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A shuttle of glyphosate applied over the top of a Roundup Ready cotton crop was recently shown to also contain a damaging level of 2,4-D impurity, resulting in significant crop injury and yield loss.
The grower involved did not accept the suggestion that the crop damage was due to poor sprayer decontamination or spray drift from a fallow application of 2,4-D, and he was able to prove the problem was due to product impurity.
2,4-D herbicide injury in cotton after the crop was sprayed with glyphosate product contaminated with 2,4-D.
Other shuttles of the same batch may have been applied to fallow weeds where the residual 2,4-D in the glyphosate would have gone unnoticed. Full rate 2,4-D in glyphosate is known to compromise glyphosate efficacy, but studies of low-rate 2,4-D impurity in glyphosate could not be found.
Where can impurities come from?
While the agricultural chemical manufacture and supply chain in Australia is considered first-class and is highly regulated, there is an acceptance that the nil-impurity requirement for the manufacture of agricultural chemicals is unattainable in facilities that use multi-purpose equipment for synthesis, formulation and packaging of products.
Companies therefore apply their own quality assurance standards before releasing products for distribution and sale. If the level of risk posed by certain residual impurities in a product is underestimated, there is potential for instances of crop injury, pesticide residue in produce or poor performance of the product on the intended target weed, fungus or pest.
Mistakes can and do happen within the manufacturing process and chemical supply and distribution chain. To ensure that risks of contamination are minimised and that quality assurance protocols are followed carefully, it is important that any breaches or errors are identified quickly, reported and investigated.
Keep good records of each spray event, including batch numbers of applied product, to help identify the cause of adverse experiences with herbicides.
There are two important things to note: firstly, the current regulations specify that crop protection products must contain nil impurities (other than manufacturing impurities listed in the APVMA standard); and secondly, companies are required to recall product batches when contamination issues are identified. The Australian Pesticide and Veterinary Medicines Authority (APVMA) oversees a highly regulated system of registration, compliance and enforcement on crop protection products.
Assess potential application issues
When misapplication (wrong product applied, incorrect mixing, contaminated product etc) occurs, symptoms of affected plants are usually uniform throughout the treated area. It is often suggested that poor application technique or poor sprayer decontamination is the reason for crop injury or poor weed control results – suggesting a grower ‘own-goal’. Such potential errors must be considered, but if best practice spray techniques and spray rig decontamination procedures have been followed, product impurity should also be considered and investigated.
The chemistry of the product will determine the risk of residues being held within the tank and spray lines of the application rig. This is why there are differences in the sprayer hygiene requirements after using particular products.
Most modern spray rigs have impervious rubber and plastic, or stainless steel components, drastically reducing the risk of chemical absorption and subsequent extraction. Residues on the rubber surfaces are the main concern, and all registered cleaners will physically remove residues when used as directed, but cracked rubber components can present a contamination risk. All filters/strainers must be cleaned and all actuators and taps musts be cycled as the cleaner is run through the spray boom and tank loading system, agitators and tank.
Crop injury or poor weed control that is associated with just one sprayer tank load would suggest sprayer contamination. Effects from contaminated tanks are usually worse at the beginning of the spray run, with damage diminishing with spraying and tank reloading. The field pattern can provide clues to the sprayer filling routine in the field where the crop damage occurred.
The other major reason commonly cited for crop injury in spray drift. Although there is always some small amount of drift when agricultural chemicals are sprayed from a ground rig, the amount is down to ‘virtually safe’ levels within a few tens of metres. If the conditions are very windy, or the boom is too high, or the droplet size too small, spray could drift a few hundred metres from the application ground rig.
Spray droplets may travel a few feet to several kilometres from the targeted area, depending on weather conditions and spray application; but the potential for drift damage decreases with distance because droplets are deposited or become diluted in the atmosphere. The pattern of injury is normally seen most prominently on the section of the field closest to the sprayer that generated the spray drift, and decreases across the field.
During inversion conditions, a similar amount of product is subject to drift, but the drifting product will not dilute as much in the air, so concentrations at specific locations can be higher than expected in non-inversion conditions.
What to do if your crop is damaged or weeds don’t die as expected?
Along with several other possible causes, unintended application of contaminated product should be considered as a potential explanation for crop injury or poor weed control.
Keep in mind that if product impurity is the problem, it is most likely due to a low-dose effect that may be difficult to diagnose or may take longer to express in the target weeds or susceptible crops.
Finding the cause of an ‘adverse experience’ with herbicide is one of the most important reasons to keep accurate and detailed spray records.
If a problem occurs:

Take detailed, time-stamped photographs of the crop or weeds and record everything you know about the crop or fallow management, weather conditions in the weeks prior to the damage being seen, spray history of the field etc. If possible, geotag the photos so they can be easily associated with the correct field.
Record the relevant batch numbers of the chemicals used, which can be checked against the retention samples at the factory if necessary. Collect samples from drums of product used prior to the injury being observed (up to 14 days prior to symptoms being obvious). When you take samples, make sure there are witnesses who can vouch for the voracity of the evidence you have collected. Testing for one impurity (e.g. 2,4-D in glyphosate) costs less than $500 per sample.
Document the injury over time. For example, injury in cotton from low rates of 2,4-D will grow out in two weeks, but injury from higher rates, could last three to four weeks and are the most likely to result in yield loss. Similarly with weeds although the impact may be more difficult to document.
Mark out the affected area in the field to help assess crop yield loss at the end of the season. Note the pattern and intensity of the problem across the field.
Eliminate as many possible causes as you can. Re-assess the application technique and equipment, consider the pattern of damage in the field, look at the weather conditions for the relevant period of time and so on.
Test for herbicide resistance in weeds.
Report the crop damage or poor weed control. The APVMA administers the Adverse Experience Reporting Program, which allows anyone to report a problem with an agricultural chemical, including crop and plant damage, for example, plant death, severe stunting or significant yield loss. This is also the way to report poor weed control outcomes.

The APVMA acknowledges there is likely under-reporting of adverse experiences. The magnitude of under-reporting is unknown and provides limitations in quantifying product risk.
Investigations of spray drift are conducted by the relevant state government body, for example: NSW EPA (call Environment Line: 131-555), Biosecurity Queensland (call 132-523) and Chemical Standards Officer (Victoria) (call 03 5430 4463). Industry organisations will also support growers impacted by chemical damage to crops.
If the damage is due to factors other than spray drift, the affected party will need to take legal action and seek compensation themselves.
Related resources
Is poor weed control due to herbicide resistance?

Chaff carts were invented in Canada in the 1970s as a fodder collection system, so they have always had an association with livestock. In the late 1980s they were introduced to Australia and used as one of the first harvest weed seed control (HWSC) tools to manage herbicide resistant ryegrass and wild radish in Western Australia.
Depending on the grower’s situation, chaff heaps in Australia have been burned, grazed then burned, sold as loose chaff for horse feed, grazed or knocked down then seeded through (unburned) and most recently, baled and used on-farm as conserved fodder.
Different model chaff carts incorporate more or less straw with the chaff, which can affect the feed value of the bales.
This latest method fits very well with the move to confinement feeding of sheep and brings the use of the chaff cart almost full circle. Given the long-standing association with fodder conservation, the value of the chaff as a feed source is well known, particularly for sheep.
A HWSC Calculator developed by WeedSmart western extension agronomist Peter Newman, now enables growers to quickly estimate the potential value of baling chaff from chaff heaps and feeding it out in on-farm confinement areas. Upper Eyre Peninsula mixed farmer, Bruce Heddle, instigated the addition of the new feature to the calculator and provided the energy figures from his own testing of different chaff types. Planfarm livestock consultant, Paul Omodei, worked with Peter to make sure the calculator took into account the important considerations for livestock production.
Although there is variability in feed value results, ballpark energy figures are in the order of 6.7 MJ/kg DM for wheat chaff, 7.7 MJ/kg DM for barley chaff, and 8 MJ/kg DM for canola and pulse chaff. Peter says the cost to deliver baled barley chaff to an on-farm feedlot is around 1.5 c/MJ compared to barley grain for 2.3 c/MJ, based on these feed values that Bruce Heddle provided.
“Chaff carts incorporate more or less straw with the chaff depending on the cart model, and while some extra straw makes baling and handling easier, it also reduces the feed value slightly, so that needs to be factored in,” he says. “Some growers are already set-up for hay making while others might choose to employ a contractor.”
Bruce has a contractor to bale their wheat, barley, canola and lentil chaff dumps, and feeds the bales along with barley grain and a lick feeder to sheep held in confinement from January to the end of seeding. All the chaff is beneficial although the canola and lentil chaff has the highest feed value.
Bruce says having the sheep confined during this period reduces their workload as the sheep are easy to look after and feeding out is only needed once a week. They are able to easily monitor the flock and attend to any problems quickly, and don’t have to spend time moving sheep around during seeding.
The pastures also benefit from a spell from grazing for these months, so the sheep are turned out into refreshed pasture after their confinement.
Jarred Tilley is another mixed farmer who has been making chaff bales for use in their family’s sheep enterprise at Kapunda in South Australia for the last two years. Jarred has been fencing off less productive areas on their farms and using them as confinement yards for their sheep in May and June. Their high density baler creates large bales that weigh 500 to 550 kg each from chaff dumps that are knocked down using a telehandler and then raked to save time when baling. Having their own hay-making equipment helps justify the chaff baling operation.
“We have fed 500 ewes in an 8 hectare paddock for six weeks with a diet of chaff bales, regular hay and a lick feeder,” he says. “The benefits are probably marginal for us using wheat chaff, but the canola and pulse chaff is a better feed supplement. The feed quality of the chaff is not always as good as we would like.”
Jarred says that the sheep spread the chaff out in the confinement paddocks and often leave more ground cover in those paddocks than when the sheep went in.
Chaff is a low-cost feed that is only sufficient for maintenance energy levels for sheep. Other feeds need to be included in the feed ration to promote growth or support lactation. Studies that Ed Riggall at AgPro Management conducted at multiple sites in Western Australia over three years demonstrated that sheep with access to chaff heaps from various crops gained an average of 2 kg in the first three weeks. This was 500 g more than sheep grazing stubbles where the chaff was spread out by the harvester.
Livestock containment paddocks boost productivity while stopping the spread of herbicide resistant weed seeds.
At the end of six weeks grazing the sheep with access to chaff piles had gained about 100 g, while sheep without access to the chaff piles had lost almost 2 kg compared to their starting weight. Additional benefits might be expected in a confinement feeding situation where the chaff is easier to forage and is potentially available to the stock for longer as part of a mixed ration.
By making use of the chaff within their own operations these growers are gaining benefit from a resource that is otherwise wasted, and avoid the risks associated with burning chaff heaps.
All harvest weed seed control methods provide similar levels of weed control – collecting, concentrating or destroying over 90 per cent of the weed seed that enters the front of a well set-up harvester. Australian farmers are spoiled for choice when it comes to the options for collecting and destroying weed seed at harvest and keeping downward pressure on herbicide resistance. The WeedSmart HWSC Calculator tool allows growers and agronomists to test different scenarios using their own figures to estimate the costs and benefits of the different systems available.
More information

