The changing behaviour of common broadacre weeds is giving the cropping sector food for thought as they look to identify six problem weeds to target for future research.
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Senior lecturer in weed science and agronomy at La Trobe University, Ali Bajwa has hosted GRDC supported forums in Adelaide, Perth and Wagga Wagga, NSW, in the past month to get insights from agronomists about what ongoing and emerging weeds are causing headaches in the cropping sector, and what weeds are changing the most in terms of their biology.
Results have been eye opening.
Later emergence of weeds and species having more dormant seeds are making them more difficult to control, while a move to lower disturbance farming systems is making weed management more critical due to the extra moisture in the soil.
In an example coming from the western region, species like wild radish have evolved to shed their seeds earlier than harvest to avoid harvest weed seed control technology.
Dr Bajwa said some of the "big ticket weeds" were the ones that were changing the most.
"Across all three workshops, annual ryegrass has been mentioned as the biggest issue," he said.
"Other weeds like fleabane, sowthistle are also very problematic and change their biology.
"The other cohort is emerging problem weeds which vary across regions.
"Feathertop Rhodes grass is pretty widespread in northern regions and is now coming into the southern regions, particularly Vic and parts of SA.
"Gazanias have been here for a while but are starting to move into cropping paddocks from roadside settings."
Dr Bajwa said the change in emergence patterns was perhaps the most difficult problem for farmers and agronomists.
"They're either emerging too late or multiple times during the season, making it very challenging to control with pre-emergent and post-emergent herbicide programs," he said.
"There's other weeds that are undergoing phenological changes at the back end - things like wild radish shedding their seeds a lot earlier than they used to.
"That's a combination of changing environments, but also management strategies.
"We often see weeds evolve to resist herbicides very quickly, but they are also able to evolve to combat some of these non-chemical management strategies which becomes very challenging."
Changing rainfall patterns have also led to fallow weed management becoming more complex, which is driving a push toward optical weed management due to cost pressures of doing multiple spray passes.
Dr Bajwa said the workshops would inform the R&D sector where the best "bang for buck" would be when it came to weed research.
"There's some variation across regions but again some of the problematic weeds mentioned are ones that are highly resistant to herbicides," he said.
The research team will compile information from the forums and do a literature review before shortlisting six weeds for further research.
"We still have some knowledge gaps in their ecology in response to changing climatic patterns," Dr Bajwa said.
"We need to better understand that area so we know when they'll emerge, what emergence patterns are, how competitive they'll be if they emerge in spring vs autumn for example.
"We also want to look at biological traits to understand why they're behaving the way they are."
Ryegrass still topping the list of problem weeds in SA
In the South Australian cropping sector, ryegrass still ranks as public enemy number one when it comes to problem weeds, according to Crop Consulting Services' Craig Davis.
The agronomist, who has clients in the Mid North, Mallee and on the Yorke and Eyre Peninsulas, said despite the addition of new management tools in the past two decades, ryegrass remained a major focus.
"Some of the more recent introductions that we haven't had to deal with in the past are fleabane and Feathertop Rhodes grass as fallow weeds," Mr Davis said.
"They've increased in prevalence and are inherently difficult to control.
"All-year round we're getting weeds like sowthistle, particularly in farming systems which incorporate a lot of legumes. It's problematic both in-crop and in a fallow situation.
"That's because we don't incorporate as much residual herbicide in the cereal phase because we need to grow a legume afterwards.
"Subsequently we're not getting that longevity of control."
With many farms bidding farewell to livestock, a fallow weed management tool had been lost, while lower disturbance farming systems were storing more moisture and giving weeds more opportunity to establish.
While more moisture is a bonus for the target cereal crop, Mr Davis said it was leading to more spray passes particularly over summer.
A bright spot on the horizon has been the introduction of optical weed management - green-on-brown spraying in summer and green-on-green in season - which should help farmers eventually reduce the spray load and cost.
Dr Bajwa's project is being funded by GRDC and is being supported by researchers Dr Aakansha Chadha and Bill Grant from Federation University.
