A Grains Research and Development Corporation (GRDC) program monitoring herbicide resistance in weed species has shown increasing rates of resistance to a range of herbicides.
University of Adelaide researchers collected ryegrass seed from 325 randomly selected paddocks across parts the southern growing region between October and December last year, which were grown in pots and sprayed with herbicides at label rates in May and June.
Results indicate growers will need to consider tactics such as double-knock spraying and harvest weed seed control to help them keep resistant ryegrass populations at bay.
Researcher Dr Peter Boutsalis said herbicide resistance had increased over the program's 2009, 2014 and 2019 surveys.
"Our results from the lower Eyre Peninsula, upper Eyre Peninsula and south-western Victoria all show increasing resistance to various herbicides," he said.
"For example, the ryegrass samples from south-western Victoria showed no resistance to trifluralin in 2009, but that increased to two per cent of samples in 2014 and reached 17 per cent in 2019.
"On the lower Eyre Peninsula, the rates of resistance to trifluralin were 10 per cent in 2009, then 51 per cent in 2014 and 66 per cent in 2019."
Dr Boutsalis said the increasing rates of resistance were not restricted to any one herbicide group or region, but as strains with a specific resistance evolved, they quickly established themselves in the local area.
"On the upper Eyre Peninsula, the incidence of resistance to imidazolinone herbicides was 30 per cent of samples in 2009, rose to 39 per cent in 2014 and comprised 88 per cent in 2019."
Dr Boutsalis said identifying and keeping herbicide-resistant weeds at manageable levels was essential for the long-term viability of chemical actives like glyphosate in Australian cropping systems.
"If the weeds grow through to flowering their pollen can carry the resistant gene across a wide area via wind dispersal, plus the resistant seeds can be spread via several methods including farm equipment and livestock."