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Diversity extends herbicide ‘life’ in triple-stacked cotton

‘Diversity’ is one of those ‘how much is good enough’ things. It is often recommended that farmers use ‘as much diversity as possible’ in their weed management program to keep a lid on herbicide resistance, but it is difficult to quantify how much diversity will achieve adequate control.

Computer models, like the new ‘Diversity’ model, have proven to give reliable predictions of the real-world outcomes likely to result from the implementation of different weed management programs.

The new Diversity computer model has enabled researchers to test the effect of different weed control programs and scenarios on herbicide resistance in three key weed species, with the aim of prolonging the effective life of the triple-stack herbicide gene technology in irrigated cotton systems.

The Diversity model tracks the simultaneous evolution of resistance to multiple herbicides, using multiple genetic pathways, in several weed species at once.

With triple-stacked herbicide tolerance traits in genetically modified cotton expected to be available to Australian growers within the next five years, the Australian Cotton Research and Development Corporation has invested in research to determine how much diversity in control tactics is required to protect the effective life of this technology.

The model suggests that using more diverse strategies in weed control can add 20 years to the effective ‘life’ of this new herbicide tolerance technology. Modelling repeatedly shows that new technologies must be supported with several other herbicide and non-herbicide tactics and survivor management given the highest priority.

Bayer’s XtendFlex™ technology confers tolerance to glyphosate, dicamba and glufosinate in cotton and was approved for commercial release by The Office of the Gene Technology Regulator in December 2016. Prior to commercial release, extensive work is underway to understand the system’s fit in the unique Australian environment and to ensure growers will get the most from the technology.

XtendFlex™ cotton varieties are stacked with the Bollgard® 3 insect resistance technology, and are expected to provide growers with a robust pest and weed management tool.

The concern for weed scientists like Dr David Thornby, Innokas Intellectual Services, is that the triple-stack of herbicide tolerance is already compromised, with glyphosate resistance well-established in several weed species on many cotton farms. To-date, the problem is greatest in dryland cotton farming systems, but is also quite prevalent in non-crop areas of irrigated farms, such as along irrigation channel banks.

Dr David Thornby suggests that growers should not put off implementing the 2+2&0 strategy in the hope that XtendFlex cotton varieties will fix their weed problems when the new technology is released in Australia in a few years.

Having previously used computer modelling to assist in the development of the cotton industry 2+2&0 weed control strategy, Dr Thornby has led a team to develop a model to test how many tactics growers will need to implement to achieve effective control of three key weed species – sowthistle, flaxleaf fleabane and awnless barnyard grass – once the XtendFlex technology is adopted.

“Real-life experience aligns with the predictions made using the DAF Glyphosate Resistance computer model, with glyphosate resistance being evident in awnless barnyard grass within 13 years of commencing zero tillage, if glyphosate is the only product used for summer weed control and survivors are not controlled,” he said.

“We also predicted that common sowthistle populations would exhibit resistance within 15 years if glyphosate was the only product, or 20 years if a few other tactics were implemented.”

“We managed to predict that sowthistle would lag behind barnyard grass by a few years, under current and historical management strategies, and that has been borne out in the real world. This suggests that the computer models do provide reliable predictions of the speed at which herbicide resistance develops in weed populations, so we have built on this earlier work to develop the ‘Diversity’ computer model.”

Using this new model has enabled researchers to test the effect of different weed control programs and scenarios on herbicide resistance in these three key species, with the aim of prolonging the effective life of the triple-stack herbicide gene technology in irrigated cotton systems.

“Using the model we can show that just using the three over-the-top herbicides in triple-stacked cotton crops will lead to a failure of the technology to control awnless barnyard grass within 10 years,” he said. “This is because of the already wide-spread incidence of glyphosate resistance in this species, and the fact that glufosinate is only marginally effective on this species and dicamba is not effective at all.”

“If a grower also implements the 2+2&0 best practice of two other practices – cultural or different herbicide modes of action – in both the crop and fallow, and zero tolerance of survivors, we can extend the effective life of the technology to control awnless barnyard grass and sowthistle to 30 years,” he said. “For example, the use of the triple stack partners in a double knock tactic at least some of the time, including a pre-emergent herbicide and chipping survivors is a practical and effective control program to support the over-the-top herbicide options available with XtendFlex.”

“Of the three key species studied so far, flaxleaf fleabane is expected to be an on-going problem for cotton growers,” said Dr Thornby. “Glyphosate resistant fleabane can be hard to control with the XtendFlex herbicide options and the model suggests that at least two additional tactics would need to be applied every year to keep numbers low. Gaining control is challenging and given the amount of seed produced on these plants, even achieving a kill rate of 95 per cent is not sufficient to manage the seed bank.”

Flaxleaf fleabane can germinate over a wide portion of the year, putting strong pressure on pre-emergent herbicides as well.

To-date, the Diversity model has only been used to test scenarios in irrigated cotton systems but plans are in place to test-run dryland cotton scenarios ahead of the release of XtendFlex cotton varieties in Australia. The model also has the capability to investigate the effect of including other crops in the rotation to assist with weed control.

The bottom line is that the triple-stack herbicide tolerance gene technology will be a useful tool for cotton growers but it is not a stand-alone weed control program. It must be supported with several other herbicide and non-herbicide tactics and survivor management given the highest priority.

Dr Thornby said growers should not put off implementing the 2+2&0 strategy in the hope that XtendFlex will fix their weed problems. It is possible to use intensive patch management to reduce the numbers of glyphosate resistant weeds, particularly awnless barnyard grass, and doing so will give growers a better starting point to maximise the effectiveness of the triple-stack herbicide technology when it is released.

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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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