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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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What to expect at WeedSmart Week 2021

Big 6 at WeedSmart Week 2021 – Double knock to protect glyphosate
The WeedSmart Forum is set for Tuesday 17 August, 2021 at the Civic Centre in Esperance WA. The program features growers, agronomists and researchers discussing ways to use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
Greg Warren from Farm and General in Esperance 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. The double knock plays a key role in preserving glyphosate (and soil moisture) and providing a clean seed bed for planting crops.

 
Big 6 at WeedSmart Week 2021 – Increase crop competition
WeedSmart Week 2021 is set for Tuesday 17 to Thursday 19 August, 2021 in and around Esperance WA. The last two days feature local growers hosting visits to their farms and discussing how they use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
One of the farms hosting a visit during WeedSmart Week is Warrakirri’s 12,800 cropping operation at Condingup. Farm manager Con Murphy has implemented a variety of tactics to combat their main weeds – annual ryegrass and wild radish. Since 2015 the farm has undergone an intensive soil amelioration program to improve the drainage and ameliorate the sandy soils across the farm.
Con says the benefits have been seen in better germination and establishment that sets their cereal, pulse and canola crops up to compete strongly with weeds. There is also a benefit at the end of the season where rain in August or September enters the soil profile without causing waterlogging, and providing a better finish for their crops.
Since 2016-17 about 80% of the farm has been ripped and a portion has been ripped 2 or 3 times because the sandy soils tend to slump after substantial rainfall events, recreating the hardpan.
Con will be showing the WeedSmart tour group how their ripping, drainage, liming and spading program has helped grow more crop and less weeds!
Listen to the podcast with Warrakirri’s Con Murphy talking about the impact of improved drainage on crop competition

Big 6 at WeedSmart Week 2021 – Implement harvest weed seed control
The WeedSmart Week machinery display is set for Wednesday 18 August, 2021 at Dave Campbell’s shed near Esperance WA. The 3-day WeedSmart Week program features growers, agronomists and researchers discussing ways to use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
We’ve saved the harvest weed seed control discussion for the machinery session on Wednesday 18 August. Ben White from Kondinin Group will host the machinery session with spray and harvesting gear on display including impact mills from Seed Terminator, Redekop and iHSD (both hydraulic and belt-driven), Emar chaff deck, and spray technologies including Goldacres’ G6 Crop Cruiser series 2, and weed detection technologies using drones and weed identifying cameras (green on green).
Ben White, Kondinin Group (Photo: Melissa Powell, courtesy of GRDC)
Growers doing the WeedSmart Big 6
WeedSmart Week 2021 is set for Tuesday 17 to Thursday 19 August, 2021 in and around Esperance WA. The last two days feature local growers hosting visits to their farms and discussing how they use the BIG 6 to beat crop weeds. You can register for the 3-day WeedSmart Week event here.
One of the growers who will open up their farm for a visit is Adrian Perks who farms at Condingup, 70 km north-east of Esperance. Adrian runs a continuous cropping program on his 4300 ha property, growing canola, wheat, barley, faba beans and lupins. This diverse rotation allows him to mix and rotate both chemical and non-chemical weed control tactics. Over half of Adrian’s farm is sandplain, on which he has implemented a soil amelioration program to address non-wetting to increase the competitiveness of his crops. He currently uses chaff decks for harvest weed seed control and is introducing an impact mill this season. Adrian monitors the tramtracks for weed growth and if he feels the weed pressure is too high, he uses a shielded sprayer to reduce seed set. The bus tour will include four farm visits and a machinery display.
Listen to Adrian on the Regional Update podcast.
Adrian Perkins, Condingup WA
 

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WeedSmart agronomist set to tackle high rainfall zone weeds

