Glyphosate Resistant Weeds in Australia

Glyphosate resistance was first documented for annual ryegrass (Lolium rigidum) in 1996 in Victoria. Since then glyphosate resistance has been confirmed in another 16 weed species. Resistance is known in 10 grass species and 7 broadleaf species of which 6 are winter-growing weed species and 11 are either non-seasonal or summer-growing weed species.  

Tridax daisy (Tridax procumbens), tropical polyploid weed of the Asteraceae family is the first broadleaf weed on the list from northern Australia. The tridax daisy came from an Indian sandalwood (Santalum album) plantation in the Ord River Irrigation Area in northern Western Australia. Anecdotal evidence suggests glyphosate resistant Tridax is now widespread around the Ord. It accompanies awnless barnyard grass as the second glyphosate resistant species from this area. 

Winter grass (Poa annua) and willow-leaved lettuce (Lactuca saligna) were confirmed resistant to glyphosate during 2017 in Australia. Winter grass has been selected on golf courses in NSW, Victoria and South Australia with one population from NSW is resistant to 5 modes of action including glyphosate.

Two willow-leaved lettuce populations from Carnarvon, Western Australia, have been selected where onions are being grown.

Northern barley grass (Hordeum glaucum) is the latest species to be confirmed glyphosate resistant. It comes from fencelines and around buildings on a farm near Arthurton, South Australia.

Winter fallow in northern NSW showing an early glyphosate resistant sowthistle (Sonchus spp.) infestation.

Please note that the glyphosate resistant weed populations listed here are those that have been submitted for testing. Currently there would be several thousand unconfirmed populations of glyphosate resistant of annual ryegrass along with many more glyphosate resistant awnless barnyard grass and fleabane populations in Australia.

The greatest number of resistant populations is in annual ryegrass followed by barnyard grass and then fleabane.

The poster of Australian glyphosate resistant weeds can be downloaded here.

Glyphosate-resistant annual ryegrass has occurred in the following situations:

Glyphosate-resistant flaxleaf fleabane has occurred in the following situations:

Glyphosate-resistant tall fleabane was collected along highways and roads in southern NSW during a glyphosate resistant weed survey.

Glyphosate-resistant awnless barnyard grass has occurred in the following situations:

The glyphosate-resistant windmill grass populations are from chemical fallows (3) and roadsides (8). The glyphosate-resistant liverseed grass, sowthistle, sweet summer grass and prickly lettuce populations occured in summer chemical fallow. Glyphosate resistant great brome, red brome and wild radish have occurred in winter grain cropping. Glyphosate resistant feathertop Rhodes grass populations are from chemical fallows (2) and roadsides (2). Glyphosate resistant tall fleabane has occurred on roadsides.

The increase in confirmed cases of glyphosate-resistance in winter weeds between 1996 and 2018 is:	The increase in confirmed cases of glyphosate-resistance in summer weeds between 2007 and 2018 is:
Glyphosate-resistant annual ryegrass populations by states: State Number of populations NSW 244 SA 235 Vic 172 WA 164 Glyphosate-resistant barnyard grass populations by state: State Number of populations NSW 72 Qld 30 WA 1	Glyphosate-resistant flaxleaf fleabane populations by state: State Number of populations NSW 38 Qld 13 SA 7 Vic 6 Glyphosate-resistant windmill grass populations by state: State Number of populations NSW 4 Vic 6 WA 1

All of the glyphosate resistant weed populations have occurred in situations where there has been intensive use of glyphosate, often over 15 years or more, few or no other effective herbicides used and few other weed control practices are used.  This suggests the following are the main risk factors for the evolution of glyphosate resistance:

• Intensive use of glyphosate - every year or multiple times a year for 15 years or more.
• Heavy reliance on glyphosate for weed control.
• No other weed controls targeted to stop seed set.

Farming practices in chemical fallows the northern cropping region are heavily dependent on glyphosate for weed control.  Therefore, it is highly likely that unconfirmed populations of glyphosate resistant summer and winter weeds are present in this system.
 
Farming practices under the vines in vineyards across Australia are heavily dependent on glyphosate for weed control.  Therefore, it is highly likely that unconfirmed populations of glyphosate resistant annual ryegrass are present in this system.
 
These unconfirmed glyphosate-resistant populations are not recorded on the register of glyphosate resistant populations in Australia.
 
Acknowledgements
The AGSWG would like to thank Dr Chris Preston, University of Adelaide, Dr John Broster of Charles Sturt University, Prof Hugh Beckie of AHRI, Dr. Peter Boutsalis of Plant Science Consulting, Dr. Abul Hashem of Department of Agriculture and Food, Western Australia, Dr. Michael Widderick and Dr. Jeff Werth of the Queensland Department of Agriculture & Fisheries, and Dr Hanwen Wu and Mr. Tony Cook, NSW Department of Primary Industries for sharing data from their testing.