Download the HSWC costs calculator
More lambs, less weeds in confinement systems
Sheep can turn weed seeds and chaff into cash
Ben & Emily Webb case study

These weeds are now the subject of a pilot ‘area-wide management’ project to trial cooperative and cost-effective methods to reduce the movement of these weeds and the herbicide resistance traits they have evolved.
The cross-industry project has Australian government funding to target weeds that are a common problem to all industries in an area and have ‘mobile’ seed and pollen – that is, they spread easily. Weed species that fit the criterion include flaxleaf fleabane, feathertop Rhodes grass and annual ryegrass.
In three distinct regions – in the Darling Downs region of Queensland, the Riverina region of NSW and the Sunraysia region of Victoria – project teams are devising and implementing area-wide management programs to tackle target weeds of concern in their region.
The University of Adelaide is providing targeted herbicide resistance testing within the pilot areas and mapping the spread of weeds, based on genetic testing conducted at the University of Queensland.
Dr Rick Llewellyn, senior principal research scientist (agricultural systems) with CSIRO is leading the ‘Area wide management for cropping systems weeds’ project to better understand the importance of weed mobility, and test the opportunities for this collaborative approach. He says the idea is to draw together industries and land managers to ‘find a collaborative solution to a common problem’ where a strong value proposition can be established.
Dr Rick Llewellyn, CSIRO, says more coordinated awareness and information sharing can channel effort and innovation into weed management improvements that benefit both the individual land manager as well as the district.
“Area-wide management has been very effective in the management of invasive animal pests and for some mobile insect pests,” says Rick. “We know that some weeds are particularly good at moving across the landscape, either as contaminants or borne on the wind or in flood water – and most farmers have experienced a weed incursion from a source beyond their farm boundary. We are testing collaborative and cost-effective ways to reduce the spread of cropping weeds across diverse farming landscapes.”
Each of the three pilot areas have identified the highest priority mobile weeds to target in their initial on-ground project. In the Sunraysia region the Mallee Sustainable Farming and horticulture organisations have partnered to develop strategies that minimise spray drift while also providing effective control of important weeds like fleabane. Dr Chris Preston, The University of Adelaide, is assisting the Sunraysia project team as they investigate application techniques and product choice for summer weed control in this diverse cropping region.
“Where a range of different crops are grown in close proximity there is a risk of damage through off-site movement of herbicides. To reduce this risk, growers using some products, such as phenoxy herbicides, must work within narrow application windows; but to prevent large populations of weeds setting mobile seeds, growers need cost-effective herbicide options,” says Rick. “The area-wide management trials led by Mallee Sustainable Farming compared weed fallow control efficacy of six alternative products registered for use in optical sprayers, as well as options for better control of mobile and resistance-prone weeds like sow thistle in horticulture.”
The area-wide management trials led by Mallee Sustainable Farming compared weed control efficacy of six alternative products registered for use in optical sprayers, as well as options for better control of mobile and resistance-prone weeds like sow thistle in horticulture. Photo: Frontier Farming Systems.
In the Riverina’s Murrumbidgee Irrigation Area, the focus of the area-wide trials led by the local Irrigation Research and Extension Committee (IREC) is to reduce the movement of weed seed within the irrigation scheme through better management of channel bank vegetation.
Establishment of weed-suppressive grassed channel banks is being tested to assess its feasibility as a weed management strategy that could bring benefits to the many industries that utilise the irrigation scheme. The area-wide activities in the Riverina also attracted extra support from the wine industry with additional trials established to prevent seed set of mobile weeds in vineyards.
The Darling Downs pilot is addressing the management of drainage line and roadside vegetation, particularly where farmers are aiming to maintain weed-free fallows. The project also included the demonstration of new innovations, such as the mechanical weed chipper for control of glyphosate resistant weeds in fallows. The focus weeds in this project are fleabane and feathertop Rhodes grass, both of which can be difficult to control with knockdown herbicides such as glyphosate.
The Darling Downs area-wide weed management project is focused on the management of road-side vegetation, particularly alongside cropping paddocks.
“In each of the pilot areas we have conducted herbicide resistance testing to build a picture of the extent and pattern of resistance across the landscape, on public lands, orchards, vineyards, and on grain and cotton farms,” says Rick. “We have also used genetic testing to map related populations to determine where weeds have come from. For example, we are interested in whether the weeds in the cropping paddocks are related to those found on adjacent roadsides, or did they originate from a distant site.”
Initial findings suggest that there is not usually evidence of ‘resistance fronts’ moving across a district, but rather resistant weed outbreaks are usually scattered. This points to the importance of localised ‘neighbourly’ action to reduce the overall cost of weeds, in addition to broader regional approaches to prevent the introduction and spread of problem weeds.
The WeedSmart Big 6 tactics can be applied to area-wide management as well as within a cropping enterprise to tackle resistance through strategic patch management and diverse control methods that result in low weed densities and prevent seed set of mobile weeds.
Rick says more coordinated awareness and information sharing can channel effort and innovation into weed management improvements that benefit the individual land manager as well as the district.
“There has been an increase in the diversity of food production industries in many districts over recent decades, so there’s more and more opportunity for a collaborative approach to reduce weed costs and risks as ‘new neighbours’ become established in many dryland grain growing areas,” he says.
Research and development partners involved with the project include Grains Research and Development Corporation, Cotton Research and Development Corporation, AgriFutures Australia, CSIRO, University of Queensland, University of Adelaide, University of Wollongong, Mallee Sustainable Farming, Millmerran Landcare Group, Irrigation Research & Extension Committee Inc, together with Wine Australia, the Toowoomba Regional Council and a range of additional local industry organisations.
This project is supported through funding from the Australian Government Department of Agriculture as part of its Rural R&D for Profit program and the Grains Research and Development Corporation and the Cotton Research and Development Corporation.
More resources

Area wide management of weeds project updates
Social attitudes to area-wide management – Preliminary report, Darling Downs

