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'Double Knocking' Answer to Australian Weed Control

08 May 2012

AUSTRALIA - Advisors are asking Central Queensland grain growers to try again if they have experimented with the 'double knock' weed control technique with mixed results.

Darren Aisthorpe, Queensland Department of Agriculture, Fisheries and Forestry (DAFF) extension agronomist for the CQ Grower Solutions project, Biloela says the double knock technique is a key strategy for management of difficult-to-control weeds such as flaxleaf fleabane and feathertop Rhodes grass.

“To date there have been some very mixed results with double knocking at a commercial level, despite excellent results at trial and demonstration levels, and this is not helping with the uptake of the practice,” Mr Aisthorpe said.

“However, double knock is a key strategy for maximising weed kill and reducing the threat of herbicide resistance.”

The Grains Research and Development Corporation (GRDC) has invested in weed management research across the northern region, including extensive work by the DAFF weeds team, CQ Grower solutions project and Northern Grower Alliance (NGA) into maximising effectiveness of the double knock technique.

Double knocking involves the planned application of two (or more) practices to manage a target weed.

Mr Aisthorpe says the most common form of double knocking is separate spray applications of two different chemical groups. A double knock could also involve any number of combinations including chemical application, tillage, burning and mulching, he says.

“The crucial element of the second knock or practice is to take out those targets that were not taken out by the first knock,” Mr Aisthorpe says.

“This is essential, particularly when using Group A and B herbicides. If any plants survive the initial application of chemical, a resistant population of weeds will develop very quickly, rendering that chemical group useless against the target species.”

Mr Aisthorpe cites several key strategies when using knockdown herbicides on feathertop Rhodes grass or fleabane:

  • Plan to double knock—spraying after plant shutdown will compromise the bipyridyl efficacy. Don’t consider application as an afterthought, timing is the key.
  • Keep water rates high—bipyridyls rely on contact to work. More viable droplets will cause more damage.
  • Time your spray applications to avoid hot, dry, stressed conditions during the middle of the day.
  • Be realistic in your expectations—double knocking works best with small, rapidly growing plants. If plants are large, mature and rank, the best you can expect is to limit seed set and reduce biomass to allow for a tillage event, ideally followed up with a residual product post-tillage to manage any future peak flushes after rain.

For more information on weed management, click here.

TheCropSite News Desk

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