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Advice from weed resistance expert: try diversity

Renowned weed scientist Steve Powles, having had to find solutions and work-arounds in resistant weed-infested crops in his native Australia, has tried to prepare American producers for their own burgeoning resistance problems.

In 2005, just as glyphosate-resistant pigweeds began to tighten their grip on fields in the American South, Powles cautioned U.S. producers against continuing practices that would only spread resistance.

“There is something Australia is No. 1 in the world at: herbicide resistance. We know about this problem and have the dubious distinction of being tops. However, the United States is about to take the top spot away from us. My prediction is you will be crowned king of herbicide resistance within the next few years.” (Read more here)

In summer 2013, Powles again visited the United States and his message took on a more urgent tone. Agronomic diversity is a must, he told Farm Press, if American producers are to head off massive cropland problems.

Here are comments from the Q&A:

It’s been several years since you were in the States. What have you been doing on this trip?

“I’m participating in seminars and the like. I began with a tour through the Texas High Plains, checked out the Arkansas pigweed situation and then headed to Illinois. That was a great opportunity to speak with many farmers, agronomists, crop consultants and university people.

I’ve spent a few days at the University of Illinois Champaign-Urbana and a day at a weed resistance symposium in Indianapolis.

“Looking at the situation in the field, talking with farmers — which is always great, because I have my own farm in Australia —and all the rest has made for a fantastic trip.”

A few weeks back, University of Illinois Weed Scientist Aaron Hager was saying Illinois’ biggest problem is still waterhemp but pigweeds are becoming a larger issue (read here). Have you noticed that shift?

“Yes. Two years ago, pigweed had become a real problem in the South. That has only spread farther afield.

“When I was visiting the Texas High Plains, they told me resistant pigweed wasn’t a problem in 2012. This year, though, it’s in as much as 50 percent of the fields.

“That’s just the exponential part of the curve, which states like Arkansas, Mississippi and Georgia have been through. Whereas, in the Midwest, the Corn Belt, the producers have had a different set of issues.

“I don’t want to overstate the situation, but on a bus ride from Chicago to Indianapolis — with many stops — you can really see a sprinkling of marestail and waterhemp through some 50 percent of the soybean crops. I’m prepared to claim that those weeds are glyphosate-resistant. Those aren’t just misses. I don’t remember seeing that several years ago.”

What about new soybean technologies?

“I’ve seen some of that. The 2, 4-D-tolerant soybeans are most impressive, actually.

“I hope none of the new technologies are clear ‘winners.’ I say that because if I was a U.S. farmer and could use Monsanto’s glyphosate and dicamba-tolerant soybeans, I’d use them. And then, I’d leave them alone for a few years. Same with Dow’s glyphosate/2,4-D-tolerant technology. Same with Bayer’s LibertyLink crops.

“Get the picture?

“These things offer diversity to the farmer. And they must be protected. Which producer wants to be in Delta Farm Press for being the first one with glufosinate/LibertyLink-resistant weeds or dicamba-resistant weeds or HPPD-resistant weeds?

Well, if they want that, if they want a photo in your publication, they can just keep planting the same thing over and over. The resistance will happen.

“Anyone who wants these technologies to keep going, if they want to keep a good thing, then they must change things up regularly.”

On resistance in Australia…

“We’ve learned the hard way that you cannot simply rely on the next jug. It’s simply unsustainable — you waste the chemicals’ efficacy.

“I love herbicides. I reckon they’re the best way to control weeds in cropping. But they aren’t much good if they don’t work.

“And the only way to keep them working is diversity. That means employing non-chemical techniques as well as smart rotation of chemicals and crops.

“We’ve learned that we must do ‘harvest weed seed control.’ That involves non-chemical, mechanical techniques at harvest.

“When I suggest that in the United States, everyone looks at me and thinks, ‘I’m not doing that.’ I can see it in their eyes, I can see them working it over in the minds: ‘We won’t do that.’

“But you know what? They’re already doing that when they march the hand-hoeing teams across the cotton fields. What they’re doing is practicing very, very expensive harvest weed seed control!”

On the objective of his trip…

“I’m here to help convince everyone involved in agriculture that they should think about harvest weed seed control. But it’s tough because people simply aren’t psychologically ready for that. All the creativity is still aimed at reaching for the next product.

“I hope everyone doesn’t have to learn hard lessons before considering these other techniques.

“But let’s not get depressed about this. It doesn’t have to be this way. We can manage the problems if we’d just be more diverse and pay more attention to agronomy and rotation.

“We wouldn’t be farming in Australia if what I’m saying wasn’t true.

“Fortune favors the prepared mind. You must actually do something more creative than reaching for the next jug.”

Is that a uniquely American mindset?

“No. It’s simply due to the fact that, until recently, there have always been new herbicide solutions.

“You know, ‘There’s a problem? Well, let’s just move on to the next product.’

“That’s been the experience of nearly everyone in our industry. Well, those days are coming to an end and change will come because it will be forced. Multiple herbicide resistance in waterhemp, pigweeds, marestail and johnsongrass is no joke. Control is not easy.

“However, control can be achieved with real attention and effort. I’m afraid without that focus, farmers will go bankrupt.”

On the ‘zero tolerance’ efforts (more here) in the Mid-South…

“I’ve been following that. That’s exactly the sort of thing that’s needed.

“I met with an excellent farmer, David Wildy, of Manila, Ark. On his land, he has very low pigweed numbers. That’s due to zero tolerance, doing what he must. He’s demonstrating success.

“And I’m willing to bet that if you talk to him, he’ll say zero tolerance is a pain in the neck. But he’s done it and made a success of it.

“I was speaking to another Arkansas farmer and said, ‘You can get on top of those Palmer pigweeds.’ He said, ‘Steve, at the moment, pigweed is like my mistress. I think of her when I wake in the morning, throughout the day, and then just before I go to sleep.’

“I thought that was pretty good. But that’s the focus you need to control a formidable opponent.”

On the importance of dealing with the soil’s seed bank…

“Say you’ve got a big cotton field, a Roundup Ready variety, and you’ve got a grown-up mess of glyphosate-resistant weeds. In that situation, simply switching to an alternative technology is a bad idea.

“What you must do is get your seed bank of those resistant weeds low. It seems that many people don’t understand the concept of a seed bank. Get the numbers low enough and your precious herbicides are much more sustainable.

“If farmers would understand that and practice it, they’d be much better off.”

This article was published online at Delta Farm Press.

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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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