By Cindy Benjamin
Knockdown herbicides opened the door for broadacre conservation farming, essentially taking the place of cultivation for weed control between crops.
Unfortunately, only a few broad-spectrum herbicides are available to growers to prevent weeds from taking advantage of stored soil moisture in fallow periods.
The minimal diversity of chemistry inevitably leads to strong selection pressure after many years of consistent use. It also leaves the industry exposed to regulatory uncertainty and changes.
Mitch Allen, the national commercial technical lead at Nufarm Australia–Crop Protection, says the grains industry is at the start of a period of adjustment to identify alternative knockdown products and use patterns for fallow weed management.
“The double-knock tactic was developed specifically to extend the useful ‘life’ of glyphosate, the first broad-spectrum knockdown herbicide,” Nick explains.
“Herbicide resistance follows fairly predictable timelines, and the only way to slow or prevent the process is to consistently remove any individual plants which survive the herbicide application before they set seed,” he says.
“This is the role of the double-knock tactic, a component of the WeedSmart Big 6 strategy.”
When the ‘glyphosate followed by paraquat’ double-knock was developed, there was a period of experimentation and refining of the practice in different regions and with different weed spectrums.
Mitch says the industry will need to undergo a similar process in the next few years to learn how to best use other knockdown herbicides.
“There was hesitancy to use paraquat in the initial stages, and lessons were taken on board to build a robust strategy around using the product effectively,” he says.
“We now need to do the same with glufosinate and potentially other products that might have a fit in the double-knock tactic for fallow weed management.”
Mitch warns against the natural tendency to rely too heavily on a new product once an older product begins to have performance issues.
For example, replacing paraquat with glufosinate (Group 10, e.g. Biffo or Basta) or a Group 14 herbicide (e.g. Terrad'or (tiafenacil) or Voraxor (saflufenacil + trifludimoxazin)) will have a place to manage fallow weeds, but this is not ‘the answer’.
“It is necessary to rethink the herbicide program for double-knocks,” he says.
“The glyphosate/paraquat double-knock is a robust combination which has worked well for a long time. Glufosinate is less robust than glyphosate or paraquat, it controls a different weed spectrum, and particular application requirements must be met for it to provide effective weed control. It will not be the best option in every situation.”
With rising glyphosate resistance in key fallow weeds and regulatory uncertainty surrounding paraquat, Nufarm and other chemical companies are reviewing their product range in search of existing chemistries which may enhance paraquat efficacy or provide an alternative double-knock partner.
“Group 14 herbicides can improve the efficacy of paraquat, so they are likely to be part of the strategy,” Nick says.
“So far, Nufarm has completed one year of work investigating alternative products and use patterns for summer weed control. We are starting work on winter fallow weeds this year, factoring in the increasing resistance in annual ryegrass to glyphosate and paraquat.”
Mitch says current Group 14 herbicides, such as Terrad’or or Voraxor, can be used in limited standalone applications but are still best tank-mixed with glufosinate or paraquat.
The indications so far are the proposed lower rate of paraquat will benefit from tank mixing with a Group 14 herbicide, like Terrad’or, in a double-knock tactic. This use pattern has shown promising activity on sowthistle, but results were less favourable on larger fleabane and summer grasses.
“More work needs to be done to identify the summer weed spectrum which will respond to various products and mixes,” Mitch says.
“We do have some insights through grower and agronomist experience in the northern region using high rates of glufosinate through optical spot sprayers, and growers in the southern regions are keen to adapt this experience to their region.”
For example, an application of 10 L Biffo (glufosinate) per 100 L water is registered for optical spot-spraying technology (OSST), providing good control of small fleabane, sowthistle and feathertop Rhodes grass.
Mitch says this strategy may have merit in the southern regions, but some weeds, such as heliotrope, will not be controlled effectively.
An alternative double-knock strategy involving a blanket application of glyphosate + 2,4-D, followed by an OSST application of glufosinate, may help overcome glufosinate’s weakness in the weed spectrum and inconsistent results due to hot, dry conditions.
A period of careful trial and error in the field is inevitable, and providing feedback to crop protection companies will help identify best-use patterns for different scenarios.
Other insights are available from the cotton industry as a result of the development of Bollgard3 XtendFlex cotton, which offers over-the-top triple herbicide tolerance for glyphosate, dicamba and glufosinate. Bayer’s trial results from 2019 to 2024 show that a double-knock of glyphosate (Roundup Ready) followed by glufosinate (Biffo) gave 90 to 100 per cent control of feathertop Rhodes grass, awnless barnyard grass, sowthistle, caltrop, tridax daisy and pigweed.
“Glufosinate is slow to penetrate plant leaves and moves slowly within the weed, so the registered higher rate for OSST applications helps increase the efficacy of glufosinate,” says Mitch. “Glufosinate is ineffective if the weeds are large or stressed or if the conditions are hot and dry.”
Tips for OSST and broadacre spray applications of glufosinate plus Group 14 in a double-knock:
In the longer term, Nufarm is working toward having a new stand-alone, non-selective herbicide registered in Australia before 2030.
The double-knock tactic is a key component of the WeedSmart Big 6 strategy designed to keep weed numbers low, preserve herbicide efficacy, and conserve soil moisture in fallow to support competitive crops. Increasing the diversity of tactics used in the double-knock is essential to stop survivor weeds setting seed. Along with alternative knockdown herbicides, growers can also incorporate non-herbicide options such as targeted tillage and potentially electric weeding in the future.
Registrations are now open for WeedSmart Week 2025 in Geraldton, WA, on 29 and 30 July, showcasing how the Big 6 tactics are safeguarding the profitability of large-scale cropping enterprises.
For more information about fallow weed management, visit the WeedSmart website: www.weedsmart.org.au
