Resistance testing knowledge is power


Written by: Peter Newman

In 1597 Sir Francis Bacon said,

“Knowledge is power.”

Then a heap of people stole his quote and changed it, like motivational speaker Tony Robbins, who said,

“Knowledge is not power. Knowledge is only potential power. Action is power”.

And then a heap of people stole Tony’s version of the quote and so on…

Dr Roberto Busi has jumped on this bandwagon with his offering of,

“Resistance testing isn’t power. Changing your herbicide application as a result of your resistance test is power”.

Okay, he didn’t really say that, I made it up, but he should have said it!

Roberto is a researcher at AHRI and also runs a herbicide resistance testing service on the side. He teamed up with agronomist David Pfeiffer and AHRI director Ken Flower to deliver some key messages that came out of the 2023 annual ryegrass resistance testing campaign data, such as:

  • Glyphosate resistance is getting worse, now in at least 10% of paddocks in WA surviving a robust rate (2L Glyph450/ha equivalent).
  • Paraquat resistance was confirmed in 2023 again after the first cases were reported in 2022 for WA.
  • Trifluralin resistance isn’t as bad as it should be.
  • Clethodim resistance is also perhaps not as bad as it should be.
  • Use fresh clethodim, and mix it with glyphosate in glyphosate-tolerant canola.
  • Some pre-emergent herbicides are in trouble, while others are going strong.
  • Resistance testing is awesome! You can learn about your resistance levels and also learn from people who analyse the results of 350 tests to come up with the best strategy for action.

In his spare time, when he is not running or playing trumpet, Dr Roberto Busi runs a herbicide testing service, offering a test with over 20 herbicides and mixes for annual ryegrass screening. This AHRI insight features the results of 350 ryegrass samples taken during harvest 2022 in Western Australia. Keep in mind that this is not a random survey, rather it is samples from paddocks where growers and agronomists suspected a resistance problem.

Dave Pfeifer is a Western Australian agronomist from Synergy Consulting who has worked closely with Roberto in recent years, giving him some insight into the practical implications of the resistance test results.

We’ll go through the products one at a time, in approximate priority order, starting with the knockdowns.

Glyphosate resistance is in 10% of paddocks. Survival from a usually lethal dose of glyphosate (900g ai/ha) ranged between 6% and 78%! Of interest, there was no substantial difference between the hi-load triple salt Crucial®600 and standard 450ai Knockout® applied at the same rate of a.i..

Paraquat (2L/ha) is still working across WA with less than 1% of any resistance.

However, a couple of populations of paraquat and glyphosate-resistant ryegrass have been confirmed in Western Australia and there are populations in other states. Monitor fence lines as this is where it is found and contact us if you suspect something.

If you ever find yourself spraying paraquat alone on ryegrass, be it on a fenceline or in a paddock, you need to ask yourself why. This is our last reliable knockdown herbicide for many paddocks in Australia, and spraying it alone is the fastest track to breaking it.

The interesting bit you need to be aware of is that paraquat activity can be much increased when mixed with a residual herbicide from groups 13, 14, 15. Our 2023 research on tough-to-kill ryegrass populations coming off fence lines evaluates how to best control dual-resistance glyphosate and paraquat ryegrass and has outcomes we can immediately adopt in the paddock. In a nutshell, the best options for glyphosate and paraquat-resistant ryegrass is switching to paraquat mixed with BoxerGold®, Mateno®Complete, Overwatch®, Sakura® + Terrad’or®, propyzamide (Rustler®/ Edge®/ Dargo®) or Voraxor®. The residual pre-emergent product is helping to do the “heavy lifting” to limit resistance.

Is Liberty® containing glufosinate the answer? The answer, at this stage, is no.

The two sequential sprays, of 2L each applied 14 days apart, resulted in an unacceptable 20% of samples not fully controlled. However, as soon as combined with Terrad’or® followed by glyphosate or when Liberty is mixed with clethodim (Nitro®) and atrazine the outcome can dramatically improve with 97% and 98% controlled (i.e., sensitive) respectively. Of interest, tank mixing with something is the strategy recommended when using glufosinate in the new stacked Invigor®LT4530P (Liberty® Triazine) and Invigor®LR4540P (Liberty®Truflex®) canola varieties. In 2024, the UWA resistance testing program will check the efficacy of Liberty + glyphosate mixes. Watch this space.

