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Gearing up to use pre-emergent herbicides

Pre-emergent herbicides are not without their challenges. Working out the best pre-emergent herbicide choice for a particular situation requires a thorough knowledge of how the herbicide works.

Dr Chris Preston, University of Adelaide (UA) Associate Professor—Weed Management says some of the earlier practices regarding pre-emergent herbicides need to be reconsidered in the light of greater experience under different field conditions and changes in sowing technology.

Competitive (right) v non-competitive canola — growers can take advantage of the canola varieties with greater competitive ability.

Competitive (right) v non-competitive canola — growers can take advantage of the canola varieties with greater competitive ability.

The latest recommendations from research in the high rainfall zones of southern Australia can be summed up as: sow a competitive crop early, on the first opening rain, with pre-emergent herbicide; sow the cleanest paddocks last and implement harvest weed seed control.The latest recommendations from research in the high rainfall zones of southern Australia can be summed up as: sow a competitive crop early, on the first opening rain, with pre-emergent herbicide; sow the cleanest paddocks last and implement harvest weed seed control.

“Pre-emergent herbicides on the market vary in their water solubility, ability to bind to soil components, behaviour under different soil moisture conditions and rate of degradation over time,” he says. “Unfortunately, the seasonal conditions that unfold can have a significant effect on the efficacy of any product applied. For example, some products are not well suited to higher rainfall zones because multiple weed germinations are more likely and by later in the season the herbicide has dissipated or moved too far down the soil profile to have any effect on later germinations.”

One solution to this problem is the pre-emergent application of trifluralin followed with the recently registered use of Boxer Gold applied post-emergent. This relies on rainfall to incorporate the Boxer Gold and has provided excellent control, even of trifluralin-resistant weeds.

The method that provided the highest level of control in the high rainfall zone was pre-emergent Sakura + Boxer Gold post-emergent.

A third option for the high rainfall areas is Sakura + Avadex. This is also expensive but provided a high level of control, and avoids using Boxer Gold in wheat, leaving it to be used in other crops in the rotation.

“Keep in mind that Sakura can only be used in wheat crops so it is good to avoid using Boxer Gold in both wheat and barley crops. Sakura + Avadex may be a better choice in the wheat crop rather than Sakura + Boxer Gold,” he says. “There is a very high risk of losing Boxer Gold as an effective herbicide if it is used frequently in a rotation, so it is essential to plan herbicide use across the crop rotation and use different chemicals in break crops.”

Early (left) v late sown cereals — ryegrass head count was lower in early sown crop.

Early (left) v late sown cereals — ryegrass head count was lower in early sown crop.

These combinations of herbicides aim to reduce seed head production through season-long control but rely on harvest weed seed control to manage any survivors.

“Herbicide resistance has been shown to occur rapidly if these new chemistries are used unwisely,” says Dr Preston. “They can be part of a weed management plan but must not be relied on without the implementation of supporting non-chemical tactics, including harvest weed seed control and competitive cropping.”

For pre-emergent herbicides to be effective the chemical needs to be in the right place at the right time—beginning with the right stubble management, sowing equipment and sowing depth. “The guiding principle is that the pre-emergent herbicide must be in contact with the soil to have any effect. Some products wash off stubble better than others and so stubble load and whether it is standing or laying flat will influence the efficacy of the pre-emergent,” says Dr Preston. “Large droplets and high water rates are generally required when stubble is present to ensure the herbicide reaches the soil.”

Dr Preston’s trial work clearly demonstrated that planting equipment, such as single disc seeders, which do not remove soil from on top of the crop row, causes an increased incidence of crop damage from pre-emergent herbicides. He says there is a need to make some compromise in a zero till system to ensure soil containing the herbicide was thrown to one side as the crop seed is sown. “It is also important that the planter closes the seeding slot, particularly if a product such as Boxer Gold is to be applied post-emergent,” he says.

Sowing date is also important. Initial research suggests that sowing early, while the temperatures are still warm, with a fast growing crop and pre-emergent herbicide will suppress early weed germinations and any later germinations will occur under the crop canopy and be less likely to out-compete the crop.

“What we saw with later sown crops was that ryegrass was able to grow above the crop canopy and competition from the crop did not affect ryegrass seed head production,” he says. “This suggests that sowing weedy paddocks as early as possible with a pre-emergent herbicide could be a very useful tactic in helping to drive down weed seed production, particularly when harvest weed seed control is added to the system.”

To hear more from Dr Preston on his trial work with pre-emergent herbicides in the high rainfall zones of southern Australia, watch the ‘Setting crops up for success’ webinar recording available here.

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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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