Conditions ripe for double knockdown weed tactics

Author: Melissa Williams | Date: 23 Apr 2015

Image of Paul Bartlett, Dr Abul Hashem and Mohammad Amjad

Early rain has created an ideal opportunity to target problem weeds with a double knockdown strategy this year.

Small weeds are cheaper and easier to kill than big ones later in the year and getting on top of these early can take the pressure off the use of in-crop herbicides.

Lowering overall weed burdens at the start of the season and then preventing seed set in any weeds present later in the year are the best strategies to ensure long-term herbicide efficacy and reduce herbicide resistance risks.

Benefits of a double knockdown

The GRDC-funded Australian Herbicide Resistance Initiative (AHRI) advises that a double knockdown of a full rate of glyphosate followed – one to 10 days later – by a full rate of a paraquat-based product is the ideal herbicide option to take the pressure off glyphosate.

But, as it is not possible to achieve a double knockdown on every paddock -particularly given that there may be shortages of paraquat this year - AHRI recommends prioritizing which paddocks require a double knockdown and ensuring there is sufficient product on-farm.

It says the top priority paddocks for the double knockdown are:

  • Where glyphosate resistance is confirmed or suspected
  • Paddocks going into glyphosate tolerant crops this year
  • Paddocks that were in chemical fallow or sown to glyphosate tolerant crops last year.

AHRI communications leader Peter Newman says growers should avoid using two or three shots of glyphosate before sowing Roundup Ready® canola, as this will be the quickest way to break glyphosate.

He says two knockdowns with the same herbicide one month apart does not constitute a double knock.

A true double knock is where the same cohort (germination) of weeds is hit with glyphosate followed by paraquat one to 10 days later.

Break the glyphosate habit

Peter says using more paraquat-based products in a double knockdown – or for a single weed knock – will reduce reliance on glyphosate and help ensure its longevity.

He says paraquat-based products are the most cost effective at controlling small weeds and will give glyphosate a break.

AHRI surveys across WA – and other surveys throughout Australian cropping regions – indicate there is escalating annual ryegrass (Lolium rigidum) resistance to glyphosate in all cropping areas of WA.

In the short term, replacing glyphosate as much as possible with paraquat-based products is a good option.

Peter says for the long-term, any over-use of paraquat-based products is not sustainable and will lead to paraquat resistance.

The best solution is to use a range of integrated weed control methods to farm with a very low weed seed bank.

Taking action in 2015 – weed control tactics in the southern wheatbelt

Department of Agriculture and Food WA (DAFWA) senior research officer at Albany, Sally Peltzer, says the first consideration should be the dormancy status of weeds that are present.

She says a wet spring and mild summer in 2014-15 means weeds seeds produced last year are likely to be reasonably dormant and germinate in several cohorts.

Past AHRI research has shown if spring conditions are wetter than usual, annual ryegrass will produce seeds with more dormancy.

Hot and wet summers tend to reduce the level of annual ryegrass dormancy, but WA’s south had quite a mild summer with little rain until February.

Sally says the same principles could apply for brome grass (Bromus ssp.) and wild oats (Avena spp.) but the research to date is inconclusive.

She recommends:

  • Not dry sowing a paddock that was weedy in 2014.
  • Allowing weeds to germinate prior to seeding, as many paddocks have weeds with high levels of resistance to post-emergent herbicides.
  • Using an autumn tickle if stubble levels allow.
  • Delaying sowing to get a germination before applying a knockdown.
  • If glyphosate resistance is suspected, using a double knockdown with a paraquat +diquat product or paraquat within 10 days of a glyphosate spray.
  • Growing a crop with different herbicide options, especially if there is resistance to all - or most - Group A herbicides.
  • Growing the most competitive crop type and use maximum seeding rates for your area.
  • For grass weeds, planning to use a pre-emergent herbicide such as trifluralin, pyroxasulfone (Sakura®) or prosulfocarb+S-metolachlor (Boxer® Gold) at robust label rates.
  • Using a range of integrated weed control tactics to reduce the seedbank for future years.

Taking action in 2015 – weed control tactics in the northern, central and eastern wheatbelt

DAFWA principal research officer Dr Abul Hashem says many growers in northern, central and eastern regions are battling both summer and winter weeds and it is possible to control them all in one hit.

DAFWA’s 2014-15 summer weed control trials, funded by GRDC, in Grass Valley, Jennacubbine, Mullewa and Geraldton found double knockdown treatments were highly effective and Abul says these will also be successful for controlling winter weeds.

Across all trial locations, a mixture of glyphosate and 2,4-D (as a tank mix), followed by a mixture of paraquat and diquat was highly effective on most weeds.

A mixture of glyphosate and 2,4-D (as a tank mix) followed by a mixture of amitrole and paraquat also achieved good control (note, rate and application time will change with location).

Dr Hashem says these treatments will control most summer weeds and new winter weed seedlings at the same time.

He says eastern States trials have found use of herbicides, such as 2,4-D, at the bud formation stage can reduce seed set in some weeds, including fleabane.

At Cunderdin, Dr Hashem is also trialling a range of integrated weed control strategies that show annual ryegrass numbers can be slashed by as much as 99 per cent using a combination of crop rotations (wheat-lupins-Roundup Ready (RR) canola or lupins-wheat-RR canola) and double knockdowns using herbicides with different modes of action (MOA).

Testing for herbicide resistance

Weeds that survive pre-seeding glyphosate and other herbicide MOA applications will be easy to spot in the two weeks post-spraying.

These are prime candidates for resistance Quicktest® testing and determining better weed management strategies in the current season, according to the Australian Glyphosate Sustainability Working Group (AGSWG).

Resistance/susceptibility testing is also important to either confirm or rule out herbicide resistance if weeds survived for another reason, such as poor herbicide application or dodgy tank-mixes.

Plant Science Consulting (PSC), based in South Australia, offers the Quicktest® weed plant sample testing service for pre and post-emergent herbicide efficacy during the growing season. See ‘Useful Resources’ below for further details.

Turnaround time for the Quicktest® is about four to six weeks, which allows time to change post emergent herbicide selection in cereals.

If glyphosate resistance is found in canola, the AGSWG recommends using late season seed set control strategies, especially if clethodim resistance is present.

AHRI says it is best to jump hard on weed problems, don’t allow set seed and develop a long term plan to declare herbicide, cultural and mechanical war on weeds.


Caption: Assessing the efficacy of range of herbicides on button grass at Mullewa as part of DAFWA’s weed control trials this summer are DAFWA technical officer Paul Bartlett, left, principal research officer Dr Abul Hashem and research officer Mohammad Amjad. Photo: DAFWA.

Contact details

More information

Peter Newman, AHRI
08 9956 8563

Sally Peltzer, DAFWA
08 9892 8504

Dr Abul Hashem, DAFWA
08 9690 2136

Andrew Storrie, AGSWG/Agronomo
08 9842 3598

Peter Boutsalis, PSC
0400 664 460


Melissa Williams, Cox Inall Communications
0428 884 414

Useful resources

GRDC Project Code UWA000124, UWA00146, DAW00196, DAW00218, DAW535, UA00149

Region West