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Controlling glyphosate-resistant grass in irrigated cotton

When glyphosate-resistant summer grass starts to cause yield losses in cotton, growers obviously need to add some non-glyphosate options to their system to protect yields and prevent further weed blowouts. The questions then become where to add non-glyphosate tactics to get most benefit, and how many are needed?

Developer David Thornby used the Barnyard Grass Understanding and Management tool, BYGUM to investigate three key questions relevant to irrigated cotton systems.

Scenario #1 – Ask BYGUM: the value of glyphosate

What’s the remaining value of glyphosate in rotations with glyphosate-resistant awnless barnyard grass?

BYGUM developer David Thornby has developed a series of scenarios to demonstrate this new decision support tool.

Since the first confirmation of glyphosate resistance in awnless barnyard grass, many other resistant populations have been found, and these populations don’t all display the same level of resistance.

While glyphosate is no longer effective as a stand-alone control measure against any of them, some populations are less strongly resistant than others. In the case of the first-confirmed population, field rates still had around a 40 percent efficacy on small seedlings. For other more recently confirmed populations, efficacy even on small seedlings is much lower.

Given that glyphosate is going to be applied to these populations anyway, it’s important not to overstate the usefulness of glyphosate by hoping to be able to rely on it for some level of control. BYGUM can test the difference for us, between populations with strong resistance and those with moderately strong resistance.

David said he used a simple irrigated rotation, with modest use of non-glyphosate options in an otherwise glyphosate dominated system. He varied the effectiveness of glyphosate from around 40 percent (‘moderate’ resistance) to around five percent (‘strong’ resistance).

Key outcomes

The results show two key things. First, both systems are still making money after five years. High levels of crop competition keep seed production per escaping weed low, and the addition of a few effective tactics reduces the number of surviving plants to moderate/know levels of between six and 14 plants per square metre at the end of the fifth season.

Secondly, however, both systems are heading towards failure. Weed and seedbank numbers are increasing, however slowly. And while gross margins are the same at the end of season one, there is a predicted difference of around $500/ha between the gross returns in season five.

David says there are three lessons here. First, strong crop competitive effects might mask the seriousness of resistance issues in irrigated cotton, should they be present. Second, there are good reasons to determine just how strongly resistant your resistant awnless barnyard grass population is, if you’re going to be sticking with a system that is predominately about the use of glyphosate. And third, allowing a slow decline with somewhat-inadequate weed management looks likely to have a substantial cost as the years pass by. We’ve made many assumptions in this example – in particular, that irrigated cotton is planted and provided with resources to allow for strong competitiveness against the weed. We’ve also made assumptions about crop and herbicide costs, average yields and prices. “You could run BYGUM with a different set of assumptions that fit your experiences, and see if the results change.

Scenario #2 – Testing the value of a cover crop

Can summer cover crops be used to get on top of weed populations?

In a one-in-one-out rotation of dryland cotton, summer fallows offer a chance to get on top of weed populations through vigorous use of non-crop herbicides. However, with no crop competition present, they can also offer weed escapes an opportunity to set a lot of seed, especially when the key herbicide in both crop and fallow, glyphosate, is no longer effective.

“Cover crops allow growers to maintain some competition even in non-crop seasons,” David Thornby says. “A good stand of millet (as simulated in BYGUM), sprayed out before seed set, allows for a combination of late season herbicide use to clean up survivors and mid-season competition with glyphosate- resistant barnyard grass, reducing seed set per plant.”

The first scenario is a basic one-in- one-out rotation. This contains the assumption that the barnyard grass population is resistant to glyphosate, and that an early season residual and mid-season inter-row cultivation are used to provide some control in crop: summer fallows use two cultivations and a double knock. In scenario 2 David replaced the second summer fallow with a cover crop.

The cover crop includes cultivation, a double knock, spray out (assuming this is with a non-glyphosate product effective on glyphosate, such as paraquat), and a late application of paraquat over the now-dead millet. “The cover crop is more expensive than the summer fallow, and actual plant numbers per square metre are not reduced all that much (1.1 to 0.8 per square metre),” David says. “But there is a substantial difference in seeds returned to the seed bank.”

The comparison scenario shows a substantial increase in the yield from the final cotton crop, due to the strong reduction in seed bank numbers at the end of the cover crop season. “The benefits of the cover crop come due, as expected, in the following crop, where the seed bank has been driven down and emerging weed numbers are low,” David said. “Over the course of the whole rotation, incorporating one cover crop every second summer fallow is predicted to be worth almost $200/ha in increased yield.