Just as Australia led the way with the development and adoption of ‘green-on-brown’ weed detection and spot spraying in fallow situations, now Australian researchers are developing technologies that will deliver ‘green-on-green’ weed recognition and targeted control in-crop.
Imagine a machine that can identify one weed species from another and apply the best treatment to each weed, even in-crop. While expert human brains can make these differentiations and decisions relatively easily, training artificial intelligence technologies to do the same thing is challenging.
With investment from GRDC, a team of researchers led by Dr Michael Walsh, director weed research at the University of Sydney, have recently completed the pilot phase of crucial work that will underpin future developments for machine learning in weed recognition.
Dr Michael Walsh, director weed research at the University of Sydney, says the WeedAI image database will underpin future developments for machine learning in weed recognition.
Dr Walsh says there are several commercial interests developing machine learning technologies for site-specific weed control in Australia, but they all need access to a collection of relevant images to essentially ‘train’ computers in the development of weed recognition algorithms that can differentiate between crop and weed plants.
“We have set out to establish protocols for collecting and annotating images that will be stored in an open-source database that anyone with commercial or academic interests can contribute to and also use for future developments in this technology,” he says. “The pilot project has centred on collecting images and developing weed recognition algorithms to detect representative grass and weed species in wheat and chickpea crops.”
The WeedAI database currently contains thousands of images of annual ryegrass and turnip weed growing in chickpea and wheat crops. These images have been manually annotated and used to develop and test weed recognition algorithms for their accuracy in correctly identifying weeds growing in-crop.
“The images are all high quality, with annotation outlining the weed shown in the image and notes about the agricultural context, such as soil colour, location, crop type, and growth stages of the crop and or weed,” he says. “We are hoping to fast-track developments and take advantage of the machine learning technologies that have capability to accurately recognise and locate in-crop weeds to ultimately provide growers with the opportunity to specifically target these weeds with a range of weed control options.”
Machine learning offers the potential for high-level accuracy in weed recognition in-crop.
“We are hopeful that this will give growers access to a range of novel chemical and non-chemical weed control technologies that will add to the existing options available for in-crop weed control. This might include herbicides that are currently too expensive for blanket spray application.”
Dr Walsh says Australia is leading the way in developing weed recognition technologies for grain production systems and he believes the open-source database will reduce replication of effort and encourage technology companies to address more challenging scenarios, such as recognition of grass weeds in cereal crops.
Like the optical spray technology that brought tractor-mounted spot spraying to fallow management over 20 years ago, the green-on-green in-crop weed recognition systems in-crop will be used for site-specific weed control in situations where weed density is already quite low.
“At densities of less than one weed per 10 square metres, the area sprayed with herbicides would be 70 to 80 per cent less than when a blanket spray is applied,” says Dr Walsh. “The opportunities to introduce different herbicide modes of action or alternate methods of weed control such as targeted tillage or laser treatment can also be considered to reduce the risk of herbicide resistance.”
Ground speed is the enemy of real-time weed recognition systems, as accuracy increases considerably with speeds slower than those currently used for blanket spraying. With increasing computing processing speeds the expectation is that in-crop weed recognition systems will be accurate at 10 to 15 km/h. The introduction of autonomous platforms is reducing the need for higher speeds, and with a light source there will be the opportunity for round-the-clock operation of weed recognition equipped site-specific weed control systems.
A number of commercial companies are bringing in-crop spot spraying to market and will be on-hand at WeedSmart Week, Esperance to showcase their technology in mid-August. Ben White, Kondinin Group’s research manager will host the machinery session with spray and harvesting gear on display including Goldacres’ G6 Crop Cruiser series 2, weed detection technologies using drones, weed identifying cameras (green on green) and a range of harvest weed seed control options including impact mills from Seed Terminator, Redekop and iHSD (both hydraulic and belt-driven) and the Emar chaff deck. This flagship event always attracts growers keen to see how other farmers are keeping weed numbers low in different systems. Early bird registration is now open.
More resources

WeedSmart podcast – Farmers can now help improve green-on-green technology
Browse the Weed-AI image database
Video of Weed-AI workshop presentations (Day 1)
Video of Weed-AI workshop presentations (Day 2)

The single cause of herbicide resistance in weeds is selection pressure through herbicide use.
Annual ryegrass leads the charge, with resistance to multiple herbicide modes of action, and demands a readjustment in weed control strategies.
Dr Peter Boutsalis of Plant Science Consulting said that the introduction of several new herbicides over recent years has provided options for controlling some resistant populations, particularly for Group 1 [A] and Group 2 [B] resistant ryegrass, but this alone will not halt resistance evolution in ryegrass populations across Australia.
“Simply changing to another mode of action when older chemistry seems less effective is not a long-term solution. Any herbicide has the ability to select for resistance, especially in a genetically diverse species such as ryegrass,” he said. “The strategy needs to centre on increasing diversity in herbicides and non-herbicide tools, not just switching from an ‘old’ herbicide to a ‘new’ one.”
In 2020, Dr Peter Boutsalis, Plant Science Consulting, was sent 83 ryegrass samples from concerned growers in NSW and the Quick Test showed 79% of individual plants that survived paddock treatments were in fact resistant to glyphosate.
The Grains Research and Development Corporation has invested in random weed surveys in different regions within New South Wales each year from 2015 to 2019. These surveys have identified differences in the pattern of resistance between regions and other states but the trend toward multiple resistance mechanisms and resistance to increasing application rates is undeniable.
Dr John Broster, Charles Sturt University said the majority of annual ryegrass populations in NSW are resistant to Group 1 [A] ‘fop’ and Group 2 [B] herbicides with some variability between the surveyed sub-regions.
The random surveys conducted in NSW from 2015 to 2019 involved the collection and testing of 608 ryegrass populations by researchers from Charles Sturt University.
To date, no populations have been found that are resistant to the newer pre-emergent herbicides, however resistance has been reported in other states.
“Of particular concern is the percentage of ryegrass populations sampled in the random survey in some sub-regions that are resistant to glyphosate,” he said. “The extent of resistance in some areas was brought home strongly in the 2020 season when many growers were confronted with significant patches of ryegrass that clearly escaped pre-seeding glyphosate applications.”
The random surveys conducted in NSW from 2015 to 2019 involved the collection and testing of 608 ryegrass populations, with the results showing 5% of these populations were resistant to glyphosate. The highest level of resistance so far was found in the 2019 results from the eastern NSW region alone, where 14% of populations were resistant to glyphosate. A population is considered resistant to a herbicide when more than 20% of the plants grown from seed collected at a single site survive applications of registered rates of the herbicide in question.
In addition to the random sampling to provide the ‘big picture’ of resistance extent, Dr Boutsalis also conducts Quick Tests when growers and agronomists experience an apparent herbicide failure. In 2020, he was sent 83 ryegrass samples from concerned growers in NSW and the Quick Test showed 79% of individual plants that survived paddock treatments were in fact resistant to glyphosate.
“This suggested that although glyphosate resistance is generally a significant contributing factor to weeds ‘escaping’ herbicide treatment in the paddock, there are potentially other forces involved as well,” said Dr Boutsalis. “Poor application technique or application onto stressed plants, incorrect timing, sampling plants that were not exposed to glyphosate, antagonistic tank mixes, inferior glyphosate formulation, poor water quality, incorrect adjuvants, or a combination of these can also result in poor weed control in the field.”
“To keep any herbicide as a long-term option it is essential that high quality products are applied correctly and that survivor plants are prevented from setting seed,” he said. “Switching products is a very short term and inadequate solution. A better strategy is to implement a diverse program of both herbicide and non-herbicide tactics and be diligent about keeping weed numbers low.”
Other than confirming resistance, herbicide testing is a powerful way to identify modes of action that a resistant population is still susceptible to. Growers who are confronted with patches of ‘survivor’ weeds this season can send live plant samples in for the Quick Test to identify herbicide options that could be used to prevent seed set in the current season. If the escapes are not seen until seed has set, seed can be collected and sent to either CSU or Plant Science Consulting for testing against a wider range of herbicides, including pre-emergent herbicides.
Testing of ‘suspect’ seed samples sent to CSU last year resulted in 30% of populations testing positive to glyphosate resistance.
Back row = glyphosate resistant biotype, Front row = susceptibleLeft to right is 1.5 L/ha, 3 L/ha, 4 L/ha Glyphosate 540.
Patch management strategies such as cutting for hay, spraying out with paraquat, or chipping can be very effective in containing a potential blow-out. The WeedSmart Big 6 strategies for integrated weed management can then be implemented to apply long-term downward pressure on weed numbers.
The WeedSmart Big 6 tactics will be the centre of discussion at WeedSmart Week in Esperance, WA in August this year. This flagship event always attracts growers from interstate keen to see how other farmers are keeping weed numbers low in different systems. Early bird registration is now open.
Resources

Causes of poor ryegrass results and paraquat and glyphosate resistance 2020 season
Resistance and susceptibility testing

Ranked as the third most costly weed in Australian grain cropping, three weedy Avena spp. – wild oat, sterile oat and slender oat – are estimated to infest over two million hectares, causing crop yield losses of 114,596 t and a national revenue loss of $28.1 million.
In the southern and western regions, the main species found is wild oats (A. fatua), while in the northern region, sterile oat (A. sterilis ssp. ludoviciana) is the more problematic species. Both have evolved resistance to multiple herbicide groups in Australia.

