New herbicide resistance discovered in annual ryegrass 05 Nov 2012

Pasture seed producers are being urged to re-think their weed management strategies following the identification of the first cases of annual ryegrass resistance to the popular herbicide, paraquat.

The discovery of paraquat-resistant annual ryegrass was a result of a research project led by Associate Professor Christopher Preston at the University of Adelaide, as part of the Australian Government’s National Weeds and Productivity Research Program.

Dr Preston said the presence of paraquat resistance in annual ryegrass - the major weed dealt with by farmers in southern Australia - would threaten the viability of weed management systems in the pasture seed industry.

“Many pasture species do not compete well with annual ryegrass and adequate control is important to allow the establishment of the crop and to reduce weed contamination of seed crops,” Dr Preston said.

“Paraquat is the main alternative non-selective herbicide to glyphosate and if weed populations evolve resistance to both paraquat and glyphosate, weed management in many industry sectors will become much more difficult.

“There is a need to determine the extent of resistance in this area and the risks of resistant populations being dispersed to other agricultural sectors, as well as within the pasture seed industry.”

Weeds cost Australian agriculture more than $4 billion dollars each year, including control costs and lost production. Pasture seed is grown throughout Australia for both on-farm use and certified seed sale, with South Australia, Victoria and New South Wales having the largest production of pasture seed in Australia.

The National Weeds Program, managed by the Rural Industries Research and Development Corporation (RIRDC), provided $12.4 million of Commonwealth funds to support more than 50 research projects, including 11 projects which dealt directly with herbicide resistance issues.

The Program, which ended on 30 June 2012, has delivered improved knowledge of herbicide resistance and superior herbicide management practices, as well as alternative management strategies such as biocontrol measures.

As part of Dr Preston’s research project annual ryegrass populations were collected from cropping fields in the South East region of South Australia to test for resistance to paraquat. Nine of the populations collected were resistant to paraquat at varying degrees, and two of the populations were found to have resistance to both paraquat and glyphosate.

Dr Preston said that rotating crops and increasing use of non-chemical weed management practices could reduce the incidence of paraquat resistant weeds.
“Minimising the risk of spread of resistant populations should be considered when hay, seed and equipment are being transported between fields and off-farm,” he said.

“Effective identification of resistant populations by growers, and careful farming practices to minimise spread of resistance will greatly reduce the cost to manage this problem.”

A fact sheet on paraquat resistance is available at