First case of glyphosate resistant Italian ryegrass found in the UK
The first documented case in the UK
Scientists at ADAS have identified a single field population of Italian ryegrass (Lolium multiflorum) which is resistant to glyphosate at the field label rate – the first documented case in the UK.
February 2025 Update: In addition to the initial confirmed case, three additional samples (Somerset, Essex and North Yorkshire) have been tested. Two of these have demonstrated significantly reduced control from appropriate doses of glyphosate. The third population also showed decreased glyphosate sensitivity in the tests. All of these cases are very high-risk situations.
Researchers from ADAS have concluded that a single population of Italian ryegrass in Kent has successfully evolved resistance to glyphosate following rigorous laboratory testing and resampling. The discovered case was in a high risk scenario where large weeds were surviving high rates of appropriate glyphosate applications in the run-up to drilling a relatively late drilling spring crop.
Signs that glyphosate resistance is developing in common UK arable weeds have been present for a number of years. Scientists from ADAS and Rothamsted first discovered reduced glyphosate sensitivity in sterile brome back in 2018. To detect early signs of glyphosate resistance developing in UK Italian ryegrass populations, ADAS and NIAB have screened over 300 seed samples as part of standard herbicide resistance testing. Although there have been a number of potential high risk cases found during that time, this is the first documented case where glyphosate resistance has been confirmed.
ADAS Principal Consultant and weed specialist John Cussans commented:
“We have come across several high risk cases before where we could rule out resistance following resampling and retesting. However, this is the first time we can confirm, after vigorous testing and multiple samples taken, that glyphosate resistance is present in a UK Italian ryegrass population.”
In addition to this one confirmed case, three suspect populations of Italian ryegrass are still under investigation, with results expected later this year. To date, Italian ryegrass is the only weed with suspect populations in the UK. Extensive testing of black-grass and a 2023 survey of 166 brome samples found no populations with glyphosate resistance.
The current state of glyphosate resistance
To put this result into a global context, glyphosate resistance has been found in 60 species in 30 countries. Most confirmed cases of glyphosate resistance in Italian Ryegrass are in North and South America. Although there have been instances both in Spain and Italy in 2006 and 2012 respectively, it is worth noting that in both European countries, resistance did not escalate and result in a widespread problem.
Katy Hebditch, ADAS Weed Science Consultant confirmed:
“This finding is a concern but not a surprise. We have known for a long time glyphosate resistance was a matter of when, not if. The main message we want people to take away from this is pro-actively assess risks and manage them to delay the development of resistance on your fields or farm. We must remain careful of how and when we use glyphosate, stay vigilant, and follow best practice recommendations.”
Industry best practice recommendations
Ahead of spring cropping, the Weed Resistance Action Group (WRAG), an independent body made up of crop protection researchers (including ADAS) and agrochemical industry experts, are re-emphasising the importance of glyphosate stewardship and encouraging all growers and agronomists to read and act on the WRAG Guidelines for minimising the risk of glyphosate in the UK, first published in 2015.
WRAG guidelines ask growers to:
- Prevent survivors: Avoid repeat applications to surviving plants.
- Maximise efficacy: Apply the right dose rate to kill all target weeds (reduced rates increase the risk of reduced efficacy), at the right timing (when weeds are actively growing, avoiding stem extension growing phase), in the right conditions (do not apply to plants under stress from waterlogging, drought, and very cold weather).
- Use alternatives: Use non-chemical options (such as cultivation), where practical, and use other herbicides in sequence.
- Monitor success: Remove survivors and report potential resistance issues to your advisor and/or the product manufacturer.
Importance of herbicide resistance testing
In the meantime, as part of the takeaway message to growers, Katy concluded:
“If you have noticed that a particular weed is surviving repeated herbicide applications, it could very well be resistant to that active. ADAS offers independent herbicide resistance testing for all grass and broad leaved weeds so get it checked to know for sure. Early intervention and pro-active mitigation is the best way to manage and reduce risk and ultimately cost.”