such as which herbicides you want to test against. Plant Science Consulting and Charles Sturt University both offer commercial herbicide resistance seed testing. Find the details under Point 4 of the 10 Point Plan on the WeedSmart website.
How to ask a WeedSmart question
Ask your questions about the spread of herbicide resistance, or any herbicide resistance management strategy, using this blog or using Twitter @WeedSmartAU.

Ask an Expert

On the lookout for new resistant weeds with Rohan Rainbow

Herbicide resistance is well-established in populations of several weed species across Australia but many more are at risk of evolving resistance if they are given the opportunity. Dr Rohan Rainbow says the two main risks are that new species will evolve resistance to the chemicals currently available and that weeds already known to have evolved herbicide resistance will spread to new areas.
“It is not likely that any new chemistry or modes of action will become available any time soon,” says Dr Rainbow. “We have to do whatever we can to conserve the chemistry that we have now and this must be done on a regional and farm by farm basis.”
“Weeds will continue to spread by wind and water and on vehicles and there is a significant risk that herbicide resistance can simply arrive in a region or on a farm even if farmers have been diligent with their herbicide management.”
“The weeds that are hard-seeded and can stay dormant for many years are the most difficult to control once herbicide resistance evolves,” he says

How do I know if the weeds on my farm are becoming resistant to the herbicides I use?
Short answer: Spray applications will seem ineffective.
Longer answer: After applying herbicide it is important to assess the effectiveness of the operation. Herbicide resistance usually becomes apparent as patches of weeds that ‘survive’ the application of herbicide. If you notice patches of weeds, take seed samples and have them tested for herbicide resistance. Even while the herbicides are still effective, implement strategies such as collecting and destroying weed seed at harvest, preventing seed set and rotating chemicals with different modes of action to reduce the risk of resistance evolving. Information about herbicide resistance testing is on the WeedSmart website.
Are there areas on the farm where herbicide resistance might evolve unnoticed?
Short answer: Yes.
Longer answer: Herbicide resistance may evolve in weeds that have always been present on your farm or in your district. These are likely to be noticed in-crop. Other weeds may spread to your farm or district and already be resistant to herbicides. Take notice of the plants growing along fence lines and road ways and on neighbouring properties that are upstream or upwind. Taking action across the landscape is the most effective way of reducing the economic and environmental damage associated with herbicide resistance.

Can I assess the risk of weeds on my farm becoming herbicide resistant?
Short answer: Yes.
Longer answer: On the WeedSmart website you will find the WeedSmart app and the Glyphosate Resistance Toolkit (for northern growers). These are free digital tools that farmers can use to assess the risk of herbicide resistance evolving in their current farming system. Answering a set of questions about your farming system and target weeds will highlight potential risks. Recommendations will be offered to minimise the risk of herbicide resistance. Take action early and implement the WeedSmart 10 Point Plan.
How to ask a WeedSmart question
Ask your questions about the spread of herbicide resistance, or any herbicide resistance management strategy, using this blog or using Twitter @WeedSmartAU.

Ask an Expert

Harvest weed seed control with Dr Michael Walsh

Weeds that ‘escape’ herbicide control can quickly contribute vast quantities of seed to a paddock’s soil seedbank.
Research Associate Professor, Dr Michael Walsh, says there are four options for weed seed control at harvest, and all growers should seriously consider implementing one of these strategies every harvest.
These harvest weed seed control strategies are known to work, and if regularly applied play an important role in keeping cropping land productive and, importantly, will extend the useful life of the herbicides currently available.

