Double-knock to beat glyphosate resistance

Growers can reduce the spread of glyphosate-resistant weeds and sustain the effectiveness of glyphosate by using two or more herbicides as part of a double-knock strategy.

The increasing incidence of glyphosate resistance in the northern region and across Australia over the past 12 months was a major topic of discussion at the recent GRDC Research Update in Goondiwindi, Queensland.

Tony Cook, a technical weed specialist with the New South Wales Department of Primary Industries, presented the latest information on issues, trials and developments associated with managing herbicide-resistant weeds in the northern region. 
His main message: kill all weeds using a double-knock strategy.

“We are seeing more and more instances of weeds becoming resistant to our most commonly used herbicides, like glyphosate,” Mr Cook said.

“Recently in northern NSW, glyphosate resistance was confirmed in sowthistle, and earlier this year the world’s first cases of glyphosate-resistant wild radish were recorded in Western Australia.”

Mr Cook said it would not be long until other glyphosate-resistant species were confirmed in the north, adding to the region’s confirmed instances of glyphosate resistance in fleabane, sowthistle, windmill grass, annual ryegrass, liverseed grass and barnyard grass.

“If we want to keep glyphosate and other systemic herbicides (for example, Groups A, B and I) in our toolkit, we need to change the way we manage weeds resistant to them. We need to aim to kill all weeds to reduce the risk of resistant weeds spreading.”

Mr Cook said there were several suitable herbicide and non-herbicide options to choose from but recommended using a double-knock strategy, with two different groups of herbicides, such as Group L in conjunction with glyphosate (Group M) on glyphosate-susceptible plants.

Target all weeds

Weeds in a paddock

Cultivation can be used strategically to control survivors of initial treatments. Shallow cultivation is recommended so that weed seed is not buried too deep; deep-buried weed seeds can persist longer than shallow-buried seed.


A double-knock strategy requires growers to apply two weed-control tactics, with different modes of action, to a single flush of weeds to kill any survivors from the first application. Tactics do not need to be herbicides. 

“Cultivation, heavy grazing or fire can also be used as a second knock, but these aren’t as practical as using another herbicide,” Mr Cook said.

The most common double-knock strategy in the northern region is to apply a systemic herbicide such as glyphosate (Group M) and then follow it with a new chemical from a different group such as paraquat (Group L).

“With a double-knock strategy, growers should be aiming to kill all their weeds,” Mr Cook said.

“Doing this will help make sure that surviving weeds, which may be resistant to the first herbicide used, don’t spread further and make that herbicide redundant.”

Mr Cook said whatever herbicides growers choose to use as part of their double-knock strategy, the first application needs to be robust and targeted at small weeds. 

The second application is then targeted at surviving weeds.

“Choosing your second herbicide may take some trial and error until you find the right one that kills the surviving weeds,” Mr Cook said.

“The critical rule when using double-knock with paraquat as a second herbicide is to make sure it is applied to very small plants, particularly grass weeds.

“Once grasses develop a few tillers, the effectiveness of paraquat rapidly diminishes.”

Double-knock success

Image of a weed

Paraquat does not work on large glyphosate-resistant grasses. It is best suited to plants no greater than 10 centimetres in diameter.


Grower Graeme Constance has been successfully using a double-knock strategy at Bellata, NSW, for the past three years to control the increasing populations of resistant ryegrass, barnyard grass and fleabane on his 840-hectare property.

“If I see anything that survives the first spray, I try to remove it,” he says. “Because I do my own spraying I have a fair handle on what is going on in the paddock. So if I don’t like the look of something I will go back and deal with it.”

Mr Constance says the double-knock strategy, where he uses glyphosate and then paraquat, or a concentrated mix of Group I herbicides (Tordon® and Logran®) on resistant patches, has worked very well for him.

“I tend to use the Group I mix and glyphosate in my wheat crops for broadleaf control, which provides my wheat crop with good residual control through summer,” Mr Constance says.

“However, I did have to resort to ploughing a small patch of resistant ryegrass last year. It wasn’t ideal, given we run a zero-till system here, but I felt it was the best option to control the ryegrass and stop it from spreading even further.”

Retro herbicides

Glyphosate has been northern growers’ herbicide of choice for more than two decades.

Mr Cook said resistance has been slower to develop in the northern region because of the diverse crop rotations, which allow for greater crop and herbicide choice.

However, the increase in the number of reported glyphosate-resistant weeds over the past year suggests that growers will need to find alternative herbicides.

“Unfortunately there are no new herbicides with the flexibility and acceptance of glyphosate on the horizon,” Mr Cook said. 
“For this reason, researchers and agronomists are investigating whether old chemistries can now be used – some of which have not been used for 20 to 30 years.”

Mr Cook said because these older chemicals have not been used as widely or continually, compared with glyphosate, there were fewer examples of weed species resistant to them.

“The classic example is paraquat (Group L), which fell out of favour 30 years ago because it was not as easy to use compared to glyphosate,” Mr Cook said. “Users still need some skill to use the chemical; however, it is being effectively used by growers, particularly as a second chemical option as part of a double-knock strategy.”

Mr Cook said there was no reason for glyphosate to become redundant. He believed it would continue to play an important role in managing weeds in the northern region for many years to come, as long as growers become more vigilant in managing weeds.

“Using herbicides to control weeds is still a viable option in the northern region. However, we have to not just rely on glyphosate. Rather we need to use a variety of herbicides so that glyphosate will still be useful in the future.”

More information:

Tony Cook
0447 651 607


A fact sheet on Herbicide Resistance is available at:

A fact sheet on Effective Double Knock Herbicide Applications Northern Region is available at:

End of Ground Cover #110 (Northern region edition):

Read the accompanying Ground Cover Supplement: Ground Cover Supplement Issue 110: Cereal foliar fungal diseases



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