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Testing for herbicide resistance

With Plant Science Consulting Director and Adelaide University weed science team researcher, Peter Boutsalis.

An application of herbicide may cost thousands of dollars, all of which might be wasted if the weeds being targetted are resistant to the products applied. Testing for herbicide resistance can identify which herbicides will work on your weeds.

Dr Peter Boutsalis said the greatest value in testing for herbicide resistance on your property is to have confidence in the products and application methods you are using.

“Testing takes the guesswork out of the equation and gives farmers baseline information that they can use to monitor changes in the weeds on their farms,” he said.

“If low level resistance is identified early there are many more management options available compared to situations where full blown resistance has taken hold.”

Dr Boutsalis said the over use and over reliance on particular herbicides will unavoidably lead to herbicide resistance developing. “We often hear of farmers applying herbicide even though they are not sure if it will work,” he said.

The $300 to $400 cost of testing is insignificant compared to the cost of wasted herbicide, lost production and the costs of driving down a large seed bank of resistant weeds.

Select dose response

What herbicide resistance tests are available to farmers in Australia?

Short answer: The ‘quick’ test using the whole plant and the ‘seed’ test.

Longer answer: The ‘quick’ test uses plant samples collected on farm and sent to the laboratory. The plants are revived and planted into pots then tested against the required herbicides. The ‘seed’ test requires the collection of ripe seed, which is planted out at the laboratory. After dormancy has been broken and the seedlings have started to grow they are tested for their response to herbicides. Both tests are equally accurate. The ‘quick’ test can not test for resistance to some pre-emergent herbicides, such as trifluralin.

Which is the most common test that farmers use?

Short answer: The seed test.

Longer answer: Collecting seed before or at harvest is the most common method used. The collected seed must be mature, from green to when the seed changes colour. Before harvest collect 30 to 40 ryegrass seedheads or several handfuls of wild oats seed. After harvest it is common to find seedheads still in the paddock or samples of contaminated grain can be sent for analysis.

Wild radish- Eclipse resistance-2

Where is the best place to collect samples?

Short answer: From suspicious or high risk areas.

Longer answer: Herbicide resistance can develop in high risk areas like fencelines or at random through a paddock. Visual observations and changes on the yield monitor in the header can indicate good places to collect seed. If collecting plant samples, look for weeds at the early tillering stage that appear to have ‘escaped’ previous herbicide treatment. Collect 50 to 100 small plants or fewer larger plants. Shake off the soil from the roots, place in a plastic bag and send to the laboratory.

What’s involved in sending samples?

Short answer: Pick, pack, register and ship.

Longer answer: Each sample needs to arrive at the laboratory with suitable identification and instructions. Register the samples online to get a unique sample number and to provide the information required, such as which herbicides you want to test against. Plant Science Consulting and Charles Sturt University both offer commercial herbicide resistance seed testing. Find the details under Point 4 of the 10 Point Plan on the WeedSmart website.



How to ask a WeedSmart question

Ask your questions about the spread of herbicide resistance, or any herbicide resistance management strategy, using this blog or using Twitter @WeedSmartAU.

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