Posts tagged herbicide susceptibility
Why herbicide resistance tests don’t always reflect paddock experience

Some growers and agronomists feel the results from herbicide resistance testing services don’t accurately reflect what is happening in the paddock. Some samples are coming back susceptible to the tested herbicides while the farmers are finding poor control in the paddock.

Glyphosate resistant annual ryegrass in winter fallowThis phenomenon was investigated in northern NSW in 2008 where there were differences between in-paddock control of wild oats compared to the testing service results. What came back as susceptible in the “lab” tests were still not being well controlled in the field. So what is going on?

Differences occur due to:

  1. Sampling
    How were the samples taken? From one spot in the paddock, several dense patches or averaged over the whole paddock. Different sampling methods will give different test results. This needs to be known by whoever is interpreting the test. 90% resistance in one patch can be quite different to 5% resistance is a bulked sample from across the paddock. Results are only as good as the original sampling.
  2. Differences in herbicide coverage between field and lab
    Testing services spray the weeds in spray cabinets using 110° 01 twin jet nozzles. These nozzles produce a very fine to fine droplet spectrum and under the controlled conditions of a spray cabinet give excellent coverage. If a grower tried this at least half of the herbicide would not reach the target.
    Spray application by the grower has to contend with widely varying temperatures, humidity, wind, droplet interception by stubble and crop, much higher travel speeds, varying boom height and potential moisture and temperature stress of the target weeds.
    By necessity growers use coarser spray quality and often lower application volumes giving less than 15% coverage compare to the lab’s over 40% coverage. Add to this herbicide rate, adjuvant, water quality there is no wonder there are differences.
    Testing pre-emergent herbicides such as atrazine add another layer of complexity. Due to atrazine’s high water solubility the composition of the potting mix and level of watering by the testing service can make huge differences to the results. Too much watering can leach the herbicide from the pots. Add to this atrazine’s sensitivity to light, insufficient light means the herbicide doesn’t work, giving a false positive to resistance.

Blank pots were susceptible samplesBottom line

If an Australian testing service says you have resistance, you have resistance. No doubt about it. What that means to you as a grower or adviser is:

  • Make sure you take a representative sample.
    Seed from a harvest sample is more likely to be an average. Take some photos of the infestation and send them in with the samples so the service can give you some more accurate advice.
  • So you have resistance? But do you know which herbicides still work?
    Next time think more broadly and test for susceptibility.
  • Make sure you use the best spray application techniques available to you and don’t cut corners. Poor application will only make ALL of your weed problems worse!
Forget séances and reading animal entrails – your last chance to REALLY know which herbicides still work in the 2016 season

With so many farmers dry seeding crops to maximise potential yield, the loss of key post emergent herbicides through resistance makes it is even more important that weed control is planned before seeding.

Unfortunately many growers are unknowingly spending tens of thousands of dollars on ineffective herbicides. Gut-feelings and wishful thinking are no substitute for hard data and sound analysis. 

This is no way to develop a weed management plan!

To put this in perspective the 2010 pre-harvest survey in Western Australia by AHRI showed that over 80% of wild radish plants tested were resistant to Group B SU’s, 50% to Group B ‘imis’, 50% to Group F, and 70% to 2,4-D. Also Group B and Group A ‘fops’ were virtually useless on most annual ryegrass populations and clethodim was failing at an alarming rate. Five years later, the situation is much worse.

AGRONOMO and Peter Boutsalis at Plant Science Consulting offer you the chance to beat the resistance challenge in 2016 by conducting weed seed testing for which herbicides still work! Combined AGRONOMO and Plant Science Consulting have 50 years of weed and herbicide resistance expertise.

Seed testing requires the collection of weed seed samples before harvest and mailed to the lab. Once received dormancy of the seed is broken and seedlings grown before spraying with the herbicides determined as relevant to your farming system. Level of susceptibility for each herbicide tested is then measured. Results are normally available in early March.

Seed testing is an effective for both pre-emergent (trifluralin, Sakura®, Boxer® Gold) and post emergent herbicides.

A range of packages are available. Packages start with DIY seed collection and submitting samples for testing with report showing the effective herbicides.

Premium packages includes on-farm consultation, seed collection, advice on herbicide selection for testing, susceptibility testing, and a detailed report and management plan for the next three to five years.

For more information on Herbicide Susceptibility Testing go to /herbicide-suscept-testing/

Or phone on (Western Standard Time)

Andrew Storrie 0428 423 577

Testing for herbicide susceptibility is the way forward for growers

With herbicide resistance weeds are now driving the farming system in many areas, reducing crop yields and limiting enterprise options for growers AGRONOMO and Plant Science Consulting P/L are offering professional herbicide susceptibility testing using the Quick-test backed up with a professional management advice service.

No grower wants to be painted into a corner and forced to make management decisions that don’t suit your short term and medium term goals. The question is how can you be sure the herbicides you are using this season are going to work and are you spending thousands of dollars on herbicides that are giving poor levels of control?

Faba beans sprayed one month earlier with clethodim. Ryegrass happily growing. What to do now? Image:AGRONOMO

Alternatively, are you using expensive herbicides to tackle resistance when older and cheaper herbicides could still be working because you ruled them out thinking you have resistance to them already? This may not be the case.

The only sure way to be certain is to TEST!

For example many growers are now experiencing reduced levels of control from clethodim (e.g. Select®) due to herbicide resistance. Will butroxydim (Factor®) give better control of these problem grasses? Maybe yes or maybe no. The only way to be sure is to either spray the paddock with butroxydim or do a herbicide resistance test.

The service provided by AGRONOMO offers the Quick-test backed up by relevant personalised management advice.


What is the Quick-test?

The Quick-test can be conducted NOW by collecting live plants which are then expressed posted to the laboratory, trimmed, re-potted then sprayed with the herbicides of your choice following discussion of the relevant options.  Results are available in 3 to 4 weeks enabling effective management decisions to be made this season to prevent viable seed being produced by these resistant weeds. This is the ONLY way to manage herbicide resistance.


The Quick-test is ideal for pre-seeding or early post emergent herbicide survivors. Why didn’t those weeds die? What will you do about it?

Please note that the Quick-test is only suitable for post emergent herbicides such as glyphosate and paraquat or in-crop selective herbicides. To test for susceptibility to pre-emergent herbicides, particularly trifluralin, you must use the seed test later in the year.

For more information on this new service go here or phone AGRONOMO on 0428 423 577.