Glyphosate resistance forces new approach

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Glyphosate-resistant fleabane and barnyard grass are now widespread in the northern cropping region

Controlling barnyard grass and fleabane

  • Monitor paddocks for new flushes especially in autumn and spring for fleabane and spring and summer for barnyard grass.
  • In fallows, spray small weeds (less than five centimetres in diameter) for fleabane and seedling to early tillering for barnyard grass. Remember: knockdown efficacy drops markedly on older and larger weeds.
  • To minimise replenishment of the seedbank, use the double-knock tactic by spraying paraquat products about seven days after the first knock.
  • Consider adding a residual to the double-knock for season-long control.
  • Follow-up sprayed survivors with robust rates of knockdowns using a weed detector sprayer as outlined in the new WeedSeeker® permit (PER11163).
  • Grow a competitive crop and use in-crop spraying to control small weeds.

FIGURE 1 Comparison of zero-tillage and strategic tillage on
fleabane emergence in the northern growing region.

A blue bar graph

FIGURE 2 Impact of double-knock (glyphosate mix followed by
Spray.Seed®) or a single application of a glyphosate mix on control
of small and large fleabane plants in the northern growing region.

A graph with four variables represented

FIGURE 3 Impact of low and high glyphosate rates on control
of small (early tillering) and large (late tillering) barnyard grass plants
in the northern growing region.

A blue bar graph

Note: This research is a combined effort between the University of
Queensland, the Queensland Department of Agriculture, Fisheries
and Forestry and the NSW Department of Primary Industries

Five of Australia’s six glyphosate-resistant weed species can be found in the northern growing region where several decades of glyphosate use to control fallow weeds have selected for herbicide-resistant populations.

Weed-free fallows are essential for conserving soil moisture and maximising yields of the region’s winter and summer crops but an over-reliance on glyphosate (some growers are using six applications over the fallow) is threatening the viability of the area’s no-till cropping systems.

Topping the region’s herbicide-resistance list is Australia’s first glyphosate-resistant broadleaf weed, flaxleaf fleabane, which in some cases has proved resistant to eight times the normal rate of glyphosate.

Barnyard grass is a close second on the list, with half of the tested populations identified as resistant.

GRDC-funded research by the Queensland Department of Agriculture, Fisheries and Forestry, the University of Queensland and the New South Wales Department of Primary Industries has identified chemical and non-chemical tactics to control glyphosate-resistant weeds and extend the life of the valuable knockdown herbicide.

Fortunately, many of the weeds that dominate northern region no-till cropping systems produce seeds that are short-lived in the soil. This means that with just a few years of preventing seed-set the weed seedbank can be reduced by up to 99 per cent.


An inability to germinate from deeper than one centimetre makes fleabane particularly susceptible to cultivation. Strategic tillage to bury weed seed ‘blow-outs’ has proved a successful control tool for this troublesome weed, which is capable of producing two or three flushes of new seedlings each year and 110,000 seeds per plant.

A strategic tillage event using harrows, tynes, offset discs or one-way discs reduced subsequent fleabane emergence by about 90 per cent of that in zero-tilled plots in recent research at Tamworth, NSW (Figure 1). 

The results show that even a harrow operation, which results in a relatively shallow soil disturbance, has a large and significant impact on fleabane emergence. However, the more disruptive one-way discs decrease seedling emergence to near zero.

A one-off tillage operation to cope with a severe fleabane blow-out would be enough to set back the weed seedbank to a manageable level, which could then be further reduced using herbicide control such as the double-knock tactic.

Fleabane double knock

A correctly used double-knock approach with applications of herbicides from two different mode-of-action groups, such as a glyphosate mixture followed by paraquat and diquat five to 10 days later, can result in complete fleabane control (Table 1).

It is important to remember that even susceptible fleabane populations are difficult to control with glyphosate, especially once the weed has grown past the seedling stage. Fleabane control using the double-knock can decline substantially on older rosettes (two months old) compared with younger rosettes (one month old) (Figure 2).

