Growers using glyphosate to clean stubbles ahead of direct drilling spring crops are urged to remain vigilant this spring, following confirmation of the first case of glyphosate resistance on a farm in Kent.

The Weed Resistance Action Group (WRAG) today announced the discovery of the first field population of any weed to have glyphosate resistance in the UK.

Discovered in several fields in Kent, suspicions were initially raised as patches of Italian ryegrass (lolium multiflorum) survived appropriate applications of glyphosate prior to drilling spring crops.

Further testing by Adas of plants grown from seed collected on the farm showed the maximum rate failed to control plants when applied in ideal conditions in a glasshouse.

See also: Video: Mechanical hoes offer solution as herbicide resistance builds

What this mean for farmers

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

“But we must retain perspective; it’s resistance in one weed species on one farm, it is not a widespread problem yet.”

For context, he explains that over the period 2018-2023, Adas has collated data for samples of Italian ryegrass which were submitted for standard resistance tests. They have been screened with glyphosate, and he didn’t find 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 blackgrass 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, John warns that other weeds can also develop resistance. Therefore, glyphosate stewardship concerns every farmer, not just those managing Italian ryegrass problems.

“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 steps that farmers using glyphosate need to take. On top of that, those running higher-risk systems are urged to be extra vigilant and consider adjusting their approach to reduce selection pressure for glyphosate resistance.

From the known cases, John says the two main risk factors are:

1. Italian ryegrass populations
2. Little or no mechanical weeding through cultivation in the system.

While cultivation reduces the risk, those who do should also follow good stewardship practice. Bayer agronomist Roger Bradbury points to existing advice:

• 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.

In conclusion, experts say farmers shouldn’t take any chances and ensure any 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’.

What should farmers with suspect cases do?

Early intervention is vital to prevent the development of more cases of resistance.

Adas weed specialist John Cussans advises growers to monitor areas treated with glyphosate and where there are any instances of poor control, investigate with help from a Basis-qualified advisor.

“The vast majority of cases of suboptimal control with glyphosate will have nothing to do with resistance. It will be due to the application rate, timing or conditions.”

John recalls that spring 2024 is a good example, where many farmers suffered from poor control due to one of these reasons.

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“If you’ve ruled out the obvious causes, then seek further advice and consider a resistance test.”

A rapid testing service will be available this spring, with further details to be announced. 

Meanwhile, farmer are urged to take a precautionary approach. The aim 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 extreme cases, ploughing, and be rigorous with machine hygiene.