Double knocking northern weeds (fa la la la)

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Deck the halls with the double knock,

Fa la la la la, la la la la.

Tis the season to conserve moisture,

Fa la la la la, la la la la.

Don the weeds with two modes of action,

Fa la la, la la la, la la la.

Or glyphosate resistance will cause problems,

Fa la la la la, la la la la.

OK, it doesn’t rhyme, but you get the picture!
Earlier in the year, we reported on glyphosate resistant awnless barnyard grass and how it is less glyphosate resistant in cool weather. We incorrectly recommended to “double knock when it’s hot”. Really, we should have recommended to always double knock because the risk of glyphosate resistance in this weed (and others) is very high. And let’s face it; it’s always hot at this time of year.
So what’s a good double knock in Northern Australia?
The Queensland Department of Agriculture and Fisheries (DAF) weed research team (based in Toowoomba) has researched the double knock for a number of the northern grass weeds.
There are differences between species in how they react to the double knock, but there are some common threads.
A good double knock for northern grass weeds is a robust rate of glyphosate applied to pre-tillering plants followed by plenty of paraquat or Spray.Seed® about 4 to 7 days later. Some weeds favour 2 to 4 days between knocks where others favour 7 to 14 days, so 4 to 7 days is a good compromise.
Glyphosate resistance is a big threat to the northern farming system, but it can be managed.

#58 blog and tweet card

Northern growers have heard plenty about glyphosate resistant weeds and the need to use the double knock, but there is some confusion as to exactly how to execute an effective double knock. The reason for this confusion is that we are dealing with multiple weed species. The weed research team from DAF has conducted a number of pot trials to understand the double knock for some of the northern grass species.

Pot experiment: Awnless barnyard grass, Feathertop Rhodes and Windmill grass

Awnless barnyard grass (Photo: Michael Widderick)

Awnless barnyard grass (Photo: Michael Widderick)

Awnless barnyard grass and windmill grass were not fussed about when the second knock came along. Any time beyond 1 day was fine.

Feathertop Rhodes grass, on the other hand, was a little more fussy. In this trial it seems that there was a sweet spot for Feathertop Rhodes of 7 days between knocks.

Timing between knocks is important. There needs to be sufficient time for the first knock to be effective and not too much time between knocks such that too much reliance is placed on the second knock. A short interval is best as delaying the second knock allows the surviving weeds to get bigger.

[Tweet “Timing between knocks is important. A short interval is best”]

Pot experiment

Figure 1. Biomass of summer grass weed species treated with a double knock of glyphosate followed by Spray.Seed® applied at different intervals after the first knock.

Group A herbicide research

There has been recent experimental work on Group A herbicides to control these summer grass species. These products are not registered for this fallow use therefore, we’re not able to share the experimental data.

Typically, Group A grass selective herbicides are designed for use in broad-leaf crops to selectively control grass weeds. However, the recent occurrence of difficult to control summer grass weeds including Feathertop Rhodes grass and awnless barnyard grass (particularly with glyphosate resistance) has resulted in some growers choosing to apply Group A herbicides in fallow. Applying Group A herbicides in fallow is a high risk practice as weeds generally evolve resistance to Group A herbicides easily.

[Tweet “Applying Group A herbicides in fallow = high risk of resistance evolution”]

If the Group A herbicide is followed by a 100% effective second knock (e.g. Spray.Seed®) then the risk of resistance is eliminated.

The key is an effective second knock, which is why spraying small, pre-tillering grass is so important.

Currently, there is one minor use permit (Permit 12941, valid in Queensland only) allowing the application of Verdict™ and other registered products containing 520 g/L haloxyfop for the control of feathertop Rhodes grass in fallow. The permit stipulates that Verdict and other registered products containing 520 g/L haloxyfop can be applied once per season in fallow preceding a mungbean crop followed within 7-14 days by a treatment of paraquat applied at a minimum rate of 1.6 L/ha (using 250 g/L paraquat product).

Just to reiterate: the use of Group A herbicides in fallow is fraught with danger as the risk for resistance development is extremely high.

If glyphosate isn’t a viable option for the first knock treatment, other tactics may need to be explored that are not chemically based. For example, targeted tillage, burning or grazing. Alternatively, utilising residual herbicides in fallow may effectively control multiple consecutive flushes of emergence.

Introducing Paul McIntosh

Paul Macca

The newest member of the AHRI communications team is Paul McIntosh (aka Paul Mac). Paul is based in Toowoomba and has 35 years of agronomy experience in the northern cropping region. He knows a few people (understatement of the century) and he knows the issues that northern growers are facing. Paul will work in the region where summer crops are grown, roughly from Dubbo, north. For this edition of AHRI insight, Paul sat down with DAF’s Michael Widderick to review some double knock research.

Some words of wisdom from Paul Mac

Awnless Barnyard grass is once again proving a difficult weed to control in our summer cropping Northern Zone. In fallow situations with invariably hot and dry conditions this very competitive annual grass plant has always been difficult to control from our initial zero till days. These days with confirmed glyphosate resistant levels in this species and its ability to produce over 40,000 viable seeds per plant, the degree of difficulty has increased.

Researchers constantly advise adoption of double knock actions that utilises the sequential application of two different weed control tactics, where the second tactic is designed to control any survivors of the first action.

The double knock has been commonly employed by applying two different herbicides with different modes of action. However growers can include nonchemical approaches like cultivation, burning, or to a lesser extent, stock grazing, as the second action.


The interaction of temperature on the efficacy of glyphosate on awnless barnyard grass has been researched, and whilst it was found that cooler temperatures enable more effective control, the ultimate conclusion is that the best tactic for awnless barnyard grass control in fallow, irrespective of temperature, is the double knock.

Both knocks effective

What all double knock practices have in common is that both control measures must be effective in their own right.

Timing between knocks

Awnless barnyard grass and windmill grass are well controlled with double knocks where the second knock is anywhere from 1 to 14 days after glyphosate application.

Feathertop Rhodes grass appears to be best controlled with about 7 to 14 days between knocks where glyphosate is applied first, but even when we get the timing right it is difficult to control.

Feathertop Rhodes grass

Small grass

As with other herbicide treatments, double knock is most effective on small weeds. For our summer grass species, aim for the first knock to be applied to pre-tillering plants.

Think outside the drum

There are other forms of double knock that don’t rely entirely on herbicides. As long as both knocks give good control in their own right it is a good double knock. Tillage, grazing and burning are a few examples.


There is more field research into the double knock that we have not reported on here, and more research is ongoing. We’ll report on this research in the coming months.

We still have a lot to learn about the double knock for northern weeds. Based on what we know now the answer is:

  1. Spray small grass: 3 – 5 leaf, pre-tillering. Don’t wait for more rain and another germination – get out there and spray
  2. High rate of glyphosate first under low plant stress conditions if possible
  3. High rate of Spray.Seed® or paraquat 4 to 7 days later, plenty of water, excellent coverage, also in the coolest possible conditions.
  4. Don’t accept any survivors: take no prisoners!

Growers that are taking an aggressive approach are having a win. You can too!


Merry Christmas!

Fa la la, la la la, la la la.


P.S. Did you know we’re on Twitter? Come and say hi!



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