Australian Herbicide Resistance Initiative (AHRI)

Susceptibility testing

Glasshouse with plants

Who uses cloth nappies (diapers) these days? Almost nobody. So how is it that the premium cloth nappy soaker, NapiSan, is still a successful product? The answer is that the company who make NapiSan cleverly repositioned their product as the premium laundry stain remover.

We at AHRI are tired of being the bearers of bad news so we have decided it is time to reposition herbicide resistance testing services.

Enter the new and improved “Susceptibility Testing” – it will tell us which herbicides will work next year rather than those that won’t, and shows us where to focus our efforts to keep the weeds susceptible. In other words, is the cup half full or half empty?

John Broster, Charles Sturt University, runs one of Australia’s two commercial herbicide resistance testing services. His 2014 report shows a long list of resistant weeds, but it also shows a long list of susceptible weeds as well.

This harvest, put an A4 envelope or two in the cab of the harvester, and plan to do a susceptibility test to give you the peace of mind that the herbicide that you choose next year is going to work, and make a plan to keep it working.

What should I test?

The summary of John Broster’s 2014 report below gives us a really good indication of what we should test for. Keep in mind that this is not a random survey, rather it is a summary of test results where growers and agronomist have suspected resistance and have paid for a resistance test.

JBroster 2014 ryegrass


JBroster 2014 Wild radish


JBroster 2014 wild oats


Susceptible ryegrass

How could there be this many herbicide options for the world champion of resistance (annual ryegrass) in Australia (the home of ryegrass resistance)? The answer is that these herbicides are either low to medium risk for resistance evolution, and / or have been scarcely used.

It is alarming to see some resistance to important herbicides such as glyphosate and trifluralin, and we don’t want to underestimate these problems. However, it is refreshing to see the levels of susceptibility to a range of herbicides and we need to keep it that way. This level of trifluralin resistance is lower than other surveys / testing surveys have found in the past. South Australia and Western Australia have higher levels of trifluralin resistance than indicated below.

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

JBroster 2014 ryegrass options

Yep, I thought so!

If you think a weed is resistant to a herbicide, it probably is. Is it really worth paying for a Group A (fop) or Group B ryegrass test when we know that the vast majority of it is resistant?

Table 2:  Results of herbicide resistance testing of annual ryegrass to Group A and B herbicides at Charles Sturt University over the past five years.

JBroster 2014 ryegrass resistant

Susceptible Wild Radish

There has been no resistance in Wild Radish confirmed to pyrasulfatole (the active in Precept / Velocity), or Paraquat in Australia. The high levels of phenoxy and diflufenican resistance in Australia means that resistance to herbicides such as Jaguar, Tigrex and Flight is possible, however, we have limited data from resistance testing services.

The bad news for wild radish is that there are high levels of resistance to many common herbicides.

Table 3:  High levels of herbicide resistance in wild radish to commonly used herbicides.

JBroster 2014 Wild radish resistance

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

Wild Oats

No resistance was found to Select (47 samples), Avadex (35 samples) or glyphosate (9 samples).

Finding susceptible herbicides for wild oats is getting harder each year for many growers as resistance to Group A and B herbicides increases. There are some pre-emergent options such as Avadex that offer good solutions. Wild Oats generally take a long time to evolve resistance to herbicides because they are a mostly self-pollinating, hexaploid species.  See previous edition of AHRI insight here.

Table 4:  High levels of herbicide resistance in wild oats to commonly used herbicides

JBroster 2014 wild oats resistance

Keeping them susceptible

So you get your test results back and discover that you have a range of herbicide options available to use. Hurrah! Susceptible herbicides are a precious resource and should be treated as such. The key to maintaining this susceptibility is to make a plan to keep them that way. That plan should be obsessed with a low weed seed bank through using a diverse range of weed control measures, including non-herbicide weed control.

Put an A4 envelope in the harvester

You don’t need a fancy testing kit to take a ‘Susceptibility Test’ sample. All you need is an A4 envelope. Record the location on the envelope and worry about postage later. The contact details for the testing services are below.

The amount of seed required for ryegrass is 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. Put this envelope in another envelope for mailing and do not use plastic bags as the seed can get mouldy if there is any moisture in it.

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 sample throughout that area. Think soil sampling methodology.

The most expensive herbicide is one that doesn’t work. Knowing that the herbicides that you plan to use are susceptible is great peace of mind. Susceptibility testing will not only help you sleep at night, it will also help run down those weed seed banks by ensuring that you are using effective herbicides.


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