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Research confirms first glyphosate resistant wild radish

The world’s first populations of glyphosate resistant wild radish will be announced at Perth’s Agribusiness Crop Updates, but researchers stress further cases can be minimised if farmers adopt diverse control strategies.

Australian Herbicide Resistance Initiative (AHRI) research has confirmed glyphosate resistance in three populations of wild radish, all from different locations in Western Australia’s far northern grainbelt.

They were identified by Mike Ashworth, a recipient of a Grains Research PhD Scholarship from the Grains Research and Development Corporation (GRDC).

Mr Ashworth is based at AHRI, located at The University of Western Australia.

“Wild radish is arguably the worst weed in WA cropping systems and causes economic losses in 45 crop species in 65 countries,” he said.

“Glyphosate is the world’s most important knockdown herbicide and a vital tool in the control of wild radish, which has evolved resistance to many selective herbicides in WA and other cropping regions.”

Mr Ashworth said two of the glyphosate resistant wild radish populations were identified and tested after survivors were found in fallows treated with glyphosate.

“These populations were confirmed to have moderate levels of resistance, exhibiting high rates of survival (63 and 86 per cent) following label rate glyphosate application on two-leaf plants,” he said.

“The third population was identified following an AHRI survey in 2010 and 2011 of 239 paddocks in WA’s northern and central grainbelt.”

mike ashworth_ ahri _3_

Mr Ashworth said the glyphosate resistant wild radish plants also exhibited resistance to other important herbicides.

“The first two populations identified also had resistance to label rates of the Group B (chlorsulfuron, sulfometuron-methyl, metosulam, imazamox), Group F (diflufenican) and Group I (2,4-D amine) herbicides” he said.

Mr Ashworth said the history of herbicide use where the three populations were discovered was likely to have been a major factor in the evolution of glyphosate resistance.

“The first two populations are believed to have been exposed to at least one and often two glyphosate applications annually over two decades,” he said.

Mr Ashworth said the good news was that finding the populations early meant that growers had the opportunity to adopt pro-active control strategies.

“Herbicides alone should not be used to control wild radish; growers and agronomists should use a range of tactics to control wild radish populations,” he said.

“The aim should be to control weed survivors, eliminate weed seed set and maximise diversity of control strategies.

“GRDC funded research has proven the effectivenes
s of non-herbicide tools including crop competition and harvest weed seed control (HWSC) used as part of an integrated weed management approach.

“HWSC has been shown to be very effective for controlling wild radish, as this weed tends to hold on to its seed at harvest.”

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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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