Glyphosate resistant annual ryegrass, NSW.
Photo: Andrew Storrie, NSW DPI

How much glyphosate resistance is there?
What does glyphosate resistance look like?
What is the mechanism of glyphosate resistance?
I’m now using more glyphosate to manage weeds; have I got resistance?
How quickly does glyphosate resistance develop?
Can the use of other herbicides lead to glyphosate resistance?
Will using tank mixes reduce the chance of glyphosate resistance developing?
Will using low rates of glyphosate increase the risk of glyphosate resistance?
Will using different types of glyphosate reduce my risk of developing glyphosate resistance?
Will glyphosate resistance spread?
Will glyphosate resistance in weeds revert to susceptible?
Will using the double knock lower the risk of glyphosate resistant weeds?     
Can tillage lower the risk of glyphosate resistant weeds?
Will spraytopping or croptopping with glyphosate be effective on glyphosate resistant plants?
Will inter-row application of glyphosate increase the risk of resistance?
Is paraquat resistance possible?
Will the use of low rates of paraquat increase the risk of paraquat or glyphosate resistance?
Clethodim - aka Select® (MOA Group A) resistance and glyphosate resistance – is there a link?
What do I do if I suspect glyphosate resistance?
What can increase the risk of glyphosate resistance in weeds?
What can reduce the risk of glyphosate resistance in weeds?
Has flaxleaf fleabane developed resistance to glyphosate?

What does glyphosate resistance look like?
In the field, glyphosate resistant populations generally begin to appear as a scattering of single plants or small patches of plants that have survived a glyphosate application. Resistant plants are likely to initially show typical glyphosate damage symptoms. Growth is affected for a few weeks before the plants recover. Early control of these small patches is critical to prevent resistant plants from setting seed.

What is the mechanism of glyphosate resistance?
We now know of two mechanisms of glyphosate resistance in annual ryegrass in Australia. Most of the resistant annual ryegrass populations are the result of limited translocation of glyphosate within the plant. In plants with this resistance mechanism, glyphosate preferentially collects in the leaf tips, leaving much lower concentrations at the growing points. These plants are therefore able to quickly grow away from any glyphosate damage. The second mechanism is through mutations within the EPSPS target site that allow a modest degree of resistance to glyphosate. Plants with this mechanism are typically less resistant to glyphosate than those with the translocation mechanism. They are also more heavily damaged by glyphosate and show the effects for longer. In annual ryegrass, it is possible to get both mechanisms in the same plants.  These plants are highly resistant to glyphosate.

I’m now using more glyphosate to manage weeds; have I got resistance?
Changes in farming practices, like the adoption of reduced tillage methods, often result in changes in the herbicide rate required to achieve total weed control. This is also the case for knockdown herbicides like glyphosate. The use of higher rates of knockdown herbicides to achieve complete weed control does not infer herbicide resistance or that resistance is beginning to occur. If resistance is suspected, herbicide resistance testing can confirm any resistance levels relative to known susceptible populations (see GSWG guide on what to do if glyphosate resistance is suspected).

How quickly does glyphosate resistance develop?
Field experience and simulation modelling suggests that the likelihood of glyphosate resistance developing can increase substantially after a population has received approximately 15 years of glyphosate applications. However, many field populations have received more glyphosate applications and have not developed glyphosate resistance. Clearly, there are several factors that can influence how quickly glyphosate resistance will develop other than just the number of herbicide applications. Most importantly for growers, management has a major influence and there are several practices available that reduce the risk of resistance developing and may extend the effective life of the herbicide (see GSWG risk guide).

Can the use of other herbicides lead to glyphosate resistance?
There is no evidence that resistance to other herbicides confers cross-resistance to glyphosate. That is, you can assume that only glyphosate can select for glyphosate resistance.

Will using tank mixes reduce the chance of glyphosate resistance developing?
To be effective in reducing the risk of glyphosate resistance, the tank mix partner must be fully compatible with glyphosate and must be applied at a rate that will kill any glyphosate resistant plants in the treated population.

Will using low rates of glyphosate increase the risk of glyphosate resistance?
Herbicides should only be used at rates that consistently give high kill. On average, using lower rates increases the chance of weeds surviving the glyphosate application. The likely result is higher weed numbers – a factor that can increase the risk of glyphosate resistance. Therefore, the recommended product label rate should be used.

Will using different types of glyphosate reduce my risk of developing glyphosate resistance?
No it won’t. The use of different brands of glyphosate or even other salts of glyphosate will not reduce the risk of developing glyphosate resistance. All glyphosate products, including all the various salts and different formulations, are the same from a resistance management point of view. They all have the same mode of action (EPSP synthase inhibitors) and therefore are all GROUP M herbicides.

