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Making pre-em herbicides work in high stubble

Whether stubble is standing or laying flat on the ground it represents a challenge for farmers using pre-emergent herbicides to control weeds early in the cropping season.

Most growers and advisors are aware that products such as trifluralin are quite tightly bound if they contact stubble during application, however the behaviour of newer products has been largely unknown.

For pre-emergent herbicides to be effective the product must be placed in contact with the soil and must provide an even layer of chemical to intercept germinating weeds. Decisions at harvest will have a direct bearing on pre-emergent choices and efficacy for the next season.

Yaseen Khalil is a PhD candidate at the UWA School of Agriculture and Environment, studying the availability of pre-emergent herbicides applied to crop residue and then exposed to different rainfall simulations.

Decisions at harvest will have a direct bearing on pre-emergent choices and efficacy for the next season. Yaseen Khalil, PhD candidate at the UWA School of Agriculture and Environment, is studying the availability of pre-emergent herbicides applied to crop residue and then exposed to different rainfall simulations.

The three pre-emergent herbicides investigated were trifluralin, Sakura (pyroxasulfone) and Arcade (prosulfocarb). As expected, trifluralin was very resistant to leaching from crop residue with minimal amounts of the product washing off stubble in rainfall events simulated just one day after herbicide application.

On the other hand, a sufficient quantity of Arcade was leached by rainfall applied after 7 days to provide some control of annual ryegrass. Sakura was the clear ‘stand-out’ when it comes to efficacy of leachate. Product applied to 4 t/ha crop residue plots provided 100 per cent control of annual ryegrass when just 5 mm of simulated rainfall was applied 14 days after the herbicide application.

“The implications for growers are that they can rely on significant amounts of Sakura and Arcade being washed off crop residues and into the soil if there is a rainfall event of at least 5 mm within about one week of applying these pre-emergent herbicides,” says Mr Khalil. “If the crop residue is already wet when the pre-emergent herbicides are applied, the products are more tightly held and less product leaches off the residue in subsequent rainfall events, compared with spraying onto dry stubble.”

Mr Khalil’s supervisor University of Western Australia senior lecturer, Dr Ken Flower says an important factor to consider is the unevenness of residue spread at harvest. “While the residue level across the paddock may average out to an acceptable 3 t/ha it is common for the residue to be as high as 10 to 15 t/ha directly behind the header,” he says. “This in itself has implications when it comes to growers’ decisions about their weed management tactics.”

Australian Herbicide Resistance Initiative communication lead Peter Newman says getting the pre-emergent product through the crop residue and onto the soil at application time is still the priority.

“It is good to know that some products remain effective and can be leached off the crop residue with rainfall following the application,” he says. “Yaseen’s research has added to the growing knowledge bank about the most effective use of pre-emergent herbicides in no-till, stubble retention systems.”

Whilst quite extensive in itself, this study only compared three pre-emergent herbicides. The chemical properties of the over 50 active ingredients that possess pre-emergent, or residual, activity vary enormously, and they are found in 10 mode of action groups. The efficacy of these products relies on different environmental, soil and crop residue conditions.

“Understanding how these products can best fit into an integrated weed management program on a farm requires considerable thought, taking many, many factors into consideration,” says Mr Newman. “There are great opportunities to use these products to add diversity to weed management but it is essential that they are not over-relied on as 100 per cent weed control is uncommon, even when the conditions at application are as good as possible.”

“We used to talk a lot about rotating herbicides from one year to the next, now we are more focused on herbicide mixes as the best way to go — mixing two pre-emergent herbicides together where possible, and then perhaps rotate to another mix next year,” he says. “They say that rotating herbicides buys you time, but mixing herbicides buys you shots. Mixing and rotating buys you time and shots, which is the best that we can hope for.”

Mr Newman recommends growers take the time to discuss a variety of options with agronomists and take on board research like Yaseen’s, along with their own observations, to build a robust and ever-changing weed management program. “The aim is to keep overall weed numbers low as this is the best way to minimise the risk of herbicide resistance,” he says.

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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.


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.


Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills.

Prevent spray-drift
The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum.

Bill Gordon’s 10 Tips for Reducing Spray Drift

Choose all products in the tank mix carefully.
Understand the product mode of action and coverage requirements.
Select (and check) the coarsest spray quality that will provide effective control.
Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY.
Use weather forecasts to inform your spray decisions.
Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas.
Set the boom height to achieve a double overlap of the spray patterns.
Avoid higher spraying speeds.
Leave buffers unsprayed if necessary and come back.
Continue to monitor conditions, particularly wind speed, at the site during the spray operation

High water rates don’t have to slow you down
Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job.
To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can:

Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point.
Use a larger pump, e.g. 2.5 inch, to make re-filling quicker.
Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution.

For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high
Water quality and mixing order
Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property.
Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results.

Don’t start mixing until the water quality is right
Podcast – Mixing herbicides

Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives.
When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.

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