Resistant silver grass discovered

Jo FulwoodThe West Australian

One of the first populations of silver grass resistant to simazine has been identified in the Great Southern, sparking a warning to farmers to look for resistance problems in weeds other than ryegrass and radish.

Silver grass, while not a major competitor to commercial crops, harbours root disease, which can ultimately affect crop yield.

The resistant population was discovered in the Great Southern by ConsultAg agronomist Garren Knell.

Mr Knell said in concentrating on wiping out resistant ryegrass, silver grass survivors from the eight applications of triazine chemicals over a 12-year period had the opportunity to flourish.

Further testing by the Australian Herbicide Resistance Initiative had shown the resistant silver grass population was more that 594-fold resistant to both simazine and atrazine.

Mr Knell said while the ryegrass received double knocks in the form of grazing, spray topping, crop topping, windrow burning and clethodim, the sliver grass generally avoided these second knocks.

"Unfortunately the silver grass has survived because we had been focusing so heavily on ryegrass control," he said.

"The strategies we have put in place for ryegrass included lots of exposure to simazine through the pasture phase, and atrazine during the canola phase.

"Since sheep don't like silver grass, while the ryegrass received a knock from the sheep during the pasture phase, the silver grass didn't."

Mr Knell said because silver grass matures earlier than ryegrass, it had already set seed by the time the ryegrass was spray topped.

"Also, as silver grass is quite short, even when we were burning the windrows, in canola some of the silver grass wasn't getting caught in the windrows, so it missed that knock as well," he said. "In effect, the only weed control some of the populations received over the 12-year period was exposure to triazine herbicides."

Mr Knell said this was clear evidence of the need for an integrated weed seed management strategy, saying a reliance on one chemical would exacerbate resistance issues.

Mr Knell encouraged farmers to send off samples for resistance testing.

He said he hoped to work with GRDC and AHRI in spring to do some further sampling across the Great Southern to determine the extent of the resistance issue.

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