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Going for the Big 6 on Yorke Peninsula farms

Chris Davey, partner and director of YP AG at Kadina has worked with growers on the Yorke Peninsula of SA for over 20 years, assisting them to devise weed control programs that reduce the impact of herbicide resistance.

His group of 20 clients farm between Port Broughton and Arthurton with annual rainfall ranging from 300 to 500 mm and very diverse soil types. Chris initiated the Northern Sustainable Soils farmer group in 2007 to provide growers with the opportunity to research farming system tactics and discuss their ‘fit’ for the highly variable soils found on the Peninsula.


YP-AG partner and director, Chris Davey works with his grower clients including Gary Bruce (left), to plan integrated weed management programs that include as many of the WeedSmart Big 6 tactics as possible.

“The soils here range from shallow sheetrock and limestone to grey calcareous loams, dune and swale systems to heavy red fertile clay,” he says. “This variability drives many management decisions and has a direct relationship with many of our weed problems.”

Chris has used resistance testing services to keep track of herbicide resistance in the main weeds, with growers managing resistance in annual ryegrass, brome grass and wild radish for some time and more recently finding milk thistle, Indian hedge mustard and prickly lettuce increasingly problematic. Annual ryegrass and brome grass on the Peninsula are known to have resistance to herbicides in Groups A (fop and dim), B, D and M and wild radish is resistant to Groups B, F and I.

“This area was the home of ryegrass resistance and growers are trying hard to avoid the same blow-out situation with brome grass,” says Chris. “Growers are well aware of the potential consequences if resistant brome grass gets out of hand so most are using some form of patch management and even chemical fallow in blocks where the brome grass has out-competed the crop.”

In response to increasing herbicide resistance, 85 per cent of Chris’ clients have adopted some form of harvest weed seed control within their weed management program. “Sixty per cent of my clients are using narrow windrow burning and 25 per cent are using either a chaff cart or chaff lining chute,” he says. “In the 2017 harvest there was also one iHSD and one Seed Terminator operating here.”

Chris is a strong supporter of all the tactics in the WeedSmart Big 6 and actively promotes the inclusion of all tactics in his clients’ integrated weed control programs.

1. Crop and pasture rotation

The close lentil – wheat rotation that has dominated farming systems on the northern Yorke Peninsula in recent years is acknowledged as a weak link in terms of weed control. This rotation has led to an increase in broadleaf weeds such as milk thistle and prickly lettuce, with bifora, tares and medic also exploding in the lentil phase in some years, leaving a high weed seed bank for the following year as well as increasing the risk of herbicide resistance evolution.

Chris says the economic drivers for the rotation can make weed control decisions difficult and there is a need for other profitable rotation options that can assist in reducing weed pressure.

The main problem with the short rotation is that weeds are exposed to the same herbicide modes of action every two years. Although imi-tolerant (Clearfield) varieties have been very useful, particularly PBA Hurricane lentils, allowing the use of Group B herbicides in the crop or in previous seasons, the alkaline soils on the Peninsula have expedited the more rapid evolution of Group B resistance in wild radish, mustard, milk thistle and ryegrass.


Lentils have provided several economic and weed control benefit to farming systems on the Yorke Peninsula but the short lentil / wheat rotation is a definite weakness when it comes to managing herbicide resistance.



2. Double knock to protect glyphosate

Glyphosate resistant ryegrass is widespread on the Yorke Peninsula, primarily along fencelines but as fences are removed to form larger paddocks, there is a significant risk that the resistance gene will be spread by headers. In 2013, the Peninsula had the dubious honour of having the first confirmed case of glyphosate resistant brome grass on a farm near Maitland, and this season, glyphosate resistance in barley grass was observed for the first time on northern Yorke Peninsula.

An annual double knock application before seeding is considered very important to help protect the efficacy of glyphosate and is widely practiced on the Peninsula. Sowing earlier to achieve a yield advantage and dry sowing can impact on the use of double knock. Chris advises his clients to avoid early or dry sowing in weedy paddocks and to hold off sowing until the double knock has been implemented, even though there could be a yield penalty. Under dry, dusty conditions most growers will choose two contact herbicides such as paraquat or paraquat / diquat rather than glyphosate / paraquat for the double knock.

3. Mix and rotate herbicide groups

There is a heavy reliance on pre-emergent herbicides on the Yorke Peninsula and in weedy paddocks growers need to use additional shots to drive down weed numbers to preserve yield. In cereals, the Boxer Gold and Sakura applications are often spiked with triallate to strengthen the pre-emergent efficacy because there are no in-crop herbicide options in wheat and barley crops.

