Australia
January 13, 2015
In an effort to find out which herbicides will work best in 2015, it is recommended weed seeds collected during or after harvest are sent for herbicide resistance (or susceptibility) testing.
In the past, testing for resistance has often confirmed suspicions and it is better to focus on which herbicides could be effective this year by testing for susceptibility - according to the GRDC-supported Australian Herbicide Resistance Initiative (AHRI).
This can save time and money and paves the way for fine-tuning weed management strategies to ensure weeds can be controlled and herbicide options are preserved.
Many agronomists would like herbicide susceptibility testing to become as common as soil testing, especially on the back of widespread problems with persistence of wild radish (Raphanus raphanistrum L.) in triazine tolerant (TT) canola crops across WA’s southern wheatbelt areas in 2014.
Susceptibility testing is important to either confirm or rule out herbicide resistance if weeds have got through the system for another reason, such as sub-optimal herbicide application.
Research shows that many weed seeds contaminating grain at harvest are from populations that are resistant to a range of commonly used post-emergent herbicides. This risks further spread of herbicide resistant weeds in cropping paddocks in subsequent years.
How to test for herbicide susceptibility
The two weed seed testing services available in Australia are provided by Peter Boutsalis’ Plant Science Consulting (PSC), in South Australia, and John Broster at Charles Sturt University (CSU), in NSW.
They take mature weed seeds collected during or after harvest, grow these out in controlled environments from December to February, treat the weed plants with the herbicide/s requested by the grower and then assess survival.
More information about these services is available at:
It is recommended samples are sent before the end of January (in paper bags/envelopes - not plastic) so results are available before the beginning of the cropping season.
Double bagging by placing the seed in a bag and then placing this inside a postage envelope is important, as it reduces the risk of seeds ripping the bags and spilling out.
Samples should come from a range of plants across a paddock if it is believed resistance could be widespread, or from several plants in patches of problem weeds.
The amount of seed required for annual ryegrass (Lolium rigidum) is about half a coffee cup full of clean seed, or an A4 envelope full of seed heads. For wild oats (Avena sterilis and A. fatua) or wild radish, a comfortably full A4 envelope of seed heads or pods is sufficient.
If collecting weed seeds after harvest, it is best to check between the crop rows or use screenings from the harvester.
What herbicides to test for
At the start of 2014, CSU tested more than double the number of weed seed samples (from the 2013-14) harvest, compared to the previous year, and the bulk were from WA paddocks suspected of having herbicide resistance.
The majority of weed samples that are sent to PSC also originate from WA.
Annual ryegrass and wild oats
It is advisable to test annual ryegrass and wild oats for susceptibility to the herbicides planned for use in coming years if resistance is suspected.
CSU’s John Broster says there is now little point in testing annual ryegrass for susceptibility to Group A ‘FOP’ herbicides or Group B herbicides due to the high levels of resistance found in these weeds across most regions of Australia.
PSC’s Peter Boutsalis points out - after testing thousands of samples – the observation has been made that if annual ryegrass is resistant to one FOP herbicide, it is usually resistant to all the FOPs.
However, he says this is not often the case with wild oats. In a significant number of cases where wild oats are resistant to cereal selective herbicides (such as diclofop-methyl, fenoxaprop or clodinafop), the wild oats are found to be not resistant to haloxyfop or quizalofop.
Also, Peter says differences between the cereal-selective ‘FOP’ herbicides are common - with higher rates occasionally improving control.
There are some pre-emergent options that offer a good solution for wild oats and it is best to test for those that are planned to be used in the next few years.
Peter says at present, there are no cases of wild oats resistant to pre-emergent herbicides such as trifluralin or triallate.
Of the 123 annual ryegrass samples tested to a Group A ‘FOP’ herbicide by CSU in 2014, only 16 per cent were susceptible – a similar finding to previous years.
Susceptibility of grass weeds to the Group A ‘dims’ and ‘dens’ is also declining and these are the next ‘at risk’ herbicides, according to John.
Random surveys of WA grainbelt paddocks by AHRI researcher Mechelle Owen in 2010 found 7 per cent of 362 annual ryegrass samples had glyphosate resistance. This was up from 1 per cent in 2003.
A targeted survey in 2013 by the Department of Agriculture and Food WA (DAFWA) found 40 per cent of 172 samples from WA grainbelt properties had some glyphosate resistance.
AHRI estimates only 2 per cent of annual ryegrass populations in WA remain fully susceptible to herbicide control and the remainder are resistant to one or more herbicide modes of action.
Wild radish
This year CSU received its highest number of wild radish samples for testing (125 from WA and three from the eastern States).
It found only 12 per cent were susceptible to Group B herbicides and it was probably not worth testing if resistance is suspected.
This reflected AHRI’s 2010 sampling results that showed only 16 per cent of 96 wild radish populations tested from Geraldton to Esperance were susceptible to the Group B acetolactate synthase (ALS)-inhibiting herbicide chlorsulfuron.
Susceptibility to the Group B herbicide imidazolinone was 51 per cent of populations tested by AHRI, although the average level of susceptibility within populations was high at 82 per cent.
John says it appears from testing that significant levels of resistance to Group C, F and I herbicides are developing in wild radish populations and it might be a good idea to test these if use is being considered within the next three years.
He warns that wild radish resistance to triazines (Group C) is starting to appear on the back of high use in TT canola systems, with 11 per cent of samples tested (mostly from WA) at the start of 2014 showing resistance to this group.
Wild radish resistance to glyphosate is rare – WA’s first case was found in 2013 - but is an emerging problem and tests are recommended if resistance is suspected.
John says wild radish remains susceptible to pyrasulfatole and paraquat and if any other new herbicide is being considered for use on a property, there is value in testing for susceptibility.
Cross resistance testing
Testing for cross resistance is not common and separate tests for each herbicide that is being considered for use is the best option, according to CSU.
Keeping weeds susceptible
Efficacious herbicides are a precious resource and should be treated as such.
AHRI says to reduce resistance risks, a plan is required with the aim of achieving a low weed seed bank using a diverse range of weed control measures - including non-herbicide weed control.
Note: (PSC and Syngenta also offer a Quick TestTM and RISQ (Resistance In-Season Quick) test, respectively, to test weed plant samples for post-emergent herbicide efficacy during the growing season).
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