Narrow windrow burning is fairly cheap. Is it an option for all crops?
Short answer: No.
Longer answer: Narrow windrow burning is best suited to non-cereal crops such as lupins, canola and field peas and cereal crops yielding less than 3 t/ha. In large cereal crops windrow burning poses a serious fire risk. Narrow windrow burning is the easiest harvest weed seed control strategy to implement and is a great place to start for many growers. Weather conditions strongly influence the efficacy of burning and  must be monitored carefully before and during windrow burning.
Is the ‘bale direct’ system an economic option?
Short answer: Yes, if you have access to a suitable market for the bales.
Longer answer: A square baler attached to the harvester collects and bales both the chaff and straw component. There is an obvious risk that weed seed can be relocated, so it is essential that the bales are only used in suitable environments such as a feedlot or dairy. Another safe option is to make pellets using the bale material; a process that will destroy the weed seed. It is important to remember that the economics of baling are very market driven. Other possible future markets may be for electricity co-generation and ethanol production.
What is the most economical option for big cereal crops?
Short answer: A chaff cart.
Longer answer: Chaff carts are a relatively inexpensive option for collecting the weed seed bearing chaff while retaining the straw in the field. Using chaff carts avoids some of the fire risks associated with narrow windrow burning of large cereal crops. The chaff heaps can be set up in an area where it is easier to put firebreaks. Incorporating some straw in the chaff heaps with a conveyor-style transfer system will help the heaps burn better.
I want to maintain the maximum amount of crop residue in the paddock, what is my best option for harvest weed seed control?
Short answer: A Harrington Seed Destructor.
Longer answer: The Harrington Seed Destructor (HSD) crushes the chaff, including the weed seeds, and returns all the chaff and straw to the paddock, maximising the nutrient and soil moisture benefits of these residues. The initial capital cost of the HSD is a barrier for many growers but there is potential for harvest contractors to offer growers the use of a HSD as an additional harvest service.
How to ask a WeedSmart question
Ask your questions about the Harrington Seed Destructor, or any other herbicide resistance management strategy, either using this blog, or using Twitter @WeedSmart.

Ask an Expert

Harrington Seed Destructor with Ray Harrington

Harvest time is a perfect opportunity to collect and destroy weed seeds. Field trials have demonstrated that between 73% and 99% of the weed seed produced during a growing season goes through the harvester and so can be processed through the Harrington Seed Destructor’s industrial-grade mill. This process kills at least 95% of the weed seed present.
The Harrington Seed Destructor (HSD) has been thoroughly tested in grain crops across Western Australia, and the machine’s inventor, Ray Harrington, believes it has application in many situations across Australia.
“Growers in Western Australia have been forced to develop harvest weed seed management techniques over the last 30 years,” says Ray. “Over-reliance on herbicide has allowed resistance to develop and growers need to use other control strategies as well.”
Ray uses two Class 7 harvesters with tow behind HSD machines in his own business and has covered about 12 000 ha in the last three years with minimal wear and tear on the HSDs.

Does the Harrington Seed Destructor have application in all cropping zones?
Short answer: Yes.
Longer answer: The HSD was used in trials at 20–30 sites across SA, Victoria and NSW, and worked just like it does in WA. The machine is designed to trail the harvester and process the chaff component, including weed seeds and residual crop seed that can become a weed in the following crop. It is designed for use in crops that are harvested low to the ground, such as barley, wheat, chickpea and the like. A slightly different system may be needed for the HSD to be effective in taller summer crops such as sunflower and sorghum in the north, but the principle remains, if you can get the weed seed into the HSD, they will be destroyed. The technology could also be applied to horticultural and grazing settings.
Does harvest weed seed control using the HSD rely on any special crop preparation or after-harvest operations such as burning?
Short answer: No.
Longer answer: The HSD method of control is 100% mechanical. If a weed seed passes through the mill it will be crushed and rendered unviable.
What is the cost:benefit of using the HSD to manage harvest weed seed?
Short answer: Similar to other harvest weed management options.
Longer answer: Cutting lower to the ground to make sure the majority of weed seed heads are processed slows harvest rates slightly and increases fuel usage. Other harvest weed seed control options such as windrow burning and using a chaff cart have similar costs associated with them. Harvest weed seed management is now an essential part of crop production and the cost of not controlling herbicide resistant weeds is huge.
How to ask a WeedSmart question
Ask your questions about the Harrington Seed Destructor, or any other herbicide resistance management strategy, either using this blog, or using Twitter @WeedSmart.