Pre-emergent herbicides are also very important in lentils as the main break crop to reduce ryegrass numbers so there is less pressure on the clethodim / Factor mix in crop.

Trifluralin susceptibility in ryegrass has been very low since the late 1990s and so is not a tank mix option, unless targeting broadleaf weeds like wireweed or three corner jack.

4. Stop weed seed set

Chris says Yorke Peninsula growers generally use their late fungicide application in August or September to scout for weed escapes in crop. Taking a nil tolerance approach, growers might hand pull small areas, or spot spray.

Using paraquat or paraquat / diquat, growers can avoid using glyphosate on potentially resistant individuals when chemically fallowing areas of their crop. The permit for Weedmaster DST use to crop-top in barley provides a useful control tactic for radish and ryegrass at the end of the season, but is often too late for brome grass, which has usually already set seed by this stage of the crop. In blow-out situations Chris often advocates for the ‘short-term pain for long-term gain’ of a chemical fallow using paraquat or Spray.Seed.

Chris suggests that the chemical fallow is the best tool to use if brome numbers are building up in a paddock. In his experience, the performance of the following crop usually makes up for the one-year sacrifice due to increased nutrients and moisture availability. He says some growers plan for the inclusion of a small portion of the rotation to be sown as a chemical fallow, while a larger number would use chemical fallow only in a failed crop or for a weedy portion of a paddock as a patch management option.

5. Crop competition

Where the soil type allows, Yorke Peninsula growers have readily adopted east west sowing having seen the benefits of this row orientation promoted through AHRI and WeedSmart. Some soil types, such as the sand swales around Port Broughton, dictate sowing direction but it is an option in other areas.

Barley is the most competitive crop grown across the Peninsula and growers usually consider choosing the most competitive cultivars available. This is coupled with high sowing rates and narrower row spacing of 22–30 cm (9–12″) spacing, although there is local research that suggests there could be benefits of even narrower row spacing.

6. Harvest Weed Seed Control (HWSC)

Chris’ trial work with HWSC shows the importance of getting the weed seed into the header. He says brome grass can be difficult as its flexible stem doesn’t always get cut and can flick back up once the harvester has passed. Wild radish generally stands up well with 70–80% of seed entering the header. Even with a 50 to 60% capture of brome grass, depending on the season and how early harvest occurs, HSWC is still an important part of any weed management program. Capture of ryegrass seed is seasonal with the ryegrass lodging in some years and not picked up by the header, while it will stand up well and achieve 80% capture in other years.

Once weed seed is in the header, Chris’ research has shown that it doesn’t matter what HWSC method is used – all are effective. Chaff carts and narrow windrow burning have been widely adopted for many years but Chris expects that chaff lining and chaff decks is likely to increase while NWB will decrease in use. Chaff lining was widely adopted in 2017 harvest as an economical and easy way to manage weed-laden chaff. He also expects some growers with larger areas and some contractors to purchase iHSD and Seed Terminator modules, with one each of these operating in 2017 harvest on the Peninsula.


Chris Davey’s trial work with various harvest weed seed control methods has shown that the amount of weed seed entering the header can vary, but once in the header all methods are equally effective at reducing the weed seed bank for the next season.

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Testing informs herbicide choice

