Advice on managing glyphosate resistance this spring
© Tim Scrivener A testing service will be available this spring to help farmers and agronomists rapidly investigate suspected cases of glyphosate resistance and act to contain any new populations.
Early intervention is vital to tackle any potential glyphosate-resistant weed populations.
To aid this, Adas, with support from Bayer, is launching a whole plant test aimed at farmers applying glyphosate ahead of spring drilling.
See also: 4 reasons Italian ryegrass is a growing threat to arable growers
Monitoring
Speaking at the recent Association of Independent Crop Consultants conference, Adas weed specialist John Cussans advised growers and agronomists to monitor treated areas and investigate where there are any instances of poor control.
“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 recalled that spring 2024 is a good example, where many farmers suffered from poor control due to one of these reasons.
“If you’ve ruled out the obvious causes, then seek further advice and consider a resistance test.”
Farmers are urged to take a precautionary approach, with the aim of preventing any suspect population which has survived a well-timed and appropriate glyphosate application from setting seed.
Consider non-chemical practices, such as mechanical weeding through cultivation, or in extreme cases, ploughing, and be rigorous with machine hygiene.
Higher-risk situations
Of particular risk is where farmers are applying glyphosate prior to direct drilling spring crops within no-till systems.
From the known cases, John said the two main risk factors for resistance developing are:
- Italian ryegrass populations
- Little or no mechanical weeding through cultivation in the system.
He explained that by spring, some Italian ryegrass plants are well rooted and well established.
They are, therefore, more difficult to kill, especially if in stem extension.
By aiming to minimise soil disturbance (to encourage blackgrass germination), there are no cultivations to take any ryegrass survivors out.
Are we likely to see many more cases?
In the UK, there has been a high level of glyphosate-resistance monitoring between 2019-2023. In that time, not a single case was found.
Therefore, Adas weed expert John Cussans believes it isn’t a widespread problem, and he is not expecting a big rise in cases.
Many point to the experiences in Australia and North America which has seen a large number of cases, but John highlighted that most of these are with a different species (Lolium rigidum). Therefore, direct comparison can’t be automatically made.
Putting it into context, in Europe there have only been three cases with Lolium multiflorum: Italy, Spain and now Kent.
However, this may rise with three suspected cases outside Kent (Essex, Yorkshire and Somerset) suggesting it’s not a local problem.
In conclusion, while it’s unlikely there is a widespread problem of glyphosate-resistant Italian ryegrass in the UK that has remained undetected ryegrass, farmers need to be vigilant to keep it that way.
In-crop herbicides
Another bit of good news is that there is no link with in-crop herbicide resistance, and that fops and dims were still effective on the glyphosate-resistant population.
Therefore, if farmers have populations that are known to be resistant to either the acetolactate synthase inhibitor or acetyl-CoA carboxylase inhibitor herbicides, they are not more likely to see glyphosate resistance.
Likewise, John warned farmers who don’t have any resistant populations that they are not safe from glyphosate resistance evolving.
This selection is separate to that affecting sensitivity to in-crop herbicides.
Adas weed specialist John Cussans, Rothamsted Research ecologist David Comont and Lincolnshire agronomist Sean Sparling were speaking at the Association of Independent crop consultants (AICC) annual conference near Towcester.
Research imagines weed control strategies without glyphosate
© GNP
Modelling work carried out at Rothamsted Research offers a glimpse into farming without glyphosate.
It also identifies potential trade-offs for measures aimed at mitigating the effects of its loss, such as the environmental impact of increased herbicide use in spring crops.
Glyphosate use is coming under greater pressure, with both regulatory and resistance threats.
The current UK approval which was extended after Brexit expires in December 2025, and while the Health and Safety Executive has yet to confirm its status beyond this, it’s expected the existing approval will be extended to allow the re-approval process to proceed.
However, the discovery of resistant weeds highlights the other key risk to its future use – weed resistance. So what would it look like if we were to lose it?
To answer this, scientists at Rothamsted Research developed a model which looked at many different fields and investigated several scenarios over a 10-year cropping cycle.
Not only did they look at the impact of losing glyphosate, they examined potential mitigation measures such as introducing grass leys, greater spring cropping, delayed drilling and physical cultivations like ploughing.
They also looked at the trade-offs from these measures.
Weed control without glyphosate
Ecologist David Comont says the first finding was that control would be poorer and would mean farming with more weeds.
Glyphosate is the only way to get zero tolerance and farmers would see outbreaks of infestations, he said.
Lincolnshire agronomist Sean Sparling recalled life before glyphosate, with couch grass being uncontrollable: “Life with glyphosate is a dark, scary place.”
It’s the most crucial substance in use. “Glyphosate also has a role in reducing carbon [by reduced cultivations] and UK farming wont be able to achieve net zero without it.”
However, David added that weed diversity would be greater too. “It’s not always the nastiest species that are proliferating.”
Mitigation measures
Only the ploughing and glyphosate scenarios avoided the risk of very high levels of weeds that would threaten crop viability.
Looking at crop yields, they were sometimes lower.
Those crops with fewer other available herbicide control options were disproportionately more affected, David said.
Integrated weed control strategies can help to maintain overall food production and profitability, but there are certain trade-offs.
Looking at trade offs
For example, in-crop herbicides would be used more frequently in spring cropping and these sometimes have a greater environmental impact than glyphosate.
Physical cultivations to control weeds can counter strategies to improve soil health, and the weather might exacerbate the unpredictability of future weed control, increasing risk.
In conclusion, a diversity of strategies would be key to mitigating the impact of its loss, and further resistance.