QAAFI weed researchers Gulshan Mahajan and Bhagirath Chauhan have recently published a series of papers on their weed ecology studies of Avena spp., providing growers and agronomists with more information to use when formulating integrated management plans for these weeds in crops.
Practical tips

Both wild oat and sterile oat can survive in soil moisture conditions of 60 per cent water holding capacity (WHC). Sterile oat even produced seed at 40 per cent WHC.
Seedlings of these weeds can emerge from a depth of 10 cm, but greater emergence occurred from 2 and 5 cm depths. Emergence commenced at the start of winter (May) and continued until spring (October).
Early emergence plants produce the most seed, but later emergence plants can still produce enough seed to support reinfestation.
In a no-till system there is low persistence of seed on the soil surface. A 2-year assault on the weed seed bank can result in complete control of infestations.
Weed density of 15 wild oat and 16 sterile oat plants/m2 resulted in a 50 per cent reduction in wheat yield. Lower weed density (just 3 plants/m2) can still support reinfestation.
Sterile oat is a better candidate than wild oat for harvest weed seed control (HWSC).
Wild oat is best managed through early weed control (pre and post sowing) and strong crop competition.
An integrated approach to weed management can reduce Avena weed biomass by up to 90 per cent.

Experimental design features
We are summarising the finding from four related research papers:

Biological traits of six sterile oat biotypes in response to planting time. https://doi.org/10.1002/agj2.20507
Influence of soil moisture levels on the growth and reproductive behaviour of Avena fatua and Avena ludoviciana. https://doi.org/10.1371/journal.pone.0234648
Seed longevity and seedling emergence behaviour of wild oat (Avena fatua) and sterile oat (Avena sterilis ludoviciana) in response to burial depth in eastern Australia. https://doi.org/10.1017/wsc.2021.7
Interference of wild oats (Avena fatua) and sterile oats (Avena sterilis ludoviciana) in wheat. https://doi.org/10.1017/wsc.2021.25

Detailed findings
Sterile oats growth and seed production for early and late emergence cohorts
Six biotypes of sterile oats were collected from sites in southern Qld and northern NSW and planted in field conditions at the Gatton research farm in the winter cropping seasons of 2018 and 2019. The weed seed was sown early, mid and late season and the growth and reproductive potential of the six biotypes was monitored.
Averaged across the biotypes, the early planted weeds produced 2660 seeds/plant. Weeds sow mid-season produced 21 per cent less seed and the late-season weeds produced 84 per cent less seed than the early-season plants.
Although seed production was more prolific from the early and mid season plants, the late season plants produced sufficient seed to support reinfestation the following season.
A clean seed bed and competitive crop environment is the best strategy to suppress sterile oat seed production.

Effect of moisture stress on biomass and seed production of wild oats and sterile oats
Seeds of wild oat and sterile oat used in this study were collected from Warialda, NSW, in October 2017 and multiplied at the University of Queensland, Gatton Research Farm in the winter season of 2018. The pot trial to investigate the effect of 20, 40, 60, 80 and 100 per cent water holding capacity (WHC) on these two Avena weed species was conducted in 2019.
Results revealed that wild oat did not survive, and failed to produce seeds, at 20 and 40 per cent WHC. However, sterile oat survived at 40 per cent WHC and produced 54 seeds/plant, suggesting that this species is likely to compete strongly with crops in water stressed situations.
In favourable moisture conditions, both species will produce copious quantities of seed, suggesting that high infestation rates for both species may be a risk in irrigated crops.

Effect of seed burial on emergence, growth and persistence of wild oats and sterile oats
The seed longevity and emergence pattern of wild oat and sterile oat were monitored in field conditions at Gatton, Narrabri and St. George. Fresh weed seed was placed into nylon bags and buried at depths of 0, 2 and 10 cm in November 2017. Bags were exhumed at 6-month intervals over 30-months to evaluate seed germination, viability and decay.
For both species, 50 per cent of seeds at the surface and 10 cm depth had decayed within the first six months. Shallow burial (2 cm depth) of the seed increased persistence, with a significant percentage of seed being viable in the following winter cropping season.
The largest cohort of both species began to emerge at the start of the winter season (May). To ensure the seed bed is clean prior to planting, consider using tillage, herbicide application and cover crops to control this early cohort of Avena weeds. Tillage will bury seeds below their maximum depth of emergence and subsequent tillage should not be performed for 3–4 years to avoid bringing seeds back to the ‘emergence’ depth. Later emerging cohorts (through to October) will be suppressed using strong crop competition or a winter fallow if the infestation is severe.
The results of this research suggest that management strategies that can control all emerged seedlings over two years and restrict seed rain in the field could lead to complete control of weedy Avena spp. in the field.

Effect of wild oats and sterile oats infestation on wheat yield
The interference of wild oat and sterile oat in a wheat crop was examined through field studies in 2019 and 2020 at Gatton, Qld. Infestation levels of 0, 3, 6, 12, 24 and 48 plants m2 of both weed species were evaluated for their impact on wheat yield.
At an infestation level of 15 and 16 plants per m2 for wild oats and sterile oats respectively, wheat yield was halved as a result of reduced spike number per m2.
At the highest weed infestation level (48 plants per m2), wild oat and sterile oat produced a maximum of 4800 and 3970 seeds per m2, respectively. At wheat harvest, wild oat exhibited lower seed retention (17 to 39 per cent) than sterile oat (64 to 80 per cent), with most of the wild oat seeds having fallen from the seed heads before crop maturity.
The results of this study suggest that harvest weed seed control is likely to be a useful tactic in paddocks infested with sterile oat. An integrated weed management strategy that uses both chemical and nonchemical tactics is required to avoid severe crop yield loss, increased weed seed production and weed seedbank replenishment when these weed species are present.
This body of research highlights the benefits of an integrated weed management program that takes the ecology of the target weed into account.

This research was conducted by researchers from the University of Queensland, a WeedSmart scientific partner, with investment from the Grains Research and Development Corporation a WeedSmart sponsor.
Research papers

Mahajan, G., & Chauhan, B. (2021). Biological traits of six sterile oat biotypes in response to planting time. Agronomy Journal,113: 42-51 https://doi.org/10.1002/agj2.20507
Sahil , Mahajan G, Loura D, Raymont K, Chauhan BS (2020). Influence of soil moisture levels on the growth and reproductive behaviour of Avena fatua and Avena ludoviciana. PLoS ONE 15 (7): e0234648. https://doi.org/10.1371/journal.pone.0234648
Mahajan, G., & Chauhan, B. (2021). Seed longevity and seedling emergence behavior of wild oat (Avena fatua) and sterile oat (Avena sterilis ludoviciana) in response to burial depth in eastern Australia. Weed Science, 1-10. https://doi.org/10.1017/wsc.2021.7
Mahajan, G., & Chauhan, B. (2021). Interference of Wild Oats (Avena fatua) and Sterile Oats [Avena sterilis ssp. ludoviciana (Durieu)] in Wheat. Weed Science, 1-20.  https://doi.org/10.1017/wsc.2021.25

Growers and agronomists in each region and on each farm can adapt the WeedSmart Big 6 principles to bring more diversity to their farming system and bamboozle weeds.
Each year growers and agronomists are invited to attend WeedSmart Week, somewhere in Australia. This year the 3-day event will be held in Esperance, WA, beginning with a 1-day forum at the Civic Centre on Tuesday 17 August. The following two days will be spent touring farms in the Esperance region to see how growers are implementing the WeedSmart Big 6 tactics to minimise the impact of herbicide resistance on their businesses. The WeedSmart Week theme, ‘Diversify and Disrupt – Use the BIG 6 to beat crop weeds’, says it all!
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Program leader, Lisa Mayer says the first WeedSmart Week event was held in Perth in 2016 and it’s now a highly anticipated annual event hosted by the WeedSmart program. Having now been held in Queensland, New South Wales, Victoria and South Australia over the last five years, this year sees the flagship event returning to Western Australia. WeedSmart Week is supported by the GRDC as the major sponsor and a wide range of herbicide and machinery companies – all with skin in the weed control game. This will be the seventh WeedSmart Week event.