Buying a new property brings with it a number of unknowns and it can take some time for the new owners to become familiar with the soils, the terrain and the weeds.
When the Birch family of ‘Catalina Farms’, Coorow, WA purchased a neighbouring property, historic ‘Koobabbie’, they had reason to expect that herbicide resistance would be less of a problem because there had been limited use of herbicides on ‘Koobabbie’ in its 114 history of farming.
Daniel Birch, who farms with his wife Jen, parents Rod and Shelley, and long-term team member, Justin Passamani, says the purchase conveniently coincided with the opportunity to be involved with independent herbicide resistance testing in 2020.
“The testing was arranged through the Liebe Group and conducted at the Australian Herbicide Resistance Initiative,” he says. “We were keen to test some theories we had about resistance status of weeds on Catalina and also to get some baseline information about Koobabbie to help us plan our herbicide program.”
Daniel Birch of ‘Catalina Farms’, Coorow, WA says the big take home message from recent herbicide resistance testing was the power of mode of action mixtures.
The testing revealed a number of things that surprised the Birches. To start with, they found there was Group 2 [B] resistant ryegrass in paddocks on Koobabbie that had no history of Group 2 herbicide use.
“This demonstrated just how easy it is for herbicide resistance to move in seed or hay, or on machinery,” says Daniel. “Obviously we can use this information to avoid using chemistry that we know has little to no efficacy on ryegrass.”
“The other major finding was that Group 12 [F] resistance in wild radish was much higher than we expected across the farm,” he says.
This has led the Birches to include more premium products in their program to target resistant ryegrass and wild radish, and drive down the weed seed bank as quickly as possible. On the flip side, they also discovered that they can save money by using trifluralin at lower rates in seasons with good growing conditions, where there is less need for a long residual effect.
“In those years where the crop gets off to a good start, the crop competition effect kicks in early to suppress weeds,” says Daniel.
By taking on board the resistance testing results from weed seed samples collected across the state, Daniel was reassured by the fact that the resistance issues they faced were essentially the same as other growers.
“The big take home message for us was the power of mode of action mixtures,” he says. “From the overall survey results across Australia, the resistance frequency to stand-alone pre-emergence herbicides ranged from 10 to 34 per cent, yet resistance to herbicide mixtures ranged from 0 to 6 per cent.”
Armed with knowledge about the resistance profile of their weeds, the Birches are combining some older chemistry with newer, premium herbicides for maximum effect.
The AHRI resistance testing program is led by Dr Roberto Busi. In 2019, annual ryegrass seed samples from 298 farms were submitted, representing 579 populations from four states in Australia, and these were tested for resistance to 21 herbicides applied at the recommended rate – 12 standalone and nine two-way mixtures.
In total, 15 876 individual resistance tests were conducted to screen two million seeds against registered herbicides and herbicide mixtures at the recommended label rate.
Dr Busi says the mixtures that growers can confidently incorporate in their annual ryegrass program are trifluralin + Sakura mix, Luximax + triallate, and clethodim + butroxydim.
“When applied at full rate for each component, these mixtures can achieve a better outcome than the same herbicides applied as stand-alone treatments against annual ryegrass with known resistance,” he says.
For wild radish, 200 samples were tested over a period of two years and resistance to Groups 2 [B], 4 [I] and 12 [F] were all over 50 per cent resistant. At 70 per cent resistant, Group 2 [B] herbicides should probably be dropped from most wild radish herbicide programs and Group 4 [I] is under threat.
“In 2021, we found that mixing Group 12 [F] with Group 6 [C] herbicide bromoxynil vastly improved control of wild radish,” he says. “From 51 per cent resistant to Group 12 [F] down to less than 15 per cent of samples resistant to the 12 [F] plus 6 [C] mixture.”
“It is important to emphasise that herbicide resistance testing is conducted on small, actively growing weed seedlings in a glasshouse environment,” says Roberto. “In the field, spray failures can easily occur, even in susceptible weed populations, if the herbicide is applied under the wrong conditions or to plants that are too large or stressed. This is particularly true for wild radish.”
AHRI Podcast: Interview with Daniel Birch about herbicide testing
AHRI Insight: Mixtures rock
Herbicide testing options

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Safe sorghum planting while controlling weeds