Every locality has its own spectrum of weeds, and growers face different opportunities and challenges regarding the control tactics they can employ.
The WeedSmart Big 6 approach is a practical way to ensure that an integrated weed management program is put in place that disrupts weed seed production and the evolution of herbicide resistance.
Commencing in January 2021, Jana Dixon has joined the WeedSmart team of extension agronomists, with a focus on applying the Big 6 to manage weeds in the high rainfall cropping systems of southern Australia – from Esperance in WA to south-eastern SA, Tasmania and south-western Victoria.
Jana will add to the dedicated and experienced extension agronomists on the WeedSmart team with Peter Newman in the Western region, Chris Davey in the South, Greg and Kirrily Condon in the East and Paul McIntosh in the North.
Jana Dixon has joined the WeedSmart team of extension agronomists, with a focus on applying the Big 6 to manage weeds in the high rainfall cropping systems of southern Australia – from Esperance in WA to south-eastern SA, Tasmania and south-western Victoria.
Jana hails from the Mid North of SA, and began working at Pinion Advisory (previously Rural Directions) while she was studying agriculture at the University of Adelaide. She has been employed full-time at Pinion Advisory since January 2019 as an agribusiness consultant, based in Clare, and spends most of her time delivering agronomy and farm business advice to clients from a wide range of cropping regions in South Australia.
Pinion Advisory is a foundation WeedSmart sponsor and Jana has been involved in two WeedSmart Week events already – the first as a participant and grower group organiser at the Horsham event in 2019 and then as the local organiser for WeedSmart Week 2020 in Clare.
In welcoming her to the WeedSmart team, program manager Lisa Mayer says Jana brings energy, commitment and insight to deliver communications focussed on the southern region’s high rainfall regions.
“Growers in the southern high rainfall zones are facing some serious issues with herbicide resistance influencing their farming decisions,” says Ms Mayer. “Jana will be engaging with agronomists, growers and researchers in each of the distinct high rainfall zones to understand the complexities and look for practical ways to apply the WeedSmart Big 6 in various cropping scenarios.”
“We plan to deliver WeedSmart Week in Esperance, part of Western Australia’s high rainfall cropping zone, in August 2021 and Jana will play a key role in the planning and delivering of our annual 3-day flagship event.”
Jana says her experience with the WeedSmart program has been very positive and she has been particularly impressed with the support the program has from all sectors of the grains industry.
Newly appointed WeedSmart extension agronomist, Jana Dixon (green cap) leading discussions with farm visit host, Ben Marshman, Owen SA, and growers and agronomists attending WeedSmart Week 2020 in Clare.
“I have spoken to many growers and agronomists who have found real value in the information that the WeedSmart program delivers,” she says. “For many it is as much about considering another operator’s philosophy on dealing with weeds, and taking a fresh look at their own systems, rather than just learning about a new tactic or the traits of a new herbicide in isolation from the big picture.”
She says the high calibre of industry people who contribute their time and expertise to the program is testament to the value WeedSmart has to agribusiness, growers, agronomists and researchers alike.
In taking on the responsibility for delivering information tailored for the high rainfall zones Jana says she is pleased to have an extensive network of contacts through Pinion Advisory, with offices in a number of high rainfall areas to provide easy access to local agronomists and growers. She is also aware that there are major differences in weed spectrums and farming systems in each high rainfall zone and plans to take full advantage of the opportunity this role presents to expand her understanding of different approaches to weed management.
“The long and favourable growing season and the associated prolonged periods of weed germination, is a key factor that I see potentially impacting on a grower’s weed management strategies in these regions,” she says. “On the other hand, access to highly diverse rotations and a focus on crop competition are two strategies that can play an important role in achieving excellent weed management in these regions.”
“I am keen to engage with anyone working and farming in the high rainfall zones to build my knowledge and understanding,” she says. “And to create opportunities to develop and extend the WeedSmart Big 6 strategies, both herbicide and non-herbicide, that work in each area and in different situations.”
WeedSmart is the industry voice delivering science-backed weed control solutions with support from the Grains Research and Development Corporation (GRDC), major herbicide, machinery and seed companies, and university and government research partners, all of whom have a stake in sustainable farming systems.
You an follow Jana on Twitter and keep up to date with the HRZ here.

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Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations.
Full rates when mixing herbicides too!
When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality.
Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL.
Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.

 
Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice.
Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.
 

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