Glyphosate resistance continues to increase as a potential problem in the northern grains region, with two new locations of resistant ryegrass being confirmed in northern New South Wales – on the Liverpool Plains and at Tamworth.

The Liverpool Plains population was identified in a broadacre chemical fallow and the one at Tamworth along a fenceline.

The national Glyphosate Sustainability Working Group (CWSG) has also identified two other new locations – on an irrigation channel, also in NSW, and in a vineyard in South Australia.

There has been no increase in the number of glyphosate resistant populations in Western Australia or in Victoria. 

The CWSG is a collaborative initiative between herbicide companies, researchers, extension networks and agribusiness, and is supported by the Weeds CRC, AVCARE and the Grains Research and Development Corporation (GRDC). 

GSWG  chairman Rick Llewellyn said the newly identified populations brought the total number in Australia to 44, with 24 of them found in broadacre cropping situations.

Dr Llewellyn said that while the new populations demonstrated the risk of too much reliance on glyphosate, the low number of new populations in broadacre cropping suggested preventative action taken by growers was making a difference.

"It's now 10 years since the first case of resistance to this valuable herbicide was discovered and many people had predicted much higher resistance levels by now," Dr Llewellyn said.

"We believe we are now seeing the positive pay-off from preventative action taken by the many growers who recognized their glyphosate use history had placed them at high risk of resistance and did something about it.

“Research from the University of Western Australia has shown that the rare nature of glyphosate resistance means practices such as the double-knock can greatly reduce the likelihood of resistance developing and be a profitable strategy.

GSWG northern region spokesman, NSWDPI weeds agronomist Andrew Storrie, said glyphosate resistance in annual ryegrass in northern NSW had developed quietly when growers were more focussed on wild oat control.

Annual ryegrass was a tough, genetically variable plant that set a lot of seed, making it a likely candidate to develop herbicide resistance. 

Another factor in the development of resistance was the reluctance of some growers to use herbicides with different modes of action to glyphosate – like paraquat – although rotation of herbicide groups was essential in any integrated weed management program. 

Mr Storrie said the Working Group had produced a guide to reducing the risk of glyphosate resistance developing, while details of glyphosate resistant populations in Australia were listed on a register, compiled by Dr Chris Preston from the GRDC supported Weeds CRC. 

The register and information on resistance management were available from the national GSWG website www.weeds.crc.org.au/glyphosate.