WRAG announces first case of glyphosate resistance in the UK

“Glyphosate is probably the most important herbicide for UK farmers and there is no obvious replacement hence any case of resistance is concerning,” says John Cussans principle weed science consultant at ADAS who led the research.

“But we must retain perspective; it’s resistance in one weed species on one farm, it is not a widespread problem yet. For context, over the period 2018 to 2023 we have collated data for samples of Italian ryegrass which were submitted for standard resistance tests. They have been screened with glyphosate, and we have not found a single resistant sample. This single case does however serve as a firm reminder of the importance of glyphosate stewardship otherwise we are likely to see more cases.”

At present, in addition to the one confirmed case, three suspect populations of Italian ryegrass are 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 of concern.

“Experience from around the world suggests ryegrass species are high-risk with regards to glyphosate resistance. Globally, there are several cases of resistance in annual ryegrass Lolium rigidum and Italian ryegrass Lolium multiflorum.

“However, other weeds can also develop resistance; glyphosate stewardship concerns every farmer, not just those managing Italian ryegrass. We know that herbicide resistance tends to develop field-by-field so farmers can work to maintain glyphosate efficacy on their farm.”

Reducing resistance risk

There are fundamental stewardship steps that all farmers using glyphosate need to take. On top of that, farmers with higher risk systems have to be extra vigilant and consider adjusting their approach to reduce selection pressure for glyphosate resistance.

“From the cases we know about, the two main risk factors are Italian ryegrass populations and little or no mechanical weeding through cultivation in the system. There are many good reasons for a reduced cultivation system, but for all the benefits, more focus on glyphosate stewardship is one of the trade-offs in such a system.

“Of course, cultivation doesn’t give farmers a free pass. They should also follow good stewardship practice, but they have more tools available.”

Use a sufficient dose of glyphosate to kill all target weeds because any weed which survives a glyphosate application is a potential resistance risk. Selecting the right rate for the size and hardest to kill species in the weed spectrum is the starting point. Correct application technique ensures the required rate reaches the intended target.

There are many aspects of sprayer setup, maintenance and operation that can influence application quality. Nozzle choice, forward speed of no more than 12 km/hour and water quality are three of the most important. 

At the time of application, weeds should be actively growing to allow translocation of glyphosate throughout the plant for effective control. Do not apply to plants under stress such as waterlogging, drought or during very cold weather. Also, avoid applications during the stem extension phase of growth because the glyphosate is translocated to the growing tip but not to the roots leading to potential regrowth.

Repeat applications of glyphosate to the same individual weed is a major resistance risk. A small number of weeds often survive a herbicide application. In all likelihood, this is not due to resistance but because of application or field effects. Take no chances, ensure survivors are controlled with another chemical mode of action or non-chemical method.

More information on effective glyphosate stewardship is available in the 2021 WRAG publication ‘Guidelines for minimising the risk of glyphosate resistance in the UK’.

Dealing with suspect cases

Early intervention is vital to minimise the risk of development of more cases of resistance. Work with a BASIS-qualified advisor to investigate any instances of poor control from glyphosate.

“The vast majority of cases of sub-optimal control with glyphosate will have nothing to do with resistance. It will be due to the application rate, timing or conditions. Spring 2024 is a good case in point, many farmers suffered from poor control for one or more of these reasons.”

“If you’ve ruled out the obvious causes, then seek further advice and consider a resistance test. Take a precautionary approach; the absolute key element of stewardship is to prevent any suspect population which has survived a well-timed and appropriate glyphosate application from setting seed. Consider non-chemical intervention, such as mechanical weeding through cultivation, or in extremis ploughing and be rigorous with machine hygiene.”

Industry response

As the market leader in glyphosate herbicides, Bayer has commented that it is following the situation closely. Nevertheless, Bayer said it anticipates that glyphosate will remain a core pillar of integrated weed management on farm which is the experience in other markets with cases of resistance.

It is important to emphasise that this is one case of resistance in one weed species. Bayer regularly screens black-grass, Italian ryegrass and other weed species within its own resistance monitoring programme and has found no populations of concern for glyphosate in the UK.

Reducing the risk of further cases of resistance should remain a priority for all farmers, advisors and registration holders. Bayer is committed to promoting awareness of the established guidelines for glyphosate stewardship. Resistance tends to develop within the field, so farmers’ actions directly affect the likelihood of resistance developing on farm.

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