Consistent seed control needed to combat ryegrass

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Herbicide resistance is on the increase, with higher levels of chemical tolerance recorded in south-east South Australia.

The University of Adelaide’s Dr Chris Preston says the university’s yearly survey of resistant annual ryegrass showed increasing resistance in the SA Mallee and south-east.

“Most concerning was the discovery that 16 per cent of paddocks in the south-east contained glyphosate-resistant ryegrass. No glyphosate resistance was detected in samples of the weed from the Mallee,” he says.

Dr Preston says while herbicide resistance is widespread across Australia, a three-year trial by the University of Adelaide at Roseworthy, in SA’s mid-north, found strategic use of oaten hay was the best tool for rapidly reducing the seedbank of annual ryegrass. However, another year of seed-set control is vital for keeping populations low.

A trial crop of KaspaPBR logo field peas show heavy
annual ryegrass infestation compared with a
healthy crop at a herbicide resistance trial at
Roseworthy in South Australia’s mid-north.

The trial found two years of seed-set control
was essential for reducing annual ryegrass
seedbanks.

PHOTO: Dr Chris Preston

Three different weed management strategies were used for ryegrass control in a four-year trial for improving weed management.

Cutting oaten hay in the first year reduced the seedbank of ryegrass by 86 per cent, from 4819 seeds per square metre to 692 seeds/m2 in one year.

Field peas were sown in the following year and three spray options used across three sections.

  • When trifluralin was used alone, seedbank levels increased from 692 seeds/m2 to 8319 seeds/m2.
  • When Select® was applied after trifluralin, the ryegrass seedbank slightly increased from 692 seeds/m2 to 806 seeds/m2.
  • When Select® was applied and the field peas crop-topped with Roundup® glyphosate, the seedbank declined to less than 500 seeds/m2.

This shows the importance of that second year of seed-set control in managing annual ryegrass.

Dr Preston says growers need to be cautious in chemical use because resistance to Select® is on the increase in SA, which is a major concern given the herbicide’s importance for providing effective control of ryegrass in pulse and canola break crops.

Crop-topping after Select® application, even if there are only a few weeds left in the paddock, decreased the risk of resistance emerging.

Where two years of seed-set control had been used, the annual ryegrass seedbank in the following wheat crop continued to decline, even where Boxer Gold® was the only herbicide used.

Dr Preston conducted the trials with Dr Peter Boutsalis, Sam Kleeman and Dr Gurjeet Gill. He says reducing the ryegrass seedbank is most successful when successive years of effective weed management are implemented, combining both herbicide and mechanical means of control.

More information:

WeedSmart,
www.weedsmart.org.au;

Dr Christopher Preston,
08 8313 7237,
christopher.preston@adelaide.edu.au

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