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Rising significance of resistant barley grass

Barley grass has a number of tactics up its sleeve to help evade both herbicide and non-herbicide weed control methods. This has made it a weed of interest for the University of Adelaide’s Weed Science Group, led by Associate Professor Gurjeet Gill, who are investigating the ecology of emerging weeds in the low rainfall zones of southern Australia

University of Adelaide researchers Dr Gurjeet Gill and Ben Fleet say that understanding weed ecology and undertaking herbicide screening will help find ways to manage increasing resistance in barley grass.

With investment from the GRDC, Dr Gill and his team analysed the dormancy traits and herbicide resistance status of 146 random samples of barley grass collected by agronomists in WA, SA, Victoria and NSW in 2018.

Of the 146 random samples collected, five per cent of populations showed resistance to Group A herbicides and 21 per cent showed resistance to Group B herbicides. There was no evidence of resistance to glyphosate or paraquat.

There were also large differences between the populations in the level of seed dormancy as seen by the timing of seedling emergence in autumn. Barley grass populations from the Eyre Peninsula in particular were much slower to establish than those from other low rainfall regions. Late emerging weeds can escape weed control with knockdown herbicides.

In a later study, growers from the Eyre Peninsula Agricultural Research Foundation (EPARF) collected samples of barley grass seed in 2019 from paddocks where growers had experienced difficulty in controlling barley grass with herbicides. These samples were tested for resistance screening in 2020.

Resistance screening of barley grass from suspect paddocks on the Eyre Peninsula, SA.

“The expectation was that most populations from the targeted survey in 2019 would be resistant to Group A herbicides,” says Dr Gill. “Resistance to the Group A herbicides was confirmed in 17 of the 22 populations from EP, or 77 per cent. Within this Group, resistance to quizalofop was 100 per cent for the suspect populations while there remains some useful activity from clethodim and butroxydim, which will help the growers in the short term.”

Herbicide resistant barley grass shows no response to a high rate of the commonly-used Group A herbicide, quizalofop (right) compared to a plant from a susceptible population (left).

The same populations were also tested with Group B imidazolinone chemistry, which offers some activity against Group A resistant barley grass, although one of the EP populations was completely resistant to the IMI herbicide. The good news is all of these populations remain susceptible to glyphosate and paraquat.
Dr Gill says that research and field observation confirm there is significant variability in barley grass populations’ ecology and herbicide resistance status.

“Understanding how different barley grass populations behave is key to their management,” he says. “The seed dormancy and seed shedding traits of a population have important implications in terms of management options. Barley grass often evades pre-emergent herbicides through delayed emergence and at the other end of the season barley grass often sheds its seed before crop maturity, so harvest weed seed control is rendered ineffective in many circumstances.”

Barley grass is susceptible to strong crop competition, and on mixed farms Dr Gill says some farmers have had success using pyroxasulfone herbicide in wheat ahead of a pasture phase, where good grazing management can limit seed production in barley grass.
Applying the WeedSmart Big 6 integrated weed management strategy to barley grass will keep herbicides working for longer and maximise the impact of cultural control tactics.

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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.  

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

About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm. Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed. Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems. In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed. Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak. Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands. If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks. Other resources It’s time for a glyphosate intervention Farm hygiene cottons on – Cleave Rogan, St George What’s new in management of herbicide resistant weeds on fencelines? Keeping the farm clean – Graham Clapham, Norwin Don’t jeopardise glyphosate for clean fencelines Keeping fencelines clean Resistance risk to knock-down herbicides on irrigated cotton farms

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