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Glyphosate Resistance

Glyphosate is the most popular herbicide used widely in Australia and around the world and is a valued component of modern agricultural farming systems. For this reason the development of resistance to glyphosate poses a serious threat to the long term viability of Australian farming systems. Growers adopting Roundup Ready® crops are encouraged to review their weed management systems to ensure that this technology is being used as part of an overall integrated weed management (IWM) plan. This approach will ensure that growers continue to receive the benefits afforded from the use of the technology while minimising the likelihood of resistance developing on their farms.

Recently, there have been 5 confirmed cases of glyphosate resistant Awnless Barnyard grass (Echinochloa colona) and 2 cases of resistant Liverseed grass (Urochloa panicoides) in northern NSW and southern Queensland. In each case, resistance developed as a result of over reliance of glyphosate in the farming system combined with no/min tillage operations thus increasing the selection pressure for resistance.

How does resistance develop?

Herbicide resistance has been defined by the Weed Science Society of America as “the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to a wild type.” In any weed population even before the herbicide is used individual plants may contain genes conferring resistance to that herbicide. Susceptible weeds will be killed when herbicide is applied, however weeds containing the resistance gene over time may become dominant in the population depending on:

  • how often the herbicide is used

  • the type of herbicide

  • the size of the weed population

  • the initial resistance gene frequency, the weed seed characteristics

  • the proportion of the weed population being treated

Integrated Weed Management

While it may not be possible to totally eliminate the likelihood of resistance developing in modern farming systems, there are ways to reduce the risk. The best way to avoid the development of herbicide resistance is to develop and implement an Integrated Weed Management (IWM) plan. The following principles are important considerations in an IWM plan with a more complete description of tactics included in Figure 1 which was developed by Graham Charles (I &I NSW) for use in cotton production systems:

  • Keep accurate paddock records

  • Scout fields before and after herbicide or weed control operations

  • Start with a clean field by practicing good fallow and whole farm management

  • Use the right herbicide product at the right time at the right rate

  • Use multiple herbicide modes of action in the weed control system

  • Control weeds when they are small

  • Control escapes: Prevention of weed seed set is vitally important to stop resistance developing

  • Ensure good farm hygiene. Clean equipment before moving from field to field

  • Use non-herbicide farming practices such as strategic cultivation and crop rotation as a standard part of the weed management system

  • Use competitive crops and maintain robust planting rates

Growers and advisor considerations

In situations where glyphosate resistance is suspected, farmers and agronomists should first consider the following:

  • Whether the application timing and rate was appropriate for the weed species and growth stage, was spray coverage adequate? Or did stubble cover prevent the herbicide from reaching the target?

  • Were plants stressed? Moisture stress is a common factor in poor control of grasses, especially liverseed grass, when the translocation of glyphosate to the roots is significantly impeded.

  • Glyphosate antagonism may occur due to tank mixes/additives, or to field conditions such as stubble cover or dust that can reduce the efficacy of glyphosate.

  • Did the suspect weed emerge before or after herbicide application?

  • Has glyphosate been the main method of fallow weed control over the past 10 or more years?

  • Has only one species survived while other similarly susceptible species died?

  • Is the survival patchy – has the weed only survived in a small area and been controlled elsewhere?

  • Does the surviving patch relate to a boom width and/or first run after filling the tank?

Report suspect weeds
If growers continue to suspect glyphosate resistance they should immediately contact their Monsanto or Nufarm representative. Plants should be tested to confirm resistance using either the seed test or whole plant test. The whole plant test is a quicker test (4-6 weeks) and is useful for testing grasses, while the seed test generally takes 2-3 months before results are obtained.


Further details for testing requirements for glyphosate resistance are available from the DPI website, www.dpi.qld.gov.au/26_14562.htm or by emailing info@plantscienceconsulting.com or alternately jbroster@csu.edu.au

Monsanto and Nufarm are committed to working with growers, regulators and researchers to address weed resistance to glyphosate. Stewardship of our Roundup and Roundup Ready products is key to preserving the effectiveness of this valuable herbicide and the benefits growers receive from these technologies.

For more information on Biotech Stewardship visit www.monsanto.com.au or call Craig McDonald 0407541665 at Monsanto