Will glyphosate resistance spread?
Resistance genes can potentially spread through either pollen or seed movement. The great majority of ryegrass pollen is generally only viable within 30m of the source plant. In the case of glyphosate resistance, gene transfer through pollen has not been shown to be a major factor. Weed seed can be moved between paddocks or farms by livestock, machinery, crop seed, fodder, water and wind and care needs to be taken to ensure that glyphosate resistance is not spread in this way. Many known glyphosate resistant populations in Australia have not rapidly increased in size, suggesting that resistance will not spread quickly if identified early and managed well.

Will glyphosate resistance in weeds revert to susceptible?
Current evidence suggests the frequency of glyphosate resistant individuals in a population will decline with time if glyphosate is not used. However, it would take many years of no glyphosate use for the proportion of resistant plants to return to the very low levels needed for glyphosate to be effective on that population again over repeated applications. Therefore, once a population is resistant to glyphosate it should be assumed that glyphosate will not be a fully effective control option for that population again. As a result, rotations of herbicides that give glyphosate a rest in some years can be very valuable in reducing the risk of glyphosate resistance. It is likely that glyphosate will continue to be used to control other susceptible weed species in the paddock even after resistance in one species has developed. There will be no decrease of glyphosate resistance if there is continued selection pressure.

Will using the double knock lower the risk of glyphosate resistant weeds?
Yes, but only when used correctly. The double knock of glyphosate followed by application of a paraquat-based product relies on both herbicides being applied at rates appropriate for the weed species being targeted. The strategy relies on control of the rare glyphosate resistant survivors by the paraquat. The glyphosate survivors are not likely to be controlled by low rates of paraquat. Similarly, large weeds are unlikely to be controlled by paraquat.

Can tillage lower the risk of glyphosate resistant weeds?
Yes, but only if the rare glyphosate resistant survivors are killed by the cultivation. For this to happen, the cultivation needs to be well timed and executed. For example, by acting as a double knock, full-cut cultivation at seeding following a knockdown glyphosate application will considerably reduce the chance of any rare glyphosate resistant plants surviving and therefore reduce the risk of glyphosate resistance.

Will spraytopping or croptopping with glyphosate be effective on glyphosate resistant plants?
No. Research has shown that seeds will remain viable if glyphosate is applied as a spraytop or croptop to glyphosate resistant annual ryegrass. Spraytopping or croptopping with glyphosate should not be undertaken on populations known or suspected to be resistant to glyphosate and an alternative mode of action herbicide should be used.

Will inter-row application of glyphosate increase the risk of resistance?
Although glyphosate resistance is rare, the use of glyphosate in-crop, such as in shielded interrow spraying, can increase the risk of resistance developing. This is because the use of glyphosate in-crop can expose a greater percentage of a weed population to the herbicide, with limited opportunities for resistant survivors to be controlled before they set seed. The use of inter-row spraying, where registered (e.g. for specific uses in cotton), should be used in conjunction with integrated weed management strategies.

Is paraquat resistance possible?
Yes. Paraquat and diquat resistance has been confirmed in many barley grass (Hordeum spp.) populations in Australia (see www.weedscience.org). These have mainly come from very intensive paraquat and diquat use in lucerne and some from no-till cropping. Paraquat resistant capeweed and silver grass populations have also been confirmed. There are no confirmed paraquat resistant ryegrass populations in Australia, although paraquat resistant annual ryegrass has been confirmed in vineyards in South Africa.

Will the use of low rates of paraquat increase the risk of paraquat or glyphosate resistance?
Yes.  Herbicides should only be used at rates that consistently give high levels of weed control. On average, using lower rates increases the chance of weeds surviving the paraquat application. The likely result is higher weed numbers – a factor that can increase the risk of paraquat or glyphosate resistance. Also studies have indicated that paraquat resistance can develop more quickly if lower rates (less than 1L/ha of paraquat) are routinely used on outcrossing weed species like annual ryegrass. This is caused by the pattern of inheritance of paraquat resistance in annual ryegrass. Therefore, the recommended product label rate for the purpose should always be used.

Clethodim - aka Select® (MOA Group A) resistance and glyphosate resistance – is there a link?
Currently, three populations of glyphosate resistant annual ryegrass in northern NSW also have low levels of resistance to clethodim (Group A).  This is the result of the recent intensive use of clethodim, often tank-mixed with glyphosate, to control glyphosate resistant annual ryegrass prior to planting a winter crop. Prior to becoming glyphosate resistant, these populations previously had some exposure to Group A herbicides.

This use of Group A herbicides in fallow is not registered or ‘permitted’. There are no crop plant back periods established for cereals and severe crop damage has been recorded following this practice.

There is no known cross resistance occurring between Groups A and M, resistance to the two groups is selected separately. In southern farming systems Group A resistance in annual ryegrass populations is common, while resistance to glyphosate remains quite rare.

The development of clethodim resistance in glyphosate resistant annual ryegrass effectively removes Group A herbicides as a glyphosate resistance management tool for grass weeds in winter crops.  Clethodim should not be relied on as the only management strategy for glyphosate resistant annual ryegrass. Instead integrated weed management and a policy of preventing seed set from any weed survivors should be used.

