Golden opportunity for a double knock on weeds

Author: Chris Wilkins | Date: 23 Apr 2015

Caption: GRDC western regional panel member Chris Wilkins says the double knock is an important aspect of good weed management strategies.

Caption: Conditions in WA are currently optimal for a ‘double knock’ prior to seeding. Weeds such as annual ryegrass (pictured in-crop) will be harder and more expensive to kill if growers delay controlling them.

By GRDC western regional panel member Chris Wilkins

Virtually all of Western Australia’s grain growing areas received enough rain in April to achieve a solid germination of winter weeds, providing growers with a golden opportunity to control weeds early.

Conditions are optimal to apply the ‘double knock’ technique, where growers use two weed control tactics with different modes of action to ensure that any weeds that survive the first application are killed as a result of the second application.

A double knock involves, in most cases, glyphosate followed by paraquat or combined paraquat and diquat (Spray.Seed®) about one to 10 days later.

Research worldwide and by the Australian Herbicide Resistance Initiative (AHRI), funded by the Grains Research and Development Corporation (GRDC), has demonstrated the importance of using full label rates – to help ensure that herbicides are lethal against weeds.

This principle also applies to double knock applications, and growers who consistently use full label rates will be part way along the track to successfully controlling weeds.

Using high water rates to achieve good herbicide coverage of weeds, especially when using paraquat and diquat products, is another key to a successful double knock.

Some growers may be tempted to skip a double knock in favour of a ‘single pass’ of knockdown herbicides just prior to seeding in late April or early May.

But this is a false economy – both in the short and long term.

Weeds grow fast at this time of the year and will be harder and more expensive to kill if growers delay controlling them, especially if they have to reach for greater amounts of more expensive in-crop herbicides during the growing season.

The long-term saving to growers from a well-executed double knock is a longer useful life for herbicides.

Many growers are already being forced to use more expensive pre-emergent herbicides due to weeds developing resistance to herbicides like trifluralin.

These growers are now having to pay up to about $40 per hectare for pre-emergent herbicides, compared with $10 or $15/ha previously.

 In WA we are now also starting to see increasing levels of glyphosate resistance in weeds such as annual ryegrass, which could force growers into using vastly more expensive and difficult weed control options.

Over the last 15 years, the GRDC has invested funding of about $1 million annually into AHRI, which is a research leader in herbicide resistance and its management in cropping systems.

Its research activities include the biology and population ecology of major crop weed species through to cultural and herbicide management strategies.

AHRI also conducts fundamental research on biochemical and molecular resistance mechanisms responsible for endowing herbicide resistance, and has a significant investment in communication.

AHRI research has demonstrated that a double knock is one of a number of aspects of good weed management strategies and that growers need to develop a long-term plan to declare war on weeds using chemical, cultural and mechanical methods.

According to AHRI, high priority paddocks for the double knock treatment are where:

  • Glyphosate resistance is confirmed or suspected
  • Glyphosate tolerant crops are being sown this year
  • Glyphosate tolerant crops were sown or where there was a chemical fallow last year
  • Annual ryegrass burdens are high

AHRI reports that there has been significant early germination of annual ryegrass this year in WA due to seasonal conditions last spring and summer.

Information about sustainable integrated weed management (IWM) practices is available at the AHRI website.

Herbicide sustainability information is available on the WeedSmart information hub.

The GRDC Integrated Weed Management Hub can assist Australian grain growers and advisers to quickly locate useful web based information on weed management in Australian broadacre cropping systems.


Contact Details

For Interviews

Chris Wilkins, GRDC western panellist
0427 940 925


Natalie Lee, Cox Inall Communications
08 9864 2034, 0427 189 827

GRDC Project Code UWA000124

Region West