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How well have your knock-downs worked?

May, 23rd 2014

Now is the time to be looking for weed ‘survivors’ and ‘escapes’.

Returning to paddocks you have recently sprayed with knock-down herbicides such as glyphosate to check for any plants that have not died is critical as these ‘survivors’ may be resistant to glyphosate.

Dr Chris Preston, Chair of the Australian Glyphosate Sustainability Working Group, has seen the number of confirmed sites of glyphosate resistant weeds climb at an alarming rate throughout Australia and is urging growers to act now to prevent seed set of weeds that have survived treatment this year.

Ryegrass_surviving_glyphosateWith the number of confirmed sites and species of glyphosate resistant weeds climbing at an alarming rate throughout Australia, Dr Chris Preston is urging growers to act now to prevent seed set of weeds that have survived treatment like these annual ryegrass plants.

“Glyphosate remains one of Australia’s most useful herbicides,” said Dr Preston.

“To lose it as an option in a weed management strategy would have a significant impact on farming systems around the country.

“There are now confirmed cases of glyphosate resistance at 757 sites across Australia,” he said.

“In the eighteen years since the first confirmed case of glyphosate resistance in annual ryegrass there has been a rapid rise in the number of confirmed cases, not only of annual ryegrass but also in another seven weed species.”

Wild radish, sowthistle and red brome are the most recent species to have been added to the list of weeds resistant to glyphosate. Unfortunately, researchers expect more species and more populations to be confirmed as resistant over coming years.

To keep track of confirmed cases of herbicide resistance the Australian Glyphosate Sustainability Working Group has set up an online, searchable database. Growers, advisors and researchers can use the database to better understand the extent of the problem in different regions and with different weed species.

“Some plants have a natural tolerance of the glyphosate mode of action,” said Dr Preston.

“This is very different to the processes involved with the development of glyphosate resistance.

“Resistance occurs when the one mode of action is repeatedly used against the same weed population,” he said.

“But if survivors are consistently and thoroughly removed then the resistant seed bank can be contained and managed.”

Dr Preston suggests that growers should be on the look-out for weeds that have survived any herbicide treatment.

“There are several reasons why a herbicide spray might fail to perform as expected but if individual plants or small patches are found after an otherwise-effective spray, it pays to have the plants tested for resistance.

“Testing will confirm if herbicide resistance is the problem and will also identify chemical groups that the plants are still susceptible to,” he said.

“This is important information because the quicker you can react and use another control method to eliminate the survivors the less long-term damage is done.

“It only takes a few seasons of ignoring survivors for herbicide resistance to take hold and to become an expensive problem,” he said.

After checking for and dealing with any survivors this season, the next step is to consider why herbicide resistance might have occurred and to decide how you might change your approach to weed management next season.

“Weed management during the fallow holds the key to reducing the impact of weeds in-crop,” said Dr Preston.

“Double knock tactics and rotating knockdown chemistry is a good place to start in the fallow. Follow this with setting up your winter crops to be as competitive as possible.”

Crop rotations can be devised to increased the number of herbicide modes of action being applied across the farm and harvest weed seed control tactics help to minimise the number of seeds entering the seed bank going into the fallow.

For more information on managing the risk of herbicide resistance, see our 10 Point Plan. For more information on the ‘Quick test’ for herbicide resistance, visit the Plant Science Consulting website.

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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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