“There is more than one way to protect future yields in a dryland rotation, but using cover crop competition certainly seems to bear looking at. “We’ve made many assumptions in this example – in particular, that planting time and summer rainfall are conducive to good cover crop growth, resulting in high competition, that the barnyard grass population is strongly resistant to glyphosate, and that the cover crop doesn’t reduce moisture availability to the following cotton crop. “We’ve also made assumptions about crop and herbicide costs, average yields and prices. “Once again, users could run BYGUM with a different set of assumptions that fit their experiences, and see if the results change.”

Scenario #3 – Residual answers to resistance

Can you control a glyphosate-resistant grass by adding a residual in irrigated cotton?

In situations where a glyphosate-resistant summer grass starts to cause yield losses in cotton, growers obviously need to add some non-glyphosate options to their system to protect yields and prevent further weed blowouts. The questions then become where to add non-glyphosate tactics, to get most benefit, and how many are needed?

David Thornby used BYGUM to investigate this question. First he compared a system with glyphosate-resistant barnyard grass where only glyphosate is used with the same system with an early-season (pre- or at-planting) residual added.

Residuals tripled gross margins

The yield results of adding a single residual are striking.

The glyphosate-only system is still producing some yields (Figure 2 – primarily due to the competitiveness of irrigated cotton), but end-of-season weed numbers are very high and the potential of the system is being seriously under-utilised. “Adding a single residual can reduce early-season weed numbers dramatically, and because this is when most of the competition effects occur, this has a huge benefit for the bottom line,” David said.

Figure 1: Irrigated cotton rotation with an early-season residual. While we’ve used ‘a residual’ in the notation here, a rotation of suitable products from different modes of action should be used in the field. The glyphosate-alone system is the same as this one, without the early season residual.

Figure 2: A comparison of gross margin and barnyard grass numbers at end-of-season between glyphosate alone (top) and glyphosate plus a single, early-season residual.

“Gross margins are almost tripled compared to glyphosate alone when the weed population is strongly glyphosate resistant.

“However, end-of-season weed numbers (and seedbank density) are still unacceptably high, so a single residual doesn’t appear to be enough of an addition, despite the dramatic effect.

“A single year of poor control from the residual (rather than the average of around 85 percent efficacy) would certainly result in a blowout.”

Adding a mid- or late-season tactic provides some insurance against weed blowouts and seed production. Because late weed germinants in vigorous cotton stands don’t produce a lot of seed per plant, the effects on yield aren’t so dramatic. However reducing surviving plants and especially reducing the seed bank size are critically important insurance against future blowouts and selection of resistance for other modes of action.

David tried two different tactics in BYGUM, adding either a layby residual to each crop or a mid-season knockdown.

Adding a layby reduces the seed bank somewhat, and cuts surviving plants at end-season down to around 25 per square metre. This still appears to be too many survivors for comfort, but it does represent a substantial improvement over an early- season residual alone, and offers insurance against future blowouts.

BYGUM predicts that it can be sustained at least for the five-year rotation. This comes at a cost, however: the reductions in late-season weed numbers are offset by the price of the extra residual and BYGUM includes a penalty due to phytotoxicity.

In comparison with the layby system, a system with a mid-season knockdown, rotating between options including Group A herbicides, shielded paraquat and inter-row tillage improves the gross margin (due to a combination of taking out some weed competition and having some options with lower phytotoxicity-related yield penalties), but leaves more end-of-season survivors.

“So this is not an ideal system either – but is certainly an improvement in all ways over a single-residual system,” David said.

“These analyses show that while a single early-season residual can do a lot of heavy lifting in terms of reducing weed competitiveness, it’s not enough on its own for long-term sustainability. Late or mid-season tactics provide some insurance.”

“BYGUM predicts that while good returns can be sustained at least for five years with this ‘plus two’ strategy, more non-glyphosate tactics would be needed to drive the seed bank to very low levels. We’ve made many assumptions in this example—in particular, that irrigated cotton is planted and provided with resources to allow for strong competitiveness against the weed, that resistance to glyphosate is quite strong, and that good efficacy is generally the case for residual applications. Pre-simulation weed numbers are assumed to be moderate and we’ve also made assumptions about crop and herbicide costs, average yields and prices.”

Case study reproduced courtesy of CRDC, following publication in CRDC’s Spotlight magazine, Winter 2016. To access BYGUM, visit: www.cottoninfo.com.au/barnyard-grass-understanding-and-management-bygum.

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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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