ISSUES with glyphosate resistance in Australia's cropping regions continue to build with another noxious weed now displaying immunity to the herbicide.
Capeweed (Arctotheca calendula) is widespread across Australian cropping and pasture zones, however up until now it has been relatively easy to control, either with glyphosate or with a broadleaf spray such as MCPA.
However, researchers from the GRDC-supported Australian Herbicide Resistance Initiative (AHRI) have detected the world's first population of glyphosate-resistant capeweed in Western Australia.
The resistant biotype was reported by an agronomist in the Albany port zone in the south of the state, who noticed capeweed that was not being controlled by glyphosate applications.
AHRI research associate Yaseen Kahlil said capeweed seeds were collected from the site and seedlings screened for resistance in Perth, with glyphosate applied at a label rate of 540 grams of active ingredient per litre.
"We recorded survival rates four times higher than susceptible control populations, which were collected from six non-agricultural sites across the WA grainbelt," Dr Kahlil said.
The next phase of the research showed how alarmingly quickly resistance can build.
"Seed from the survivors was subjected to glyphosate dose response testing and this second generation exhibited a resistance level 13 times higher than the control population," he said.
And it is not just glyphosate that it under threat.
The AHRI screening also found glyphosate resistance was co-occurring with resistance to the ALS-inhibiting herbicide metosulam (Eclipse) and to the phytoene desaturase (PDS)-inhibiting herbicide diflufenican (Brodal), with survival rates of more than 80 per cent in both cases.
However, Dr Kahlil said there were still options to kill the broadleaf weed, easily identified by its distinctive yellow flowers in the spring.
"The good news is that a paraquat-diquat mixture herbicide (Spray.Seed) was 100 per cent effective against this resistant population, so there is a control option," he said.
The weed is problematic as it competes for water and nutrient resources and the large, flat plant can inhibit crop emergence.
Seven to 90 capeweed plants per square metre have been demonstrated to reduce grain yield by 28 to 44 per cent respectively in wheat crops.
Dr Kahlil says capeweed is a prolific seed producer with a persistent seed bank that will germinate under a wide range of conditions, making it extremely hard to control.
He warned it is likely there are other glyphosate resistant populations about.
"No other glyphosate-resistant populations have been reported, however they probably already exist," he said.
Peter Newman, AHRI consultant, said the capeweed news showed farmers needed to concentrate their herbicides on where they get best bang for buck.
"We're blowing up our most valuable herbicides on the least productive part of the farm such as fencelines, roadsides and drainage areas," Mr Newman said.
"The resistant capeweed was found in a non-cropped drainage area that was regularly sprayed with glyphosate, then it spread into the nearby paddock," he said.
"We're not pointing the finger here - we know that this is common practice on many farms around the world, but we need to learn from these problems and work out how we can add some diversity and competition to these areas to reduce repeating the same thing happening with other weeds."
Dr Kahlil recommends a double-knock strategy to control capeweed, using glyphosate followed by a paraquat-diquat mixture herbicide to eliminate survivors.
He said growers should also practice an integrated weed management program based on the WeedSmart 'Big 6':
- Rotate crops and pastures
- Double-knock to preserve glyphosate
- Mix and rotate herbicides
- Stop weed set by including crop topping, patch management and cutting hay in the pasture phase
- Use crop competition with wheat, barley and especially oats.
Harvest weed seed control (HWSC), which is also one of the 'Big 6' strategies, is not considered effective for controlling capeweed due to the weed's lack of height and significant seed dispersal.
The story World first glyphosate resistant capeweed found in WA first appeared on Farm Online.