[Weekly Times 23 August 2011 by Peter Hemphill} -- Surveys by University of Adelaide weed management expert Chris Preston had shown a third of Wimmera paddocks are showing resistance to the pre-emergent weed killer trifluralin. Dr Preston said this was a big increase in prevalence of resistance since the last survey, in 2005. The most recent survey showed there was also high resistance in ryegrass populations to Group A and Group B herbicides in the Wimmera. In the Mallee, resistance to trifluralin and Group A herbicides was much lower than the Wimmera, while it was high for Group B chemicals.Dr Preston said about 80 per cent of Western District paddocks were showing ryegrass resistance to group A and B herbicides, but not much resistance to other chemicals.

The main survey in the Western District was taken in 2009, with some ryegrass samples taken last year when a further survey was carried out on broadleaf weeds. Dr Preston said resistance to group A and B chemicals was not surprising in the Western District, given the high-rainfall region's reliance on post-emergent herbicides.

"With plenty of winter rain, the ryegrass emerges quite late, so pre-emergent herbicides don't work as well," he said.

"A lot of canola is also grown in the Western District, so there is a lot of pressure on the Group A chemicals."

Dr Preston said the mainstay of ryegrass control was pre-emergent herbicides due to lack of break crop options. Only one in 10 Mallee paddocks had shown ryegrass resistance to trifluralin. Dr Preston said rising resistance in ryegrass was also a function of the increasing popularity of the Clearfield technology. He said the survey results showed the need to reassess weed management practices to reduce herbicide resistance. That could be through alternating different chemical types to stretch out the lifetime of susceptible herbicides. He said there were also new chemicals coming on the market but they were all pre-emergent herbicides. The new chemicals were Sakura, Boxer Gold and Outlook.

"They are all quite pricey, so growers won't want to use them in their whole cropping program," Dr Preston said.

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