Knockdown strategy

If you have some glyphosate resistance, consider applying glyphosate alone at a high rate followed by paraquat plus a group 14 (G) plus a pre-emergent herbicide. Even better appears to be a double paraquat strategy which is also becoming more common in problem paddocks, and the mixing partners are imperative to maximise the useful life of this practice. Time to give glyphosate a rest and use paraquat mixed with a residual product!

Clethodim – 16% of paddocks are demonstrating low level to field resistance levels. Previous surveys have shown much higher levels of resistance.

Roberto has recently discovered the importance of using fresh product every year when it comes to clethodim. His early research shows that it degrades in the drum (more to follow soon). There is a chance that past resistance surveys were impacted by using old product.

The combination of clethodim and glyphosate, now considered “the norm” in the first RR spray, is clearly beneficial. 99% of the paddocks are sensitive to the combination.

Let’s break that down a little. Clethodim alone – 16% of paddocks with resistance. Glyphosate alone – 10% of paddocks with resistance. Mix the two together – only 1% of paddocks with some survivors.

This is great news provided we remind ourselves not to use this on fallows or knockdown sprays, as it seems the longevity of this practice in canola may rely on the crop competition effect of the crop against weakly competitive glyphosate-resistant annual ryegrass.

Pre-emergent treatments

Luximax® continues to remain bulletproof from a resistance perspective with 100% of samples tested being sensitive. The adoption of long coleoptile wheat reducing crop safety concerns is likely to aid in the adoption of Luximax®.

Ultro® (carbetamide) and Propyzamide remain very effective with 100% sensitivity. Ultro® is an option limited to our pulse crops only. Propyzamide was not tested in this survey, but other research shows that it remains a reliable performer.

Sakura® at the label rate was of concern in less than 1% of paddocks with 12% of paddocks showing early signs with low resistance. The challenge now with it being off-patent and becoming much more attractive in terms of $/ha is to keep it effective as long as possible! Mixing Sakura with another herbicide will delay resistance.

Mateno® Complete as an early post-emergent at 1L/ha was very robust. However, 9% of samples did demonstrate a low level of resistance (similar to Sakura®), so this is one to keep an eye on.

The “don’t cut the rate” message is well and truly relevant here if you check resistance at 0.75L/ha rate of Mateno® Complete pre-sowing.

Trifluralin amazingly continues to perform with 93% of the samples being sensitive. The test clearly makes trifluralin perform at its best with high activity when applied directly to the ryegrass seed. Of note, there was a rate response from 1.5 to 2L/ha highlighting it is worth aiming for a 2-2.5L/ha rate where the seeding system allows.

Overwatch® standalone showed 3% of samples with high levels of resistance and 18% with low/questionable levels. Further investigation is required to monitor Overwatch resistance in the next few years. In combination with trifluralin, the outcome improved and in conjunction with atrazine, Overwatch® was one of the top options tested. It is a good idea to mix Overwatch® with another herbicide for resistance management and the atrazine partner is making the TT canola option rather simple. Notably, Overwatch® was effective on Sakura® resistant annual ryegrass which will be a focus of further work in 2024.

Prosulfocarb (Arcade®) and Triallate (Avadex®), similarly affected by resistance, may be running out of legs and delivering subpar control of annual ryegrass. Previous research has demonstrated cross-resistance between these two products. Greater than 1 in 3 paddocks exhibited developing to high-level resistance for both products. Given the high level of prosulfocarb used over the past decade, this realistically comes as no surprise.



This information is so critical to our growers and agronomists, helping them stay one page ahead of the weeds. There are three resistance testing services in Australia, and they are currently under-utilised. Knowledge is knowledge!

Summary of the 2022 harvest samples of annual ryegrass tested at UWA for herbicide resistance against 10 post-emergence (POST- EPE) and 11 pre-emergence (PRE – IBS) herbicides. Results are ranked from low to high based on the category of high resistance (%). Sensitive samples are those with 0-5% survival, low resistance = 6-19% survival and high or field resistance = 20% or higher survival.


If you need herbicide resistance testing done, send the seeds to Roberto Busi at: 35 Stirling Hwy, Crawley, 6009 WA, Mail Bag M086. Cost: 20 herbicides for $220 per seed sample. More information on resistance testing.

Posted in: AHRI Insight, Herbicide evolution and technology

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