Click here to download a printable brochure of Glyphosate Resistance: Frequently asked questions

How much glyphosate resistance is there? There are currently 44 confirmed glyphosate resistant annual ryegrass (Lolium rigidum) populations in Australia (October 2005). Twenty-four of these populations have been confirmed in broadacre cropping. Glyphosate resistance has not been confirmed in other weed species in Australia. For more details of the current status of glyphosate resistance in Australia visit the Australian Glyphosate Resistance Register

What does glyphosate resistance look like? In the field, glyphosate resistant annual ryegrass 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. Investigations into this second mechanism are ongoing.

I’m now using more glyphosate to manage ryegrass; 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 ryegrass regress? 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 an effective control option for that population again. It is also possible that glyphosate will continue to be used to control other susceptible weed species in the paddock after resistance in one species has developed. There will be no regression of glyphosate resistance if there is continued selection pressure.

Will using the double knock lower the risk of glyphosate resistant ryegrass? 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 ryegrass plants 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 ryegrass plants are unlikely to be controlled by paraquat.

Can tillage lower the risk of glyphosate resistant ryegrass? 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 & 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. Paraquat, like glyphosate, is regarded as a low risk herbicide for resistance in Australia.

What do I do if I suspect glyphosate resistance?

Contact
Rex Stanton (Chair)
Phone: 02 6938 1618
Fax: 02 6938 1861

The national Glyphosate Sustainability Working Group acknowledges the financial support of the CRC for Australian Weed Management, CropLife Australia and GRDC. April 2005

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.