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Acting early pays off in the war on weeds

If nothing else, herbicide resistance is predictable, but efforts to prevent seed set do pay off, particularly if action is taken early.

NSW DPI weeds specialist Tony Cook says that growers are becoming increasingly aware of the potential impact of herbicide resistance on their farming operations and they are seeing the benefits that come from early and decisive action.


NSW DPI weeds technical specialist, Tony Cook, has documented the strategies used on four farms to successfully patch-manage resistant weeds using a variety of weed control tactics over a period of at least five years.

Many factors may influence the process but if an individual plant that is resistant to a herbicide sets seed, it is only a matter of time before a small number of resistant plants becomes an ever-increasing weedy patch.

Mr Cook says that growers can build farming systems that have a level of ‘immunity’ to herbicide resistance. “If growers concentrate on minimising or preventing weed seed set, they can win against herbicide resistance,” he says. “The trick is to use a variety of means to keep weed numbers low and to keep pressure on seed set. If resistant plants are prevented from setting seed, then the problem is contained.”

“Eight out of the twelve glyphosate resistant species in Australia are present in weed populations in NSW and Queensland, particularly near the state border,” says Mr Cook. “In this region resistant awnless barnyard grass, annual ryegrass, liverseed grass and feathertop Rhodes grass are the main problems.”

Mr Cook interviewed four northern region farmers to find out what strategies they had used to successfully manage patches of these four glyphosate-resistant weed species. The take-home message from these four farmers was that growers can drive down weed seed banks through a strong commitment to consistently preventing weed seed set for at least five years. “The costs associated with treating the patches is an additional expense but this pales in significance against the cost of doing nothing and allowing the patches to spread across paddocks and beyond,” he says.

More details about the strategies used on each case study property can be found in Mr Cook’s webinar presentation with AHRI’s Peter Newman.

While glyphosate resistance is becoming more widespread across the region, many resistant populations are still in patches up to one hectare in size. “This gives growers the opportunity to use paddock-wide tactics combined with more intense patch management in problem areas,” says Mr Cook. “Some growers have been successful in completely eradicating herbicide resistant plants from patches of one hectare or less through very focused efforts to prevent seed set.”

Strategies such as full cultivation, optical weed detection, brown manuring, double-knocking, strategic use of pre-emergent herbicides, using livestock as the second knock and close attention to the removal of survivors have all been used in different situations to treat patches of resistant weeds. While some of these strategies are often applied across the whole paddock, some can be directly applied to the patch to minimise the cost while maximising effect.

Glyphosate resistance in weeds is becoming widespread across the northern grains region, particularly around the NSW Queensland border, however these resistant weeds are often found in small patches, less than one hectare in size.

Glyphosate resistance in weeds is becoming widespread across the northern grains region, particularly around the NSW Queensland border, however these resistant weeds are often found in small patches, less than one hectare in size.

“A shift in cropping rotation and well-timed use of paraquat in place of glyphosate is another useful strategy to drive down weed numbers for these key species,” says Mr Cook. “While spot spraying can be a good option it is easy to miss the outlier plants in a small patch. The optical sprayers can give better coverage of the weeds and often involve an alternative herbicide to glyphosate. Including a second knock to remove survivors is key to success with this and all other weed control tactics.”

“Pre-emergent herbicides are being used to good effect in the fallow, provided that growers are very aware that these herbicides generally achieve only 80 to 95 per cent weed control,” he says. “To gain any benefit from these herbicides it is very important to manage the small number of weeds that are likely to survive the pre-emergent application. There are quite a few herbicide modes of action that have a residual activity and each needs to be applied correctly to achieve the best results.”

“We know that routine use of glyphosate every year in the fallow will cause glyphosate resistance in the weed population within 15 years if no follow-up action is taken to remove survivors,” he says. “This has been demonstrated in trials and is evident in the field. It may then take another five or six years for the glyphosate resistant weeds to dominate in a paddock, again if no follow-up action is taken.”

The spread of resistant weeds across a paddock or around the farm is quite easily done through movement of vehicles, machinery, people and animals as well as wind and water flow across the paddock or along irrigation channels.

Mr Cook says being aware of new weeds on the farm, such as feathertop Rhodes grass deposited on farms around Dalby during flood events over the last few years, growers can get on the front foot with effective tactics such as double knocking before the new weeds are firmly established.

“Glyphosate resistant patches are frequently associated with fencelines and other non-crop areas on the farm and can spread into cropping fields,” says Mr Cook. “Identifying alternate strategies for managing these areas needs to be a high priority on all farms, even if glyphosate resistance is not yet evident.”

Want more? Check out Tony’s webinar and GRDC update paper.

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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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