THE world’s first population of glyphosate-resistant capeweed has been detected in Western Australia.

The resistant biotype was reported by an agronomist in the Albany port zone, who noticed capeweed that was not being controlled by glyphosate applications.

AHRI research associate Yaseen Kahlil with capeweed grown for herbicide resistance screening. (Photo: AHRI)

Australian Herbicide Resistance Initiative (AHRI) research associate Yaseen Kahlil, who is based at The University of Western Australia, 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,” he said.

“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.”

The AHRI screening also found glyphosate resistance was co-occurring with resistance to the ALS-inhibiting herbicide metosulam (Bayer Eclipse 100) and to the phytoene desaturase (PDS)-inhibiting herbicide diflufenican (Bayer Brodal Options), with survival rates of more than 80 per cent in both cases.

“The good news is that a paraquat-diquat mixture herbicide (Syngenta Spray.Seed 250) was 100 per cent effective against this resistant population, so there is a control option,” Dr Kahlil said.

Widespread weed

Capeweed (Arctotheca calendula) is widespread across all grain growing regions of Australia.

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 Khalil said capeweed was a prolific seed producer with a persistent seed bank that would germinate under a wide range of conditions, making it extremely hard to control.

“No other glyphosate-resistant populations have been reported, however they probably already exist,” he said.

Control strategy

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.

Source: GRDC