Sixteen per cent of randomly-collected annual ryegrass populations in the south-east of South Australia have tested resistant to glyphosate, the grains industry’s most important herbicide.
The random survey, conducted by the University of Adelaide and funded by the Grains Research and Development Corporation (GRDC), has shown it is time for farmers to make significant changes to weed management to halt this problem.
“We knew there was some glyphosate resistance in the area from previous work,” stated Associate Professor Chris Preston of the University of Adelaide and project leader, “but this surprised all of us and has set the alarm bells ringing to bring in changes to weed management.”
The majority of the resistant populations were from paddocks in the Naracoorte region, however, isolated examples occurred near Coonawarra, Padthaway, Frances and Bordertown.
In the 2012 survey, 122 paddocks were randomly sampled in the agriculture-intensive south-east of SA. Seed was germinated in pots and treated with glyphosate at the two to three-leaf stage. Samples were considered resistant if more than 20 per cent of individuals survived the glyphosate application.
This blow out in resistance was on top of the 2010 discovery of two populations in the same area resistant to both glyphosate and paraquat, the major alternative herbicide mode-of-action, Group L.
“The good news from the 2012 survey was we sampled 243 paddocks in the South Australian Mallee and didn’t detect any glyphosate resistance,” added Dr Preston. “This is because there is less continuous cropping in drier areas, hence less herbicide use and it usually takes a bit longer for problems to appear.”
Dr Preston recommends growers must use a range of tactics to ensure that the survivors of herbicide application don’t survive and set seed. Growers who are continuously cropping need to be rotating their knockdown herbicides, using the double-knockdown of glyphosate followed by paraquat where practical, and employing seed set control practices. These practices will help minimise the impact of glyphosate resistance.
The south-east of SA has the good fortune of being able to grow a range of crops and pastures. Diverse crop-pasture sequences give plenty of opportunities to use different tactics such as hay, silage, manuring, different herbicide modes-of-action plus combinations of these with heavy grazing.
Dr Preston noted that: “Most paddocks visited during the survey did not have ryegrass out of control, so growers are managing the problem at the moment. However, failure to keep on top of the weed could lead to major problems managing ryegrass.”
Now is a good time to start scouting crops for spray failures so planning can begin to stop weed seed set this spring.
Over the past 15 years the University of Adelaide team has been conducting random surveys of major weeds species present in cropped fields in late spring across SA and Victoria. Seed samples are collected from fields at random during harvest. The samples are
tested for resistance to several herbicide modes-of-action to determine the patterns of resistance in the different areas.
For more information on managing glyphosate resistance visit the AGSWG web site www.glyphosateresistance.org.au
For information on herbicide sustainability and harvest weed seed control practices, visit the WeedSmart information hub at www.weedsmart.org.au
Chris Preston, University of Adelaide
08 8313 7237 or 0488 404 120
Andrew Storrie, Agronomo
08 9842 3598 or 0428 423 577
GRDC Project Code
UA00124, UA00121, ARN00001