“The herbicide and non-herbicide tactics that form the WeedSmart Big 6 have been researched and demonstrated in the field – we know they work,” said Ms Mayer. “Low weed seed banks underpin all profitable farming enterprises. Keeping weed numbers low and quickly regaining control of blow-outs is the sole purpose of the WeedSmart program.”
WeedSmart is committed to exploring and promoting farming systems and technologies that produce ‘more yield, fewer weeds’ every year.
WeedSmart Week brings together a wealth of knowledge and experience from local and inter-state growers, researchers, advisors and technology experts – putting the spotlight on herbicide resistance and weed management. Growers can see first hand what is and isn’t working and consider how key principles can be applied directly to their own farming operation.
At the forum and on the bus trip growers, agronomists and researchers put all the options and ideas on the table for discussion. Greg Warren from Farm and General in Esperance is one of the local agronomists assisting with the planning for 2021 WeedSmart Week. As one of the forum speakers Greg will be sharing his thoughts on the control of weeds like summer-germinating ryegrass, marshmallow, fleabane and portulaca.
He says the growers around Esperance are tackling glyphosate resistance in annual ryegrass, along with brome and barley grass and other emerging weeds using a range of integrated control tactics.
“We know we can’t take the foot off the pedal when it comes to weed control,” he says. “Growers are always assessing their options and making decisions based on good science and demonstrated benefits – and that’s what events like WeedSmart Week bring to a district.”
Greg is encouraging local growers to register their interest early and is keen to welcome growers from other regions and inter-state to look, learn and discuss tactics that work.
There will be a focus on both herbicide and non-herbicide tools and plenty of chances to see how mechanical tactics like harvest weed seed control can fit into a variety of farming systems to drive down weed numbers.
The growers, agronomists and researchers speaking and participating in expert panels at the Day 1 forum will spark important discussions about herbicide resistance and how the Big 6 tactics can be used to target the weed species and farming systems of the high rainfall zones of southern and western Australia. There’s one thing for sure – doing nothing is not an option.
Day 2 and 3 will be bus tours to farms in the Scaddan and Howick areas surrounding Esperance. The bus trips will highlight how growers in the region are implementing the Big 6 weed management tactics in a variety of farming systems and environments.
This year, Ben White, Kondinin Group’s research manager will host the very popular technology and machinery field demo, where attendees will have the opportunity to see and discuss cutting-edge innovations such as the latest sprayer and weed detection technology and a range of harvest weed seed control implements, including impact mills and chaff decks.
Register for this important 3-day event for the ‘early bird’ single ticket price of $190 (GST incl), guaranteeing a seat on both the bus tour days as well as the forum, all fully catered. Early bird price is available until 31 July, 2021.
WeedSmart is committed to the health, safety and well-being of everyone working in, and in support of, the Australian grains industry. WeedSmart Week may be postponed in response to any coronavirus outbreak, and will be held in accordance with Australian Government advice in relation to social distancing.

Weeds can exploit situations where crops fail to germinate or grow less vigorously. This does not usually mean that the weeds prefer soils that have constraints such as acidity, compaction or low nutrition status.
While crop responses to changes in soil pH are extensively researched, there is far less research available that quantifies the impact of the amelioration of soil acidity on weed growth.
Gauz Azam and Catherine Borger
To help fill this knowledge gap, research scientists Catherine Borger, Gaus Azam, Chris Gazey, Andrew van Burgel and Craig Scanlan from the Department of Primary Industries and Regional Development, Western Australia (DPIRD), have recently published the results from long-term studies measuring the impact of ameliorating soil acidity on the growth of annual (rigid) ryegrass (Lolium rigidum) in wheat.

Practical tips:

In acidic soils, the application of lime increases soil pH and improves the crop’s competitive ability against annual ryegrass.
Lime applications increase initial growth of both wheat and ryegrass.
The application of lime in previous years reduced ryegrass density, biomass, and seed production in wheat crops in 2018.
Lime increased wheat tiller number and, at one location, increased yield.
Crop and weed establishment may be poor in the season following soil amelioration. The crop often ‘catches up’ later in the season.
Reacidification is common. An ongoing liming program is likely to be required to maintain the competitive edge of crops over weeds such as annual ryegrass.

Most crops and pastures grow best in soils with a pH between 5.5 and 8, but some crops, such as barley, are more sensitive to soil pH than others. Similarly, some weeds are able to grow in hostile environments but will often grow better when the pH is in the optimal range for crop growth. For example, annual ryegrass competes very strongly with wheat in low pH soils, but actually grows best in the same pH range as crops. On the other hand, there is some evidence that wild radish prefers acidic soils.
Identifying soil constraints can involve detailed investigations and there are commonly multiple constraints at play. With approximately half of the agricultural soils in Australia having a surface pH of 5.5 or less, this constraint alone can be responsible for significant yield loss. Conversely, South Australian farmers are more likely to have to contend with high pH soil constraints, with 60 per cent of agricultural soils in that state being highly alkaline.
Experimental design features
These experiments were conducted at field sites in the Merredin and Wongan Hills shires in Western Australia. The scope of this research included two experiments:

A field experiment was conducted from 2016 to 2018 at DPIRD’s Merredin Research Facility on naturally acidic soil to investigate the effect of crop rotation (continuous wheat and wheat–chemical fallow), lime incorporation (nil and to 15 cm) and lime rate (0, 2, 4 and 6 t/ha). Wheat and annual ryegrass production was measured in the 2018 season.
A field experiment at DPIRD’s Wongan Hills Research Facility was established in 1994 on soil with low pH as a result of agricultural practices. The trial investigated the long-term effect of lime rate (0, 0.5, 1, 2 and 4 t/ha applied in 1994) and top-up applications of 0 or 1.5 t/ha in 1998 and 0 or 3 t/ha in 2014. In 2018 soil was cultivated to a depth of 0, 15 or 25 cm prior to seeding. Wheat and annual ryegrass production was measured in the 2018 season.

Detailed findings
Crop rotation and lime at Merredin
Within the continuous wheat rotation at Merredin, increasing rates of lime increased surface soil pH (0–5 cm) from 4.9 to 6.0 and pH at depth (10–15 cm) from 4.3 to 4.7 with no incorporation. Increasing rates of lime reduced density, biomass, and seed production of ryegrass and increased wheat tiller number and yield.
Incorporation of lime had no significant effect on wheat yield or ryegrass biomass, even though incorporation increased pH at depth (10–15 cm) from 4.2 to 5.1.
A wheat-fallow rotation reduced ryegrass density, biomass and seed production and increased yield compared to the continuous wheat system. Lime rate and incorporation within the wheat-fallow system increased soil pH (0–5 cm) from 4.9 to 5.8, but had no effect on ryegrass due to uniformly low weed pressure. Fallowing is a very effective weed control measure, but is unlikely to be a profitable option unless weed pressure is very high.
Long-term effects of lime application at Wongan Hills
Cumulative lime application at the Wongan Hills site increased soil pH from 5.6 to 6.4 (0–10 cm), 4.6 to 5.4 (10–20 cm), and 4.1 to 4.9 (20–30 cm).
Lime applications in 1994 and 2014 had long-lasting impact on weed growth, resulting in reduced ryegrass density, biomass and seed production in the 2018 crop. The lower rates applied in 1998 had no significant impact on ryegrass density and seed production.

Wheat density was not affected by lime, but tiller number increased with increasing rates of lime applied in 1994 and 2014. The slight increase to wheat yield following application of lime was not significant and incorporation of lime in 2018 did not affect ryegrass or wheat production.
Deep tillage increased pH at depth (20–30 cm) from 4.2 to 5.2. The interaction between lime application in 2014 and incorporation of lime in 2018 was significant for ryegrass, with weed density, biomass and seed production decreasing with increasing depth of tillage in those plots where lime was not applied in 2014 (0 t/ha treatment). Deep tillage did not significantly affect ryegrass in plots where 3 t/ha of lime was applied in 2014, as ryegrass density was already very low across all tillage treatments. By 2018, the lime applied in 2014 had already done the heavy lifting in terms of reducing weed pressure in the 3 t/ha plots.

WeedSmart conclusion
Applying and incorporating lime is the best way to increase the pH of acidic soils, but it usually takes several years before a surface lime application has a measurable effect on soil pH at depth. Incorporation is the best way to speed up the process and also releases other soil nutrients to boost crop growth.
By default, the incorporation of lime by tillage or inversion also buries weed seed, placing at least a portion of the seed bank deeper in the soil profile and prohibiting germination. Annual ryegrass seed has optimal emergence from a depth of 1 or 2 cm. Emergence reduces with increasing depth and ryegrass does not emerge from depths of 10 cm or more. Even when buried, some seed can remain viable and emerge if the next sowing operation brings the seed back near the soil surface.
This trial work confirms the importance of crop competition in a diverse weed control program. Addressing soil constraints, such as low pH (and the associated aluminium toxicity), enables the crop to compete strongly with weeds such as annual ryegrass – reducing weed growth and seed production.

