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Herbicide resistance is manageable, says Australian expert

GREENSBORO, N.C. — Long before herbicide-resistant weeds were making headlines in the U.S., Australian wheat producers were taking steps to solve the problem.

Stephen Powles, weed expert and professor of plant biology at the University of Western Australia and the director of the Australian Herbicide Resistance Initiative, recently spoke at Syngenta’s headquarters on diverse weed management options.

“Basically what’s playing out here in the U.S. is similar to what played out with in Australia some time ago,” he said. “It wasn’t glyphosate. It was a different set of circumstances, but big-time resistance occurred in Australia from 1990 or so on.”

The herbicide resistance in Australia came about when the country was transitioning land from primarily large sheep farms to crop production.

“Wool was the big industry. There were a lot of sheep, and the profitability went out of that when people stopped wearing wool,” Powles said. “We planted ryegrass coast to coast when the sheep was king.”

As the wool industry declined, wheat became the major crop and high densities of ryegrass had to be removed.

Powles said to transform the pastures to cropland, one herbicide chemical was used “with no diversity in the system — and, of course, you get an evolution of resistance in the ryegrass.”

“As you contrast that to the U.S., it’s where the whole south and the Midwest got covered in one great big glyphosate field,” he said.

“So we got big-time resistance in the 1990s onward, and it has multiple resistances across several herbicides, so you couldn’t just reach for another jug to fix the problem. We just had to change our ways, and the U.S. farmer hasn’t learned that yet.”

In an interview with AgriNews , Powles said there were multiple strategies Australian farmers used to control resistant weeds.

“The first thing is the herbicides remain the single-best tool. The herbicide is the bit of crucial technology, but you can’t just rely on it, so we had to diversify our system,” he said.

“We had to never keep using the same chemical. We had to rotate crops. And we had to put some non-chemical tools in there that made sense.”

Diversity and not relying on any one herbicide are recommended, but make sure the strategies are within an economic reality.

“You might like to do something, but if you can’t do it profitably, you can’t do it,” Powles said. “As an example, we always use a pre-emergent residual herbicide — those that still work. We always use a burndown, but we wouldn’t rely just on glyphosate. We’d rotate it.

“The big thing that we do is at the end of the season we have some techniques to kill weed seeds during harvest time. That’s something no one does here.”

Harvest weed seed control provides an opportunity to target future weed populations. Problematic weed species are prolific seed producers capable of establishing a large viable seed bank in just one season.

However, very high proportions of weed seed are retained in upright stems and tillers of the weeds at crop maturity. This creates the potential to target these seed during harvest, thus restricting the inputs to the weed seed bank.

One method of weed seed control is with a chaff cart towed behind the headers during harvest to collect the material as it exits the harvester. The material is then either burned after harvest or used as a feed source for livestock.

Another option used in Australia is the Harrington Seed Destructor that collects the chaff behind the combine and destroys any weed seeds present. The nonviable weed seeds the exits the harvester.

“There are a range of techniques that we do to try to stop the weeds from producing seed,” Powles said.

Another option would be returning to the days of walking fields with hoes. During his recent visit to central Illinois, Powles said he heard of hand crews performing their own harvest weed seed control by chopping weeds at a cost of between $30 and $100 an acre.

“We have a range of mechanical ways of doing that,” he said.“I’m sure U.S. farmers are going to become much more familiar with all those sorts of things over the next few years.”

The key to all weed management is multiple modes of action and more.

“Don’t get me wrong. I love herbicides. I think herbicides are the absolute best way to control weeds. But they’re not much good when they don’t work,” Powles said.

“So the only way to insure they continue working is to use as much diversity as makes economic sense. I find that here in the U.S. pretty much the creativity is confined to one herbicide can I fix this problem with.

“‘Oh, I’ve got a problem. I have glyphosate resistance.’Well, what herbicide can I use to fix the problem when in fact the better question is how can I make my herbicide use sustainable in the long term.

“That just involves thinking about all of the possibilities. What good agronomy can you do? We keep seeing these very wide row soybeans. I bet the weeds love that.

“Some agronomic things will have a big impact. One of the problems right now is that the U.S. farmers don’t fear the weeds. They don’t have much respect for them. Well, a couple million years of evolution and they’re a pretty formidable opponent.

“I’ve learned to respect these weeds. You get something like waterhemp or pigweed, it’s a formidable opponent. You better be using all the control tools at your disposal if you want to get on top of plants like that.

“You should fear them and respect them because if you use any single tool against them, they’ll overcome it.

“I just spent a week in Illinois. What fabulous crops. What fabulous cropping country. What fabulous soils. They’re not going to stop farming, but they’re going to have to be more creative than they have been. And they can do it. That’s the main thing.

“Resistance is entirely a manageable problem. No need to get depressed. Just get on it and start handling it and don’t just rely on the next chemical.”

The original article was published by Agrinews.

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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 Adjuvants 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.

Clean borders – avoid evolving resistance on the fence line

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