GRDC crop protection manager Ruth Peek said results from testing will help inform growers' integrated weed management strategies ahead of the 2021 cropping season and beyond.

"Herbicide resistance surveys conducted across the southern cropping region have shown that resistance is increasing," Mrs Peek said.

"It's a costly problem for growers for whom management of weeds and herbicide resistance now requires year-round, multi-tactic approaches."

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The GRDC said determining the status of herbicide resistance provides growers with valuable information on the effectiveness of herbicides on target weeds, potentially preventing the wasteful use of ineffective herbicides and reducing the spread of herbicide resistance.

University of Adelaide herbicide resistance researcher Peter Boutsalis said collection of samples of weed seeds can occur during and after harvest or cutting crops for fodder.

"If resistance is widespread, seeds should be collected following a W-shaped area every 10-20 metres across the suspected paddock or problem area," Dr Boutsalis said.

"Alternatively, collect seeds from suspect areas."

"It's important that growers and advisers don't bias the samples by collecting seeds from a small number of plants.

"They should instead aim to collect a similar number of seeds from each plant."

If the seeds are not completely dry, they should be sent in paper envelopes to avoid rotting in plastic packaging.

If growers wish to have annual ryegrass seed tested, about one cup equivalent of clean ryegrass, or about 50 seed heads, is required.

"For species with larger seeds such as wild oats, brome, barley grass and wild radish, an ice-cream container full is sufficient, equivalent to an A4-sized envelope full of seeds," Dr Boutsalis said.

A long-term GRDC program monitoring herbicide resistance in significant weed species has revealed increasing rates of resistance to a range of herbicides.

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Dr Boutsalis and UoA researchers collected ryegrass seed from 325 randomly selected paddocks across the region between October and December 2019.

These were sprayed with herbicides in May and June 2020, with herbicide resistance clearly increasing over the program's 2009, 2014 and 2019 surveys.

Dr Boutsalis said identifying and keeping herbicide-resistant weeds at manageable levels is essential for the long-term viability of chemical actives such as glyphosate in Australian cropping systems.

"Undetected resistance on one farm can lead to that strain getting established, which creates a problem for surrounding paddocks," he said.

Weed seeds collected at harvest time can be sent for testing over summer and reporting in early autumn.

Weed seed resistance testing services are available by contacting Dr Boutsalis at info@plantscienceconsulting.com.au or John Broster from Charles Sturt University, Wagga Wagga at jbroster@csu.edu.au

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