This research was conducted by researchers from the Department of Primary Industries and Regional Development, Western Australia and was supported by the Grains Research and Development Corporation, a WeedSmart financial partner, through the Soil Constraints Initiative—Innovative Approaches to Managing Subsoil Acidity (DAW00252) project.
References
Borger CPD, Azam G, Gazey C, van Burgel A, Scanlan CA (2020) Ameliorating soil acidity–reduced growth of rigid ryegrass (Lolium rigidum) in wheat. Weed Sci. 68: 426–433. doi: 10.1017/wsc.2020.38

Having completed a two-year demonstration of chaff decks with investment from the GRDC, Tim, along with cropping officers from adjacent LLS regions, are capitalising on the interest in integrated weed management tactics to counter the insidious rise of herbicide resistance in weeds.
“Annual ryegrass is one of the main weeds causing growers concern in-crop,” he said. “There is known resistance to Group 1 [A] and 2 [B] herbicides, and there are strong indications that glyphosate resistance is evolving on some farms.”
Tim Bartimote, Local Land Services (LLS) in Dubbo, says many grain growers in the Central West region of NSW are keen to see the benefits of integrated weed management tactics demonstrated in their area.
Harvest weed seed control has been commonly practiced in the region for many years, primarily as narrow windrow burning or simply broadacre stubble burning. Tim says there is a definite shift in interest toward technologies such as impact mills, although the price of these machines is a barrier to immediate and wide-spread adoption.
“Through discussions with grower groups we found that a few growers had moved into using chaff decks and chaff-lining, but these options were not well-known to others in the area,” says Tim. “We decided to demonstrate chaff decks, which are less expensive than impact mills and are well-suited to the controlled traffic systems used on a few properties in the region.”
The two growers who demonstrated the use of chaff deck systems both identified resistant ryegrass as their main weed target for harvest weed seed control.
“At the demonstration site at Parkes, the ryegrass population was evenly spread across the paddock at a density of 26 plants per metre square,” says Tim. “For the purposes of monitoring the effect of the chaff deck operation, we chose four sites within the paddock and found 4, 19 and 9 plants per m2 away from the wheeltracks and 68 plants per m2 on the wheeltracks.”
“This clearly demonstrated the shift of ryegrass seed from being spread across the paddock to being concentrated on the wheeltracks where seedlings can be controlled with other tactics as required.”
Chaff decks help concentrate the weed seed onto the wheel tracks during harvest.
At the second site, near Gilgandra, the weed population was found concentrated in patches. Tim and the grower, Daniel Volkofsky, GPS-marked sites within the paddock following the 2020 harvest and will monitor the shift in weed density over the next few years.
The growers used both commercial and home-made chaff deck systems in the demonstrations and found both options were effective. In addition to the traditional use of HWSC in winter crops, Daniel also tried using his chaff decks in a sorghum crop but ran into trouble with blockages on the leading edge of the baffle plate. Some growers have added cameras to help monitor stubble flow over the baffle plate and pre-empt blockages.
Tim says the LLS team wants to achieve a ‘weed management legacy’ from the investment of GRDC funds in the region.
“One of the outcomes of the GRDC-funded project was to build a network of growers with experience using different tactics in their integrated weed management programs,” says Tim. “We are now able to direct interested growers to speak to and visit growers in their region who can talk to them about what they have tried and what has worked well for them.”
“Some of the growers who have manufactured various harvest weed seed control devices on farm are willing to share their low-cost designs with others who are not ready to invest in the commercial models. There is also a pool of experience when it comes to the modifications to baffles and chutes required for different header makes and models.”
HWSC is one of the WeedSmart Big 6 tactics that under pin integrated weed management programs across Australia. Within each of the tactics growers are implementing a range of different methods that suit their own systems to keep weed numbers low.
Tim says they are capitalising on the interest generated through the project to now test and compare the efficacy of a range of pre-emergent herbicides on the market.
Resources:

WeedSmart podcast with Tim Bartimote

Glyphosate has been the go-to product for keeping weeds in these areas under control for a long time but unfortunately it is often the only product used and the weeds are commonly quite large when they are sprayed. The result is that glyphosate resistance can, and does, quietly build up in these zones in a wide variety of weed species.
Fencelines will always be a potential source of weed seed but there are ways to ensure that the seed from these areas is not already resistant to the herbicides when it blows into the production areas.
Farmanco agronomist, Brent Pritchard, collected the suspect capeweed samples on a farm near Borden in Western Australia. The capeweed had evolved resistance to glyphosate in an un-cropped drainage area, where it had routinely been sprayed with glyphosate, and had then invaded the adjacent field. The cropped area had been managed with a diverse rotation of wheat, TT canola, pasture and fallow over a 17-year period.
The capeweed samples also showed signs of resistance to metosulam (Eclipse®) and diflufenican (Brodal®), but were susceptible to a range of other herbicides including clopyralid, MCPA, bromoxynil, diuron, metribuzin, simazine, Spray.Seed® and Velocity®.
Dr Yaseen Khalil, a researcher in the agronomy team at the Australian Herbicide Resistance Initiative (AHRI), conducted the resistance screening and confirmed the resistance status of the capeweed population.
AHRI’s Dr Yaseen Khalil confirmed the resistance status of the capeweed samples and is urging growers to take a more diverse approach to weed management in non-cropped areas around the farm
“There is no doubt that an integrated approach to weed management needs to be applied to non-production areas such as fencelines, around buildings, along tracks and roads and around irrigation infrastructure,” says Dr Khalil.
“Probably the first step is to stop using glyphosate alone in these areas unless you are able to reliably apply a double knock to every application. Evolving resistance to this useful herbicide in non-productive zones is counter productive at the least.”
Wherever possible, apply glyphosate in a mix with other herbicides effective on the target weeds, then follow with a second knock.
The main problem on fencelines is the lack of competition to weeds. If pastures are part of the crop rotation it may be possible to establish the pasture species along the fenceline and leave them in place when the paddock returns to the cropping phase. Similarly, the crop can often be sown right up to the fence and the first round or two mown or baled for hay prior to harvest. If there are livestock in the production system they can be used to graze the perimeter in the fallow or in suitable crops.
Mowing or baling the perimeter of the crop can halt the incursion of weeds into the crop area.
Establishing cover using desirable perennial species and eliminating fenceline spraying could be a long-term solution to stop fencelines being a source of herbicide resistant weeds.
If this is not practical, or if the non-crop area must be kept bare for other reasons, such as managing insect pests, close attention must be paid to using alternative herbicides, double knocking, mixing and rotating herbicides and eliminating survivors.
Applying the WeedSmart Big 6 tactics to non-crop areas is a pre-emptive strike on ‘home-grown’ herbicide resistance.
More resources:

AHRI Insight – World-first: glyphosate resistant capeweed
Management of herbicide resistant weeds on fencelines
Don’t jeopardise glyphosate for clean fencelines

You can listen to the article being read above!

We all know that old saying – ‘one year seeding, seven years weeding’ or some variant of it, and know it is true. But it is easy to overlook just how important weed seedbank management is, until herbicide resistance begins to reduce the efficacy of previously reliable tools.
For a few decades herbicides really took the focus away from seedbank management because the chemical options were so effective at killing weeds that they appeared to be a complete solution to weed management.
But all along, growers, agronomists and researchers have known it was too good to last. The WeedSmart Big 6 strategy has struck a chord because it is a useful check list that can be used to prompt growers to consider using a selection of the many available weed control tools.

No one tool will do the job – just as herbicides alone have failed, so too will harvest weed seed control or crop competition if they are not part of a planned and multi-pronged assault on the weed seedbank. This is the underlying principle for integrated weed management.
In economic simulations conducted using the RIM and WeedRisk models in 2006, agricultural economists Randall Jones and Marta Monjardino showed that although many things impact on the economic assessment of weed management practices, there is strong evidence that when seasonal risk is taken into account, and the economic assessment is for a period of 20 years, integrated weed management consistently out-performs herbicide-only systems, regardless of the weed in question.
Herbicides provide high level control and are considered an essential component of broadacre cropping systems, however, other tactics that specifically target weeds that have escaped herbicide control are what make IWM systems more profitable in the long-run (see Table 1).
For weeds like wild radish, which produce large quantities of seed that can remain viable in the soil for many years, taking a non-integrated approach of using post-emergent herbicide only has the potential to ‘crash the system’, from an economic point of view.
It will always be a numbers game and IWM consistently wins, usually by a considerable margin, primarily due to lower weed seedbank numbers and conservation of the highly effective herbicide resource for tactical use over time in integrated weed management systems.
TABLE 1 The economic impact ($/ha) of different crop and IWM systems on meana annualised discounted returns for wild oats, wild radish and annual ryegrass in a southern New South Wales cropping system (4-year crop phase followed by 3-year perennial pasture phase).

Economic return ($/ha)a

Wild oats

Wild radish

Annual ryegrass

Continuous cropping
 
 
 

No IWM

268 (± 35)

-9 (± 27)

284 (± 34)

IWM

332 (± 38)

315 (± 37)

335 (± 38)

Crop + pasture rotation

No IWM

288 (± 29)

157 (± 25)

284 (± 28)

IWM

319 (± 32)

300 (± 30)

320 (± 31)

a The shown in brackets following ± are the standard deviation.
Source: Jones R, Monjardino M and Asaduzzaman Md (contributors) (2019). Section 1: Economic Benefits of Integrated Weed Management, in: A.L. Preston (Ed) 2019. Integrated weed management in Australian cropping systems. Grains Research and Development Corporation.
Use the WeedSmart Big 6 to prepare an IWM plan for your farm
To develop an integrated weed management plan (IWM), it is useful to collate some historical information about past weed control activities, test weeds for herbicide resistance and use the WeedSmart Big 6 to match opportunities and weeds with suitable and effective control tactics, remembering that there are many weed control tools at your disposal.
With your agronomist’s assistance, aim to create a plan that maps out when each tactic will be applied. Ideally, try to include three or more of the Big 6 tactics in each crop, fallow or pasture phase.
Diversity is key. Some people prefer to have a set cropping sequence while others choose the crops in response to seasonal or market conditions, but either way it is important to look for ways to add as much diversity to your farming system as possible and to keep downward pressure on weed numbers at every opportunity.
While preventing weed seed production completely is unrealistic in the real world, a focus on the weed seedbank will pay dividends in the long run.

