A CONCERNING milestone has been notched, with confirmed populations of ryegrass resistant to both glyphosate and paraquat found for the first time in broadacre cropping environments.
The two herbicides are the grains industry's major broad spectrum weed killers and wide ranging resistance would mean a serious shake-up in management practices.
However, weed resistance specialist Peter Boutsalis, Plant Science Consulting director and researcher with the weeds group at the University of Adelaide, said while the finding was concerning it was not a widespread issue as yet.
"It's a worry when you lose both of your main knockdown options, but thankfully this double resistance has only been found in an isolated population of ryegrass," Dr Boutsalis said.
He said the sample had come from an undisclosed farm in south-west Victoria which had recently changed hands with the new owners wanting to get a handle on any potential resistance issues.
"We've seen a similar issue in the vineyards here in South Australia but never in a broadacre cropping context."
Glyphosate resistance is relatively common in regions of high intensity cropping, with one set of samples sent to Dr Boutsalis from NSW and Queensland showing 79 per cent resistance to glyphosate over 83 samples, but paraquat resistance is far less common.
While both chemicals are used as part of the Grains Research and Development Corporation approved double knock strategy, Dr Boutsalis said the resistance is unlikely to have come from a farm using a double knock system for weed control.
"Using a double knock you'd have the glyphosate killing 99 per cent or whatever then the paraquat cleaning up the survivors, the chances of resistance with a double knock is really low," he said.
"It is far more likely to have happened with growers using sub-lethal doses of the products and resistance slowly grew.
"The low rates are the killer in most cases of resistance."
Dr Boutsalis said farmers with potential double resistance would have to look at non-chemical methods of seed control
"Hay crops definitely come into it, although there is the risk of late germinating weeds setting seed after it is cut, and options like harvest seed management are very important."
He said if resistance problems were bad enough growers would have to consider options that may not fit with their general farming system, such as cultivation.
"Things like a pass with a mouldboard plough can bury the seed and be very effective, but it might not be something farmers in no-till systems are keen to do unless they really had to."
Dr Boutsalis said selective pre-emergent products such as Boxer Gold, Sakura, Luximax and Overwatch were also strong performers on ryegrass but said they were not a sustainable replacement to broad spectrum knockdown herbicides.