Pre-emergent herbicide, metolacholor and S-metolachlor, have been used in Australia primarily to control grass weeds for over 40 years. To date, no instances of resistance to this active ingredient have been documented in Australia and even internationally the few cases of resistance are in broadleaf weeds only. Resistance in northern summer grasses is probably low at present.
Syngenta’s field biology manager, Rob Battaglia, says some recent changes to the label use patterns and the launch of a new seed safener product will further expand the usefulness of S-metolachlor products, such as Dual Gold® and Primextra Gold®, in summer crops while protecting sorghum seedlings from herbicide damage.
Syngenta’s field biology manager, Rob Battaglia, says the new use patterns for Dual Gold® in sorghum, cotton and fallow centre on extending application flexibility and residual activity of the herbicide on target weeds.
“S-metolachlor is registered for use in a wide range of crops, including sorghum, maize, sweetcorn, soybean, sunflower and cotton, as well as in fallow situations,” he says. “The new use patterns in sorghum, cotton and fallow centre on extending application flexibility and residual activity of the herbicide on target weeds.”
In sorghum, a rate of 1.0 to 2.0 L/ha can be applied either as a single application before the crop or weeds emerge, or as a split application pre and post crop emergence (up to 6-leaf stage). Similarly, the full rate can be applied after harvest to establish the fallow or 1.0 to 1.5 L/ha can be applied pre-emergent to weeds at fallow establishment followed by 0.5 to 1.0 L/ha within 4 weeks. Be sure to observe replant intervals when planning the next crop.
In cotton, a single application of 1 L/ha either pre-emergent (before, at, or immediately after, planting), over-the-top or directed stand-alone spray or over-the-top mixed with Roundup Ready herbicide on Roundup Ready FLEX® cotton.
“There are several factors that affect the length of effective residual activity of S-metolachlor, some of which are outside the grower’s control,” says Rob. “The product needs to be incorporated and has interactions with rainfall, temperature, soil type, soil organic matter and stubble. Having more flexibility in the application timing gives growers the ability to compensate for some of these factors and control new waves of weed germinations for longer.”
Untreated strips are a good indicator of the seed bank and potential weed burden in a field. The pre-emergent herbicide treated area on the left has far fewer weeds than the untreated section of the field on the right, which suffered significant yield loss due to feathertop Rhodes grass infestation.
S-metolachlor is only active on weeds that have not yet emerged and has no effect on weeds that have already emerged from the soil or that do not come in contact with the herbicide as they emerge from the soil. It is essential to control already-emerged weeds first and then apply the pre-emergent to control subsequent flushes of weed germinations.
“There are some situations where the residual herbicides will appear to be less effective than they should, but this can often be attributed to compromised application timing, limited rainfall and soil conditions, rather than herbicide performance,” says Rob. “The difficultly in achieving perfect application timing and incorporation with rainfall is one of the main reasons why the new use patterns for Dual Gold® have been registered. Pre-emergent herbicides are best used within the WeedSmart Big 6 integrated weed management program and not as a stand-alone control measure.”
While other crops listed on the S-metolachlor label can metabolise the herbicide, sorghum requires a seed safener to protect the germinating plants from the herbicidal effects of S-metolachlor. The new Epivio C® seed safener replaces Concept II® and provides better overall protection for sorghum seed.
S-metolachlor damage is seen as distorted and twisted growth in sorghum, the rightmost plant is unaffected (protected by seed safener).
In Syngenta’s product development trials in Queensland and NSW, they measured an 11 per cent increase in plant stand and a 7 per cent increase in sorghum yield in crops where the seed was treated with Epivio C®compared to Concept II®. Epivio C® has also demonstrated improvements in seed safety and shelf-life for carry-over seed.
“Epivio C® is applied as seed treatment and when the seed is planted into moist soil the product is taken up into the plant and improves the metabolism of the herbicide in the seedling, resulting in no crop symptoms,” says Rob. “The result is better crop establishment and healthier plants that can better compete against weeds that germinate later in the season.”
Epivio C plot trial demonstrating the benefit of the seed safener in supporting robust germination and establishment. No safener (left) compared to Epivio C safener (right).
There is 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.
The WeedSmart Big 6 includes diversity in crops and pastures, crop competition and mixing and rotating herbicide mode of action groups.
More information

Podcast: Seed safener explainer (starts around the 22 min mark) 
Article: Weaponise sorghum crops to take out feathertop Rhodes grass and awnless barnyard grass

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Investigate adverse experiences when using herbicides

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?

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This webinar was hosted by Jana Dixon, WeedSmart’s High Rainfall Zone extension agronomist.

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WeedSmart Week Forum Day Videos

List of videos

Interviews with the Esperance Pioneers. Chair: Lisa Mayer, interviewing Neil Wandel & Theo Oorschot
Rotating buys you Time, mixing buys you shots
Efficacious use of the new pre-ems, Brent Pritchard
Delivering regionally focused research
Crop competition in wheat and canola, Hugh Beckie
Summer weed control
Strategies for control of ryegrass, marshmallow, fleabane, portulaca, Greg Warren
Farmer Experience
Rotations to stop seed set and preserve chemistry, Tom Longmire
Soil Amelioration, Tom Edwards
Crop competition: Reduced row spacing, higher seeding rates, east-west sowing, precision seed placement & competitive varieties, Theo Oorschot
Farmer Experience – Utilising crop competition strategies and the Big 6, Mic Fels
Weed control – farmer systems discussion panel – Chair: Peter Newman, with Mark Wandel and Laura Bennett
What’s next in spray technology? Andrew Messina
What’s next in spray technology 2? Guillaume Jourdain
Innovation Panel – Chair: Ben White, with Guillaume Jourdain, Andrew Messina
Stacking the Big 6 in farming systems in WA presented by Greg Condon, with Peter Newman

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Ryegrass management in the High Rainfall Zone – What have we learnt?

This webinar was hosted by Jana Dixon, WeedSmart’s High Rainfall Zone extension agronomist.

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