You can listen to this article being read above!

WeedSmart extension agronomist, Peter Newman is urging growers to think twice before mixing glyphosate with 2,4-D when treating weeds with low to medium levels of resistance to glyphosate.
Antagonism between the two products has been widely known for some time and in most circumstances careful product choice and a slight rate adjustment is all that is required to compensate for the compromised performance of glyphosate.
New research from the Australian Herbicide Resistance Initiative (AHRI) has shown that high label rates of glyphosate can often control grass weeds with low level glyphosate resistance; but adding 2,4-D amine or ester to the glyphosate can result in these weeds surviving the spray.
Once low to medium glyphosate resistance has established in a population of awnless barnyard grass, using a tank mix of glyphosate and 2,4-D is not going to work.
With investment from the Grains Research and Development Corporation and others, AHRI researcher Jingbo Li and others studied two populations of awnless barnyard grass with relatively low-level glyphosate resistance and found this phenomenon is due to the 2,4-D dramatically affecting uptake and translocation of glyphosate from the tank mix.
“For awnless barnyard grass a susceptible population was 100 per cent controlled using 0.5 L/ha glyphosate 540 while 11 per cent survived when the same rate of glyphosate was mixed with 1 L/ha of 2,4-D amine 700,” says Peter. “For the low-level resistant population, a higher rate of 1 L/ha of glyphosate was required to achieve 100 per cent control but when this rate of glyphosate was mixed with the 1 L/ha of 2,4-D amine, 90 per cent of the weeds survived. A similar result was found using 2,4-D ester.”
Survival of awnless barnyard grass seedlings with low level resistance to glyphosate. Left: Zero survival from 1 L/ha Glyphosate 540 application. Right: 85 per cent survival to 1 L/ha Glyphosate 540 + 1.03 L/ha 2,4-D Ester 680 mixture.
In another, more resistant, population of awnless barnyard grass the same scenario played out, albeit with an even higher rate of 3 L/ha of glyphosate to achieve 100 per cent control. In this population the survival rate was 77 per cent for the tank mix.
“What this means for growers is that once glyphosate resistance has established in a grass weed population, using this particular tank mix is not going to work,” says Peter. “A grower with glyphosate resistant grass weeds would be better served by applying the higher rate of glyphosate on its own, or perhaps with a different mixing partner, to achieve maximum control. It is then necessary to look at building in additional tactics to keep weed numbers low into the future with less reliance on glyphosate.”
While not examined in this study, 2,4-D antagonism of glyphosate is reported on several other species including Johnson grass, wheat, barley and wild oats. 2,4-D is also reported as antagonistic of Group A herbicides on species such as wild oats and annual ryegrass. Although mixing these two herbicides can provide a valuable multi-shot control of both grass and broadleaf weeds, the pros and cons need to be carefully evaluated.
“The other thing to remember is that the maximum level of control when using glyphosate is achieved when the best formulation is applied to young weeds at higher label rates,” says Peter. “These factors are generally within the grower’s control, even if they cannot control the weather conditions or plant stress levels, which also impact on glyphosate efficacy.”
To keep glyphosate as a viable option into the future Peter also recommends applying a double knock tactic after each application of glyphosate. He says following glyphosate with paraquat has been an effective double knock for many years but there are other options to consider, including strategic tillage and alternative herbicides.
Other resources

AHRI Insight: 2,4-D antagonises glyphosate, especially in glyphosate resistant weeds

Listen to the article being read above!

The common practice of planting sorghum on wide rows has made this crop notorious as a weak link crop that can allow key summer grass species to set bucket loads of seed.
The take home message from four years of research at Narrabri, NSW and Hermitage, Qld, is that halving sorghum row spacing can halve weed seed production in both feathertop Rhodes grass and awnless barnyard grass.
With investment from the GRDC, researchers from the University of Sydney and Queensland Department of Agriculture and Fisheries (DAF) have conducted a range of field trials to identify ways to increase the competitiveness of sorghum and summer pulses.
Dr Michael Widderick, DAF principal research scientist says the findings from these field trials have shown that a change to narrower row spacing for sorghum greatly suppressed weed growth and seed production, without reducing crop yield.
“This is very significant for sorghum growers who have struggled with controlling these grass weeds in wide-row configurations,” he said. “These weeds are difficult to control with herbicides and there are few chemical options available to growers to control grasses in a grass crop. Any non-chemical strategies that reduce seedbank replenishment are very valuable to growers.”
Dr Widderick said sorghum is often grown on one metre row spacing with an expectation that the crop will have access to more soil moisture. A considerable downside to planting on the wider row configuration is that canopy closure does not occur, allowing weeds to proliferate in the inter-row.
With soil moisture at a premium, there is nothing spare to waste on growing summer weeds. For a sorghum crop to be competitive against weeds it requires adequate stored soil moisture (or access to irrigation) to establish the crop and achieve canopy closure as quickly as possible. This is most reliably done at a row spacing of 50 cm and this trial demonstrated that row spacing did not significantly impact crop yield within a season.
 
Left: weeds growing uninhibited in the inter-row space of sorghum sown at 1 m row spacing. Right: Fewer weeds can establish when the canopy closes in sorghum sown at 50 cm row spacing.
Two of the most difficult to control summer weeds, feathertop Rhodes grass (FTR) and awnless barnyard grass (ABG) can produce 40,000 and 42,000, seeds per plant respectively. Other studies have found these numbers could be even higher, so every effort to reduce seed production is worthwhile.
Both these species have populations confirmed as resistant to glyphosate, and recently a population of FTR was confirmed to be resistant to haloxyfop (Group A). Including a poorly competitive sorghum crop in the crop rotation provides a weak link in any strategy to reduce the weed seedbank for these weeds, and potentially allows a blow-out in herbicide resistant biotypes, making future control in other crops or summer fallows very difficult.
Dr Widderick said sorghum competitiveness across all seasons and both sites was increased with narrow row spacing (50 cm) and a plant density of 10 to 15 plants/m2.
In the 2017/18 season at Hermitage, the researchers demonstrated that planting sorghum at a density of 10 to 15 plants/m2 reduced seed production of both weed species reduced by over 50 per cent compared to the seed production at the low crop density of 5 plants/m2. In the same season, cultivar choice, sorghum density (5, 10, 15 plants/m2) and row spacing (50, 75 and 100 cm) had no statistically significant effect on crop yield.
Armed with this information, the 2018/19 sorghum trial at Hermitage was sown at a crop density of 10 plants/m2, and the effect of row spacing (50 cm and 100 cm) on weed production was measured. Biomass and seed production of ABG was reduced by 55 per cent and 65 per cent, respectively when the sorghum was sown at the narrower spacing. Similarly for FTR, the 50 cm row spacing reduced biomass and seed production by 48 per cent and 56 per cent, respectively.

Graphs: Awnless barnyard grass (ABG) (left) and feathertop Rhodes grass (right) biomass and seed production as affected by sorghum row spacing at Hermitage, Qld 2018/19. Within each graph, different letters indicate significant (P<0.05) difference after pairwise comparison.
Crop competition is a ‘free kick’ non-herbicide tactic in the WeedSmart Big 6 strategy to manage herbicide resistance in weeds. There is now solid evidence that growers can maintain crop yield and reduce summer grass seed production by planting sorghum crops at a density of 10 plants/m2 and a row spacing of 50 cm.
There are some residual herbicide options for the control of these summer grass weeds in sorghum. However, their efficacy can differ greatly depending on the season and will rarely provide full control of FTR and BYG. A combination of residual herbicides and a competitive crop is likely to have an additive effect and reduce seed production on surviving weeds.
This research project also demonstrated that mungbeans are more competitive on 50 cm row spacing, making any changes to seeding equipment worthwhile as it would suit the whole summer crop program in the northern region.
Central Queensland sorghum grower experience
Organic grain producers Paul and Cherry Murphy have always relied on crop competition as an integral component of their weed management program in all crops, including sorghum, at ‘Kevricia’, near Capella in Central Queensland.
With many years of experience growing sorghum on 50 cm row spacing Paul says the suppressive ability of the closed canopy certainly reduces weed growth and seed set in-crop.
Paul Murphy, Central Queensland, has been sowing sorghum on 50 cm row spacing for many years to suppress weeds in their organic farming system.
“We have been working off a plant density of around six plants per metre square as a rule of thumb that seems to work in most seasons on our farm,” he says. “In seasons where soil moisture might be limiting we have seen higher density crops fall over, and so have leant towards the lower planting rate. But the 10 plants per metre square would certainly increase the competitiveness of the crop in seasons where there is sufficient moisture.”
Paul is pleased to see researchers doing more work on row spacing and plant density, which is difficult to really tease out in commercial settings where there are too many potential variables. “In sorghum there is a complexity associated with plant density, tillering and row spacing that needs scientific trials like this to really determine the optimal combination for maximum yield and weed control in a variety of seasonal scenarios,” he says.
This season Paul will be breaking with tradition and planting sorghum on wider row spacing as he now has a Garford camera-guided inter-row cultivator. He hopes the wider spacing will only be required for this season while he makes the adjustments required to have the machine suit their controlled traffic configuration.
The Murphy’s inter-row cultivator is capable of working in crops planted on 50 cm row spacing once it has been adjusted to suit their CTF configuration.
“Once we are ready to plant the winter crop I hope to be able to plant on 50 cm spacing again and still use the inter-row cultivator,” he says. “The cameras on the cultivator guide the alignment of the tynes to follow the plant row with a 1 cm accuracy, and can be used when the crop is 10 to 40 cm high.”
As organic growers the Murphys don’t use any herbicides and so early weed control can be difficult, but this inter-row cultivator will help remove any weeds that emerge with the crop and then crop competition can suppress any later germinations.
Other resources