What do I do if I suspect glyphosate resistance?
When glyphosate resistance is first suspected we advise that growers contact their local agronomist. The following steps are then recommended:

1 Consider the possibility of other common causes of herbicide failure by asking:
> Was the glyphosate applied in conditions and at a rate that should kill the target weed?
>  Did the suspect plants avoid herbicide contact or emerge after the glyphosate application?
> Does the pattern of surviving plants suggest a spray miss or other application problem? If resistance is still suspected:

2 Contact one of the following members of the national Glyphosate Sustainability Working Group for advice on sampling suspect plants for testing and confirmation of the resistance status:

 South Australia:
Chris Preston, University of Adelaide:
Phone: 08 8303 7237 Fax: 08 8303 7109
Email: christopher.preston@adelaide.edu.au 

New South Wales:
Southern: John Broster, Charles Sturt University:
Phone: 02 6933 4001 Fax: 02 6933 2924
Email: jbroster@csu.edu.au  

Northern: Tony Cook, NSW Department of Primary Industries:
Phone: 02 67631250 Fax: 02 6763 1222
Email: tony.cook@industry.nsw.gov.au

Victoria and Tasmania:
Chris Preston, University of Adelaide:
Phone: 08 8303 7237 Fax: 8303 7109
Email: christopher.preston@adelaide.edu.au 

Queensland:
Michael Widderick, Queensland DPI&F:
Phone: 07 4639 8856 Fax: 07 4639 8800
Email: michael.widderick@deedi.qld.gov.au

Western Australia:
Steve Powles, WAHRI, University of WA:
Phone: 08 6488 7870 Fax: 08 6488 7834
Email: spowles@plants.uwa.edu.au  

Abul Hashem, Department of Agriculture and Food Western Australia:
Phone: 08 9690 2000 Fax: 08 9622 1902
Email: ahashem@agric.wa.gov.au 

3 Ensure suspect plants do not set any seed.

4 If resistance is confirmed, develop a management plan for future years in consultation with an agronomist.

What can increase the risk of glyphosate resistance in weeds?
There are a number of predisposing risk factors for glyphosate resistance. Practices that may increase the risk of glyphosate resistance in weeds include:
> Frequent glyphosate based chemical fallow
> Continuous reliance on glyphosate pre-seeding
> Lack of tillage
> Lack of effective in-crop weed control
> Inter-row glyphosate use
> Frequent crop-topping with glyphosate
> High weed numbers

For a guide to managing glyphosate resistance risks visit: [Add new link here]

 What can reduce the risk of glyphosate resistance in weeds?
Practices that may be used to decrease the risk of glyphosate resistance in weeds include:
> The double knock technique*
> Strategic use of alternative knockdown groups
> Full-cut cultivation at sowing
> Effective in-crop weed control
> Use alternative herbicide groups or tillage for inter- row and fallow weed control
> Non-herbicide practices for weed seed kill
> Crop-topping with alternative herbicide groups
> Farm hygiene to prevent resistant seed movement

For a guide to managing glyphosate resistance risks visit: [Add new link here]

* The double knock technique is defined as using a full cut cultivation OR the full label rate of a paraquat-based product
(Herbicide Group L) following the glyphosate (Herbicide Group M) knockdown application.

Has flaxleaf fleabane developed resistance to glyphosate?
An extensive random survey of flaxleaf fleabane in southern Queensland and northern NSW showed that populations differed in their response to glyphosate when grown under the same conditions. Notably, all populations from cropping paddocks in southern Queensland and most from cropping paddocks in NSW required a significantly higher glyphosate dose (3 to 6-fold higher) for similar control than populations from non-cropping situations remote to cropping areas. It is highly likely that these cropping paddocks had a history of glyphosate use for fallow weed control. In contrast, the non-cropping situations were not likely to have been treated as extensively with glyphosate. 

This survey provides a strong indication that flaxleaf fleabane populations, that have been repeatedly sprayed with glyphosate, are less susceptible than those that have not been repeatedly sprayed. Populations from limited sampling in South Australia and West Australia, which have not had as long a history of treatment with glyphosate as the northern NSW and QLD crop paddock populations, responded the same as those from non-cropping situations in southern Queensland.

This weed has been classed as resistant to glyphosate in several countries overseas. However, this weed has generally been considered difficult to control in Australia with glyphosate alone.  It is also not on Australian glyphosate labels, so technically it can’t be classified as resistant to glyphosate in this country. 

The increased tolerance to glyphosate present in fleabane populations from crop paddocks in northern NSW and southern Qld means glyphosate alone will not control this weed.  A double knock strategy or alternative knockdown herbicides should be employed.   

While all due care has been taken in compiling this information, no responsibility is accepted for its contents, any error or omission, nor for any loss, damage or consequence whatsoever for any person or body relying on the information.

Last Updated: 16/04/2010