The paraquat and diquat application is used to ‘finish off’ any survivors. The aim is to achieve a large (more than 80 per cent) control rate using the first knock (glyphosate mix) and to kill the remaining 20 per cent or so of survivors with the second knock (paraquat and diquat).

Including a residual herbicide in the second knock can provide excellent long-term control of fleabane in fallows.

It is critical to monitor paddocks post-spraying and clean up any survivors.

Fleabane can be more reliably and more readily controlled using a double-knock approach instead of a single glyphosate-mix application. In recent research, fleabane control was increased by five to 10 per cent and variation in control reduced by as much as 30 per cent using a double-knock approach compared with a single knock of glyphosate mix (Table 1).

Table 1 Impact of a glyphosate mix (first knock only) or a double-knock of glyphosate mix followed by paraquat and diquat on level of fleabane control in the northern growing region.
 First knock only  Double-knock
(first knock followed seven days later
by a second knock of Spray.Seed®)
 Herbicide  Mean (%)  Range (%)  Herbicide  Mean (%)  Range (%)
 Glyphosate + Tordon 75-D®  95.7  86-99  Spray.Seed®  99.5  97-100
 Glyphosate + 2, 4-D amine  89.2  62-99  Spray.Seed®  98.1  93-100

Barnyard grass

Fallow control of barnyard control is critical to controlling the weed’s seedbank. While glyphosate has been the mainstay of barnyard control, increasing glyphosate-resistant populations of the weed has forced a rethink of its management.

In a survey of fallow paddocks between Dalby, Queensland, and Tamworth, NSW, that had recently been sprayed with glyphosate, nearly 90 per cent of barnyard grass survivors were identified as glyphosate resistant. The resistant populations were concentrated between Narrabri, NSW, and Goondiwindi, Queensland.

With its staggered emergence, highly competitive growth habit and ability to produce up to 20,000 seeds per plant, barnyard grass can quickly become a significant problem.

As with fleabane, effective barnyard control centres on using multiple tactics such as a double-knock, residual herbicides and strategic tillage to bury weed seed at depth. Monitoring for survivors regardless of their scarcity and preventing them from setting seed is a necessity – especially in herbicide-resistant populations.

Strategic tillage reduces barnyard emergence, but to a lesser extent than fleabane because the grass can emerge from deeper in the soil (two to four centimetres) and can also remain viable for longer at depth. Tillage can bury seeds below 5cm but some of those remaining above this depth will still emerge.

Glyphosate applied alone at robust rates on young (early-tillering) plants can lift kill rates on glyphosate-susceptible barnyard grass to 100 per cent (Figure 3). Timing of the glyphosate application is critical, with a delay of just one week reducing weed control dramatically because larger or moisture-stressed plants do not absorb the glyphosate effectively. The double-knock is needed to control survivors of larger or glyphosate-resistant plants.

Adding a residual herbicide to the double-knock can provide good control of the weed in fallows. Several registered residual herbicides, such as Flame® and atrazine, have been shown to have good control (83 to 96 per cent) of barnyard grass for a minimum of a month in fallow.

Row A
Row B
Image showing glyphosate resistant barnyard grass sprayed with a single application of glyphosate
Row C
Row D
Bird's eye view photo of twenty experimental pots

Barnyard double-knock approach. The double-knock approach is now a necessity for controlling widespread herbicide-resistant barnyard grass and fleabane populations across the northern growing region. Pictured are glyphosate-resistant (rows A and C) and glyphosate susceptible (rows B and D) barnyard grass populations that have been sprayed with either a single glyphosate application (left photo) or a double-knock of glyphosate followed by paraquat and diquat (right photo). Resistant populations are not controlled with glyphosate alone.

PHOTOS: Steve Walker

More information:

Associate Professor Steven Walker, University of Queensland,

Dr Michael Widderick, Queensland DAFF,

Next: Mechanical approach to resistance

GRDC Project Code UQ00062

Region North, South, West