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Stark reality of herbicide resistance demonstrated

Following a survey of weeds present at harvest last year, grain growers in central NSW were stunned at the severity of herbicide resistance in the district.

Survey organiser and Grain Orana Alliance (GOA) CEO, Maurie Street said awareness and concern over herbicide resistance had been mounting in the lead up to the survey, although he expected only 30 or 40 samples to be submitted for testing through the GRDC-funded survey.


“The response was overwhelming, with over 120 samples of annual ryegrass, black (or wild) oats and wild radish submitted for testing in just two days,” he said.

“Growers and their advisors were obviously very concerned and wanted to know how widespread and severe the problem was in the district.”

The weed survey was by no means random and the results are skewed toward defining the worst case scenario because the weeds that are present at harvest time are the most likely to have survived herbicide application and therefore the most likely to have some herbicide resistance.

“Proving that these samples were resistant was not really surprising,” said Mr Street.

“What we identified, and what has rocked many growers’ perceptions, is the extent of resistance to multiple herbicide groups in the weed populations present in the district.”

All 78 samples of annual ryegrass submitted and tested against 6–7 mode of action (MOA) groups were resistant to at least one MOA group. Alarmingly, the vast majority (87 per cent) were resistant to two or more MOA groups and 54 per cent were resistant to four or more MOAs, leaving very few herbicide control options available to growers.

“These results suggest that single resistance is rare and the survey also uncovered a number of cases of ryegrass populations being resistant to herbicides that had never been applied on the paddocks where the seed was collected,” he said.

“This phenomenon is well-known and highlights that hygiene is important to prevent introducing resistant weeds by accident and that resistance to an MOA can occur as a result of resistance existing to an alternate MOA.”

With half or more of the herbicide options lost to growers, Mr Street was particularly concerned about the samples that tested resistant to glyphosate. He said that although the percentage of samples resistant to Group M products was very low, about 6 per cent, these products are an essential component of zero-till farming and losing them would have wide-reaching consequences.

The story was not as severe for the 41 black oats samples submitted for testing against four herbicide MOA groups. All samples were suspected to have herbicide resistance but 25 per cent were still susceptible to all MOA groups tested. The remaining 75 per cent were resistant to one or more MOA group and half of these were resistant to two or more groups.

“It is generally accepted that black oats are slower to develop resistance but with fewer MOA groups available to control black oats compared to annual ryegrass, this level of multiple resistance is worrying,” said Mr Street.

While the problem can no longer be denied, Mr Street said these results do not spell the end of cropping in the district.

“There are non-herbicide options available to growers that have proven their effectiveness against herbicide resistant weeds,” he said.

“On my own farm, we have high levels of resistance to multiple groups but have been able to turn the situation around within a few years to get back on the front foot.”

Mr Street uses a combination of narrow windrow burning after harvest, high sowing rates and pre-emergent herbicides to turn up the heat on weeds and reduce the amount of seed returning to the soil seed bank each year.

“We realised that we couldn’t ‘rotate our way out’ of the situation and had to take decisive action,” he said.

“There has been strong interest in harvest weed seed control measures, like narrow windrow burning, to start the campaign against resistant weeds and I expect wide adoption of non-herbicide tactics over the next few years.”

This grassroots project demonstrated the value of testing suspect weeds. Compared to the cost of applying an ineffective herbicide, testing is relatively cheap and gives clear direction to the grower regarding the control options available to them.

Budgeting for a few resistance tests each year will provide valuable management information for planning rotations and selecting chemicals.

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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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