Northern resistance

Photo of collected awnless barnyard grass

Different responses of collected awnless
barnyard grass samples to glyphosate applied
12.8L/ha, showing very strong levels of
resistance in some biotypes.

PHOTOS: Tony Cook

Photo of Tony Cook

Tony Cook: surveys now measuring the rapid
spread of herbicide resistance

PHOTO: Nicole Baxter

Building a picture of the herbicide resistance problem across a large area – such as the whole northern grains region – is an exhaustive and difficult task. The diversity of weeds, cropping systems, soils and environments creates variable circumstances throughout the region.

Until recently, no one has even tried to catalogue the general distribution of herbicide resistance. However, we do know that the most widespread resistant species in the northern grains regions are wild oats (Groups A, B and Z) and fleabane (Group M). Awnless barnyard grass and annual ryegrass, both resistant to glyphosate, are found mainly in the north and south of the region, respectively.

Glyphosate-resistant (GR) windmill grass is in the infancy of its spread and is mainly confined to the central-west plains.

Since the discovery of resistance in the northern region in the mid-1990s, the number of species affected by resistance has steadily increased (Figure 1). Further, the level of resistance has escalated due to repeated use of the same mode-of-action herbicides. This has been compounded by the spread of these weeds by human assistance or natural forces.


Figure 1

Increase in species and frequency of
glyphosate-resistant weeds since 2007

Source: Australian Glyphosate Sustainability
Working Group


Resistance is usually patchy, especially with weeds such as wild oats and barnyard grass. This patchiness could be on a farm-to-farm, paddock-to-paddock and/or within-paddock scale.

Flexible and effective weed management usually controls these patches, whereas sticking to the standard control options that led to resistance will ultimately see the patches expand.

Many growers in the northern region are, or will be, dealing with summer and winter weeds with various types of resistance. No longer can growers deal with one weed issue separately. There will be a need to solve more complex problems, because solving one resistance issue has the potential to make another one worse.

Three key species to focus on are wild oats, fleabane and awnless barnyard grass.

Wild oats

Regardless of resistance status, wild oats is one of the most widespread weed species in the northern region. Although not a significant issue in Central Queensland, its numbers steadily increase from the Northern Downs into New South Wales.

A comprehensive herbicide resistance survey was undertaken by the Northern Grower Alliance (NGA) in 2007 for the central part of the region. Although the random survey indicates a concerning proportion (22 per cent) of paddocks with some form of resistance, the figures for 2013 are expected to be much worse than this. Since 2007, the incidence of any form of resistance in a paddock is likely to be at least 50 per cent due to the continual use of selective post-emergence herbicides.

This distribution and frequency of post-emergence resistance was found to be generally common throughout the NGA survey region.

It is likely that the incidence of this form of resistance would also steadily increase in the southern part of the region as the frequency of winter cropping increases (more selective post-emergence herbicides are used)

Of concern is the number of populations with multiple resistance. More than half the samples collected from ‘paddocks of concern’ had resistance to three different herbicide products.


 Biotype Location of source biotype
 Glyphosate (450g/L) rate per hectare
Table 1 Percent control of green awnless barnyard grass biomass 28 days after treatment for various rates of glyphosate (450g/L) relative to nil rate of that biotype.
0.8 1.6 3.2 6.4 12.8
 Susceptible  Tamworth  0.0
 100 100
 Resistant  Bellata  0.0  –1.6  23.4  18.5  49.2  78.0
 PXX1  Moree  0.0  –8.0  20.9  42.8  88.7  100
 LXX4  Croppa Creek
 0.0  37.4  37.7  81.8  95.8  100
 TXX3  Bellata  0.0  27.8  50.4  37.7  72.3  89.2
 PXX3  Boggabilla  0.0  –104  –151  –162  –17.2  42.6
 TXX4  Gurley  0.0  –103  –6.9  20.8  30.6  44.4
 GXX2  Bellata  0.0  –54  –7.1  21.2  57.6  75.0
 JXX1  Westmar  0.0  –144  –4.2 –15.3
 22.9  11.0

In some cases there are populations of wild oats that have resistance to all the three modes of actions that have selective post-emergence activity. In these cases wheat production is seriously threatened.

Group A herbicides are the only post-emergence option for chickpea crops. The spread of herbicide resistant wild oats will be a very significant challenge in the management of chickpea crops. A crucial finding of the survey was that no samples were found to be resistant to a trifluralin/Avadex® tank mix.


Fleabane has become the most widespread weed species in the northern region. This can be attributed to the widespread adoption of minimum or no-till farming and the large number of wind-borne seeds produced. Three comprehensive herbicide resistance surveys were completed in 2010, 2011 and 2012.

The first survey resulted in six populations from the Darling Downs and two from northern NSW being placed on the glyphosate resistance register. It was shown that there is considerable variation in response to glyphosate depending on the history of glyphosate use. Samples collected from paddocks in predominantly non-cropping areas were susceptible to glyphosate, whereas seed sourced from grain-growing regions had a higher incidence of resistance.

In 2011, a national non-cropping survey was completed. It targeted areas such as roadsides, railways, irrigation channels and around buildings such as silos. A further 35 populations from the northern region were confirmed as glyphosate resistant. All these were sourced from roadsides, an area commonly treated with glyphosate, hence the abundance in this area.

The 2012 survey in the northern region was aimed at trying to find any Group-I-resistant fleabane. Seed was collected from surviving plants in the summer fallow period. None of these populations were found to have Group I resistance.


Table 2 Summary of the latest 2012-13 glyphosate resistance national survey results.
   Resistant  Marginal resistance
 Number of samples

Awnless barnyard grass

Collaborative survey work between the NSW Department of Primary Industries (DPI), the Department of Agriculture, Fisheries and Forestry Queensland (DAFFQ) and the NGA started in late 2011. The aim was to better understand the extent of glyphosate-resistant awnless barnyard grass in the central part of the northern region.

Of the 78 samples received, 45 samples (58 per cent) were confirmed to be resistant. These new confirmed cases are now on the Australian Glyphosate Sustainability Working Group register.

Prior to this work the register had 21 confirmed cases, so the new cases represent a trebling of the confirmed resistance cases of this species. The location of the resistant awnless barnyard grass populations is well spread within the area surveyed. Resistance occurs from Dalby, Queensland, down to Tamworth, NSW, with a greater concentration of cases between Goondiwindi, Queensland, and Narrabri, NSW. As of March 2013, the number of confirmed cases stands at 76, all but one of these being within the northern region.

Photo of chickpeas, under threat from Group-A-resistant wild oats

The chickpea industry is under threat from
Group-A-resistant wild oats.


Evidence from glyphosate response experiments indicates that some of the biotypes tested are more resistant than the ‘resistant standard’ collected from a confirmed resistance site. Many populations demonstrated survival even at very high rates of glyphosate application. (Table 1).

The successful 2011-12 glyphosate resistance survey was repeated in the 2012-13 season. It was widened to be a national survey (Table 2). More than 90 samples were received and about 60 samples were sprayed and assessed for resistance.

One-third of samples received were classed as resistant. Most of these resistant biotypes were sampled in the heartland of the glyphosate resistance region, between Dalby and Narrabri. However, isolated populations were confirmed as resistant from Gatton in Queensland and Wellington, Gunnedah and Warren in NSW.

Low summer rainfall in the central-west region of NSW limited the number of samples from that region.

Funding for this research was provided predominantly by the GRDC, the NSW DPI, DAFFQ and the University of Queensland.

More information:

Tony Cook,
02 6763 1250,;

Northern Growers Alliance,

A fact sheet on Fleabane is available at:


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