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What herbicides still work?

A cheap herbicide quickly becomes very expensive if it is not effective.

Multiple resistance is beginning to show up in wild radish outside WA and testing is recommended if you plan to use certain herbicides again within the next three years.

Multiple resistance is beginning to show up in wild radish outside WA and testing is recommended if you plan to use certain herbicides again within the next three years.

Testing for herbicide resistance has played an important part in understanding how resistance occurs in field situations but now that researchers have a good understanding of the mechanisms involved it is becoming more important for growers to find out which herbicides are still effective in controlling target weeds.

John Broster, senior technical officer, Charles Sturt University (CSU) is recommending growers collect weed samples for ‘susceptibility testing’. Dr Broster heads up the herbicide resistance testing service at CSU and the data he has collected over the last 24 years has helped identify resistant populations of a wide variety of weeds across Australia.

“If a particular weed, say annual ryegrass, is present in reasonable numbers at harvest time, having survived in-crop treatment, say with a Group A (fop) or Group B herbicide, we can assume with some confidence that herbicide resistance is the reason for survival,” he says. “From a grower’s point of view then, it is not cost effective to test those plants to see if they are in fact resistant to Group A or Group B.”

“The big question is ‘what herbicides are those plants still susceptible to?’ and that is what we can help answer through susceptibility testing.”

Testing of suspected herbicide resistant ryegrass samples at CSU in 2014 suggest that if you think your ryegrass is resistant to Group A (fop) or Group B herbicides, it probably is and testing is not necessary. If you think it could be resistant to glyphosate, always have it tested. Annual ryegrass is almost always expected to be susceptible to atrazine, so only test if you think your weeds could be resistant. For all other herbicides it is worth testing if you are suspicious of herbicide resistant or cross-resistance.

Although annual ryegrass has a reputation for its resistance to important herbicide groups there are herbicides that are still effective options for annual ryegrass control. These include trifluralin, glyphosate, atrazine, propyzamide, Boxer Gold, paraquat, Sakura and Avadex.

Having several options still available is good news for growers and it is essential that these herbicide options are protected and incorporated into a diverse weed control plan that includes non-herbicide options.

Table 1. Low levels of herbicide resistance in annual ryegrass to a range of herbicides tested in 2014.


Herbicide group No of samples in 2014 % of samples resistance


D 452 3


M 403



C 375



D 278


Boxer Gold

E, K 75



L 65



K 17


Avadex J 4


For wild radish the story is similar. There are high levels of resistance to Group B (e.g. Glean, Logran), especially in Western Australia, so testing for resistance is generally not warranted, although in other states it may still be worthwhile. Resistance to Group C (e.g. atrazine), Group F (e.g. diflufenican) and Group I (e.g. 2,4-D ester 680) is beginning to show up and testing is recommended if you plan to use these herbicides again within the next three years. Similarly with glyphosate, testing is recommended if you suspect resistance. To date there is no resistance to pyrasulfatole (e.g. Velocity, Precept) in Australia, so only test if you are suspicious. For all other herbicides it is worth testing for cross resistance, particularly for herbicides you plan to use over the next few years.

Table 2. High levels of herbicide resistance in wild radish to commonly used herbicides tested in 2014.


Herbicide group No of samples in 2014

% of samples resistance

Glean / Logran

B 130



C 158



F 128


2,4-D ester 680

I 114



M 122


**This sample is being re-tested to confirm the result.

For wild oats, if you suspect Group A (fop) resistance you are probably right and testing is unnecessary. Most samples are susceptible to atrazine (Group C) and glyphosate, so only test if you suspect there is a problem. There is a 50:50 chance of samples being resistant to Axial or Achieve so testing for susceptibility is worthwhile if you plan to use these herbicides over the next few years. For all other herbicides you plan to use over the next few years it is worth checking for cross resistance to determine which products the wild oats is still susceptible to.

Table 3. High levels of herbicide resistance in wild oats to commonly used herbicides tested in 2014.


Herbicide group No of samples in 2014

% of samples resistance

Fops (Wildcat, Verdict, Topik)

A (fops) 53



B 49



A (dims) 11



A (dens) 30



Z 9


CSU researcher Dr John Broster is encouraging growers to collect samples for herbicide susceptibility testing next season.

CSU researcher Dr John Broster is encouraging growers to collect samples for herbicide susceptibility testing next season.

Dr Broster urges growers to not be complacent about herbicide resistance in these weeds. “The situation is becoming increasingly serious for a wide spectrum of weed species,” he said. “These results suggest that growers have time to implement herbicide and non-herbicide strategies to avert a crisis situation, but immediate action is required.”

In preparation for next season Dr Broster suggests growers allow for susceptibility testing in the budget and be prepared to collect samples at harvest, even carrying sample bags in the header if possible.

To have samples tested simply collect seeds or seed heads from the weeds of concern, often those present at harvest. Place the sample into a paper bag inside an A4 envelope. Double bagging the seeds protects the sample. Don’t use plastic bags because the seeds can easily become mouldy if there is any moisture in the bag. Post the samples to either of the herbicide resistance testing services—Graham Centre for Agricultural Innovation at CSU, or Plant Science Consulting at University of Adelaide.

For ryegrass, collect about a coffee cup full of clean seed or an A4 envelope full of seed heads. For wild oats or wild radish, a comfortably full A4 envelope of seed heads or pods is ample. Record information on the sample bag about where the sample was collected and what herbicides you would like the sample tested against.

Make sure the sampling occurs across the area of interest, not just at one spot. If you are interested in the entire paddock you need to sample the entire paddock, if you are looking at the area around a blow-out then sample throughout that area.

For more information on herbicide testing services visit or

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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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