It costs southern winter crop growers $93 million a year in revenue and now annual ryegrass is being called an "emerging threat" to northern areas and summer crops.
In a study published last month, University of Queensland's Professor Bhagirath Chauhan, Michael Thompson and Gulshan Mahajan said the loss was likely to increase significantly if the summer form of ryegrass became widespread.
"The expansion of Lolium rigidum into summer cropping systems [such as cotton] could exacerbate the incidence of herbicide resistance, further complicating management options for the species year-round," the report said.
"It presents the potential for devastating effects on Australian agriculture, mainly if seasonally adapted populations can increase their geographic spread."
Australia's costliest weed primarily affects grain growers in WA, SA and Victoria, competing with winter crops like wheat and canola and reducing yield.
According to another recent study, ryegrass cost the country $1.2 billion since 1960, whereas it was ranked the seventh costliest species for Queensland at $42 million.
The typical lifecycle for ryegrass is to germinate in autumn, grow during winter, and set seed in spring.
The seeds remain dormant in the soil until the following autumn, when the cycle begins again.
The weed is dormant in the summer months when rainfall is typically too low in Australia to sustain further growth after germination.
But the researchers say ryegrass has been found in irrigated fields in southern NSW.
"Whether irrigation is the sole determinant of the capability of ryegrass to grow in summer or whether an altered dormancy duration is also at play is currently unclear."
The researchers want to determine whether it is driven more by adaptive changes in seed dormancy, by changes in agronomic practices, or global warming.
Prof Chauhan's concern was amplified when he was driving from Toowoomba to Cecil Plains this week and found a 200 to 300 metre roadside infestation near Norwin.
"The infestation was not a surprise as I was informed that last year a truck dropped hay bails, which had ryegrass plants," Prof Chauhan said.
"I thought there were only a few plants so I tried to uproot them but there were too many."
He said the plants were found near a drain, which could help spread ryegrass if existing plants were allowed to produce seeds.
The weed expert said ryegrass was a prolific seed producer and seeds could germinate at temperatures ranging from 5 to 35C, suggesting that it could germinate throughout the year in Queensland.
He also said ryegrass had been reported to evolve resistance to most commonly used herbicides, including glyphosate.
"Fortunately, it is still predominantly limited to winter conditions. However, we cannot assume that this will always be the case."
Prof Chauhan said the message was clear.
"Don't allow this weed to flourish in Queensland. Don't allow this weed to produce seeds - seed bank management is very important."
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