GRDC Update paper: Growing competitive sorghum and mungbean crops to suppress summer weeds 
Creating stiff competition against summer weeds 
Managing barnyard grass in summer crops and fallow 

With investment from GRDC, researchers led by Professor Bhagirath Chauhan at the University of Queensland, have shown that both windmill grass and feathertop Rhodes grass can greatly reduce yield in mungbean, yet both weed species retain a large portion of their seed when the mungbean crop is ready for harvest.
This gives growers the opportunity to use several tactics to reduce the seedbank of these two species while growing mungbean.
Professor Chauhan says that even at the most competitive row spacing of 50 cm, mungbean yield was halved when there were around 40 windmill grass plants/m2 or just 11 feathertop Rhodes grass plants/m2 growing in the crop.
Feathertop Rhodes grass competes strongly and produces masses of seed if it gains a foothold in a mungbean crop.
“The good news is that both species have a high level of seed retention at harvest because mungbean is such a quick growing crop,” he said. “This gives growers the chance to vastly reduce the amount of new seed entering the seedbank.”
“Even though these weeds have high seed retention at harvest they also produce a huge quantity of seed,” he says. “At peak weed density in our field trials feathertop Rhodes grass produced over a quarter of a million seeds per metre square and windmill grass produced around 100,000 seeds per metre square. So, even if a small portion of this seed enters the seedbank it can still equate to a large number of seeds to potentially germinate the following spring.”
Feathertop Rhodes grass is known to begin germinating in late winter and early spring, well before a mungbean crop is planted so every effort should be made to eliminate all flushes of this weed prior to planting mungbean. Haloxyfop is currently registered for fallow control of feathertop Rhodes grass ahead of mungbean production and can be used to reduce the weed burden prior to planting mungbeans in the most competitive configuration of 50 cm row spacing. To reduce the risk of Group A resistance, use a double knock in this pre-plant situation to control any Group A herbicide survivors of these difficult grass weeds. Paraquat is the usual chemical double-knock partner in these situations and should be applied to small, unstressed weeds within 7 to 10 days after the application of haloxyfop.
Both these weed species can germinate close to the same time as the mungbean crop, so early weed control is essential to maximise yield and minimise early weed competition. Although these two grass species are susceptible to several pre-emergent herbicides, only flumioxazin (Valor) is registered for use in mungbean. This Group G herbicide can be applied at least two months pre-sowing to provide enhanced knockdown and residual control of feathertop Rhodes grass in mungbeans, taking care to follow the ‘critical comments’ to avoid crop injury.
Extra emphasis should be put on ensuring the paddock is as clean as possible prior to planting mungbeans. Inter-row cultivation may be an option provided the young plants are not injured, as wounds can allow entry of diseases such as tan spot or halo blight.
Clethodim applied before the mungbeans begin to flower will provide effective in-crop control of small, late germinating grass weeds.
Mungbean crops are commonly desiccated prior to harvest using either Reglone or glyphosate. Both of these Chloris weed species are generally unaffected by these herbicides as mature plants, so the desiccation of the crop is unlikely to stop weed seed set. Mechanical options such as swathing are currently under investigation and may provide a more reliable way to stop seed set on these weeds prior to harvest.
Professor Bhagirath Chauhan, University of Queensland, says windmill grass and feathertop Rhodes grass both retain a large portion of their seed at the time of mungbean harvest, making harvest weed seed control an practical option to help reduce the weed seedbank.
“Mungbean is a good candidate for harvest weed seed control, using chaff lining, impact mills and the like, because the crop is harvested at ground level so any weed seed held on the plants should enter the harvester front,” says Professor Chauhan.
The WeedSmart Big 6 approach to help manage resistant and hard to control weeds combines the power of multiple tactics throughout the year and across a full crop sequence to reduce weed seed set.
Although feathertop Rhodes grass and windmill grass both produce vast quantities of seed, the seed is very short-lived. If left on the soil surface the seed remains viable for only one to two years. All efforts to prevent seed set will be rewarded with a rapid decline in the weed seedbank for these two difficult grasses.
GRDC has recently updated the ‘Integrated weed management of feathertop Rhodes grass’ manual, which provides detailed information on the ecology of this important weed, along with the tactics and strategies that can be used throughout a cropping sequence to manage the seedbank.
Other resources

Giving summer legumes the competitive edge
FTR grass demands attention to stop seed set 
Creating stiff competition for summer weeds
GRDC manual: Integrated weed control for feathertop Rhodes grass 2020 update

You can listen to the article being read above!

Whether they are resistant to herbicide or not, weeds generally do not compete well with vigourous crops, but in the fallow they can rapidly take advantage of the lack of competition for resources.
Department of Agriculture and Fisheries researcher, Dr Annie Ruttledge has been running experiments at Kingaroy to investigate the benefits of bringing crop competition into the fallow phase of cropping systems in southern Queensland.
Dr Annie Ruttledge, Department of Agriculture and Fisheries, Queensland weeds researcher is conducting trials to identify cover crop species suited to southern Queensland that have weed-suppressive traits.
With investment from GRDC the cover crop project, led by Charles Sturt University, is investigating the weed-suppressive power of various cover crop species suited to either summer or winter fallows at three locations in the northern grain growing region – Kingaroy, Narrabri and Wagga Wagga.
At the Kingaroy site, both winter and summer-growing cover crops were shown to suppress weeds by over 85 per cent and up to 95 per cent, compared to an untreated fallow where the sown weeds were not inhibited by a cover crop. While this level of control is worthwhile on its own, it is also backed up with either a chemical or non-chemical tool to terminate the cover crop and kill any survivor weeds.
“In winter in Kingaroy, the best cover crops for weed control were grazing oats and tillage radish,” says Annie. “These species provided early season ground cover and suppressed our mimic annual grass weed, Italian ryegrass, by up to 94 per cent relative to the weeds-only fallow. None of the cover crop species we tried were able to suppress the quick-growing mimic broadleaf weed, Oriental mustard.”
Winter-growing cover crop monocultures and mixtures.
In summer, Annie says the best cover crop options for Kingaroy were white French millet, Japanese millet, forage sorghum and buckwheat. Again, early-season biomass and ground cover was the key to suppression of both grass and broadleaf weed mimics by up to 95 per cent when compared to the weeds-only fallow.
Summer-growing cover crop monocultures and mixtures.
So far in this trial, there has been no measurable weed suppression benefit in sowing mixed species cover crops rather than monocultures. However, a mixed species cover crop may be preferred if a grower is wanting to achieve multiple outcomes. For example, grazing oats may be selected as a fast growing and highly competitive species and teamed with a less competitive legume to boost soil nitrogen stores.
“Obviously, the species selected will depend on the growing region and soil type,” she says. “Cover crops also provide many other services to the farming system and so the grower could select a cover crop species, or mix of species, that would also provide a break from disease or insect pressure, increase moisture infiltration, build up organic matter or break down compaction.”

Source: Charles Sturt University
Cover crops are an extension of the WeedSmart Big 6 tactic of providing crop competition to suppress weed growth and reduce the weed seed bank in an integrated weed management program.
Annie says that light interception is a critical driving force in the effectiveness of cover cropping for weed control. In selecting cover crops for weed suppression, choose species that grow well in your locality and that restrict light penetration to the soil through strong early growth and the development of a dense canopy. For greatest benefit, terminate cover crops at maximum biomass, which should coincide with the beginning of flowering; however, earlier termination may be required if soil moisture is limiting.
There is a large body of research work now underway to investigate other aspects of incorporating cover cropping into farming systems in various regions. While this work focuses on weed suppression, other researchers are looking into soil water and nutrient use efficiency under different conditions and in various cropping systems.

Other resources

Summer cover crops video DAF
Day family case study
Cotton cover crops
Cover crops research update video presentation
GRDC Update paper – Cover crops to provide groundcover in dry seasons