HERBICIDE manufacturers are closely monitoring the discovery of a weed that is resistant to glyphosate and could become a future concern for potato growers, while the emphasis of good stewardship and avoidance of panic-applications is being re-emphasised.
While some potato farmers who have barley in their crop rotation may inter-seed it with cultivated commercial Italian ryegrass, wild-growing Italian Ryegrass (Lolium multiflorum) is a less welcome presence in potato fields.
Whereas cultivated varieties can sometimes help to preserve soil quality, with their prolific roots scavenging nutrients and adding carbon to the soil until it is ploughed back in, wild populations can create problems in fields used for potato growing and require herbicide treatment.
Glyphosate is a herbicide applied to the leaves, as a favoured active to kill broadleaf weeds and grasses, including Italian ryegrass. It comes in liquid and solid forms and the sodium salt form of glyphosate is also used to regulate plant growth and ripen some crops.
The Weed Resistance Action Group (WRAG) recently announced that a glyphosate-resistant population of Italian ryegrass (Lolium multiflorum) had been confirmed on a farm in Kent. This is the first field population of any weed to have glyphosate resistance in the UK.
The discovery was made in Kent by a team of scientists from ADAS, an independent provider of agricultural and environmental consultancy, policy advice, and research and development, in a high risk scenario where large weeds were surviving high rates of appropriate glyphosate applications in the run-up to drilling.
In addition to this one confirmed case, three suspect populations of Italian ryegrass are still under investigation, with results expected later this year.
ADAS Principal Consultant and weed specialist John Cussans said: “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.”
Dr Faye Ritchie, Associate Managing Director of sustainable agricultural systems at ADAS, said: “The issue here is managing arable weeds in field where potatoes are grown. Where the crop is grown on rented land, understanding the history and management and existing weed issues that will affect the potato crop is very important. In terms of growing potatoes where glyphosate resistance has developed, it will be a real challenge.”
However, the likelihood of resistance becoming widespread was currently very low, as only one isolated case had been reported, she stressed.
“Ensuring that the seed bed is free from weeds pre planting / post-planting pre-emergence is a very high priority for potato weed control. Glyphosate (before crop emergence) not being effective against an aggressive weed would make weed control after the crop has emerged really difficult. But it is important to note that this is very unlikely to be a widespread scenario at the moment.”
As a major producer of glyphosate herbicides, Bayer is closely following the discovery, but also anticipates that glyphosate will remain a core pillar of integrated weed management on farm, as it is 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,” a company announcement stated.
“Reducing the risk of further cases of resistance should remain a priority for all growers, 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 growers’ actions directly affect the likelihood of resistance developing on farm.
WRAG, an independent body made up of crop protection researchers (including ADAS) and agrochemical industry experts, is 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.
Both growers and advisors have been advised to refer closely to the 2021 WRAG publication ‘Guidelines for minimising the risk of glyphosate resistance in the UK’ and to material on Bayer’s website about stewardship.
Technical Specialist at Bayer, Roger Bradbury, who gave a presentation at the WRAG briefing, has long advocated the need to apply glyphosate at the right time, with the right dose for the target weed and with good application technique.
“Ensure effective control from glyphosate by using the correct rate, at the right timing applied with good technique in suitable conditions. Any failures of control should be closely investigated to understand the reasons. In all likelihood, it will be due to a field or application effects but if these are ruled out, more detailed investigation into possible resistance should be considered,” he advised.
Repeat applications to weeds that have survived a glyphosate application should also be avoided. Instead, growers are advised to use other chemical modes of action or non-chemical control methods such as cultivation or mechanical weeding to manage survivors.
“Appropriate stewardship steps on all farms, and effective steps to prevent the reproduction and movement of seed on farms with populations of concern will help ensure continued efficacy of glyphosate in the UK,” Bayer goes on to add in its guidance statement.
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.
ADAS points out that where instances of similar resistance were previously discovered in both Spain and Italy, in 2006 and 2012 respectively, resistance did not escalate and result in a widespread problem. Most confirmed cases of glyphosate resistance in Italian Ryegrass are in North and South America.
Katy Hebditch, ADAS Weed Science Consultant said: “This finding is a concern but not a surprise. We have known for a long time that glyphosate resistance was a matter of when, not if. The main message we want people to take away from this is to 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.”
She added: “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.”
Faye added: “Pre planting or post planting pre-emergence weed control is an essential part of weed management in the potato crop (after the withdrawal of other options) and that largely comes down to glyphosate.
“Other options might be used in that context, including carfentrazone, but are not as effective against grass weeds. The crop is associated with lots of mechanical weeding through the soil movement, so land where potatoes are grown a lot in the rotation are in quite a good place to avoid glyphosate resistance in the future since all the soil movement is reducing the selection pressure on glyphosate.”
When used as part of a mix and incorporated, Italian Ryegrass might be used as one part of a green manure within potato growing, but Faye said where that approach is adopted, there are better options for species to include in mixes.
“Companion cropping in potatoes is minor, perhaps focused on lighter soils where there’s a risk of topsoil erosion. Most companion crops will not include Italian Ryegrass, with preference for a cereal or a legume,” she said.
“We would be advising people not to start using Italian Ryegrass as part of any mix used as a companion crop. This advice also considers that the use of Italian Ryegrass in these such mixes will make weed control in general more difficult, rather than a direct threat from introduced seed and resistance.”
