Glyphosate resistance: Frequently Asked Questions

1. How many cases of glyphosate resistance are we aware of in the UK?

WRAG has recently announced the first case of glyphosate resistance in the UK in a field population of Italian ryegrass. There are three more high risk populations in Italian ryegrass currently under investigation. There are no current investigations or confirmed cases of resistance in field populations of other weed species at present in the UK. Learn more about the resistant case here. (link to summary report or press release)

2. How is herbicide resistance confirmed?

Scientific work is necessary beyond simply observing that some plants appear to have survived a dose of glyphosate in the field. There are several criteria that need to be met for a population to be officially classed as resistant, these include:

• Confirming the field observation observations under a validated, standard scientific protocol.

• The plants need to represent a field population (i.e. not plants that are selected through successive generations artificially in the lab / glasshouse).

• The resistance trait must be heritable.

• The research needs to be peer reviewed and scrutinised before being published in a reputable scientific journal.

3. Do we know the factors that influenced the development of resistance in this Italian ryegrass population?

Repeat applications of glyphosate to Italian ryegrass plants that survived a previous application. In addition, relatively low levels on in-crop herbicide were recorded.

4. Is there any other evidence of resistance in Italian ryegrass?

Bayer regularly collects and tests Italian ryegrass samples from UK fields as part of its monitoring of resistance to selective herbicides. All these samples are screened for glyphosate sensitivity and none of the samples from the UK tested by Bayer are under suspicion of being resistant, this includes 198 samples collected as part of the 2021 ryegrass survey, conducted in partnership with NIAB

However, a 2019 study into 50 ‘difficult populations’ did expose sensitivity shifts (not resistance) in a handful of populations. These results were communicated in 2021 and highlight the importance of following the established resistance management guidelines published by WRAG. Furthermore, ryegrass species tend to be relatively high-risk with respect to resistance development, with several instances reported from across the globe (albeit in a different species, annual ryegrass, Lolium rigidum)

Data for graph taken from weedscience.org

5. How common is glyphosate resistance globally?

According to the Internation Herbicide-Resistant Weed Database (available on weedscience.org), the first recorded case of glyphosate resistance was in Australia in 1996 in field population of Lolium rigidum. Since then resistance has evolved to 60 species in 31 countries globally

The first case of glyphosate resistance globally in Italian ryegrass was reported in Chile in 2001, with the first reported case in Europe being in Spain in 2006 (orchards), the first case in arable cropping was recorded in Italy in 2008. In Europe, another case has been reported in Italy (wheat); Switzerland (orchards) and Portugal (grapes) have each reported a case. It’s notable that in the European cases it has not escalated into a widespread problem.

6. Are there any weeds (apart from Italian ryegrass) likely to develop resistance?

Experience from around the world suggests that ryegrass species are relatively high-risk when it comes to glyphosate resistance risk. Therefore, it is not a surprise that this is the first species affected by resistance in the UK.

Tests of 166 field populations of brome collected in summer 2023 found no populations of concern. Black-grass is the most screened weed for herbicide resistance in the UK and once again no populations of concern for glyphosate have been found.

However, glasshouse selection experiments conducted on weeds other than ryegrass show that it is possible drive populations in the direction of reduced sensitivity to glyphosate with repeat use, meaning, the genetic potential for resistance lurks in many weeds.

Farmers actions at field level drive the risk for selection of less sensitive plants building in a population over time. By taking appropriate resistance management steps outlined in later questions, it should be possible to reduce the risk of the appearance of resistance in other species and arising independently to spread the problem in more populations of Italian ryegrass.

7. What are the consequences of glyphosate resistance for UK cropping systems?

Glyphosate remains a unique non-selective herbicide; it is unlikely an alternative as effective and broad spectrum will be available any time soon. Alongside cultural tools like rotation and cultivation, responsible glyphosate use is a key component of a successful integrated weed management programme.

As such, resistance to glyphosate would increase cost, management challenges and inevitably result in more cultivations / soil movement being necessary to manage weeds and place further pressure on the remaining in-crop selective chemistry.

The appearance of further cases of resistance through the spread of resistant seeds or independent development must be avoided.

However, perspective is needed, it is a handful of cases affecting one weed species only. Glyphosate remains fully effective against the overwhelming majority of ryegrass populations as well as all the other weeds it is used to control. Globally, even in situations with resistance, glyphosate remains an important management tool depending on the specific context on farm.

8. If I have poor control, should I be worried?

Instances of poor control are most likely not resistance. Incorrect rate and timing, application technique, shading of target plants and weather / environmental conditions can all cause control to fall below expectations.

Spring 2024 is an example where several factors caused sub-optimal performance, in some cases having a cumulative effect.

Firstly, the applications rates may have been too low for the size of target. There was frequent wet weather throughout spring 2024 so there could have been a long delay between recommendation and application during which the weeds grew much bigger.

There were few good application windows, especially in the early part of spring. It was difficult to apply at the correct timing plus workload pressure may have affected application quality due to high forward speed or marginal conditions such as applying too close to dusk. There were also very few days without rainfall so rainfastness may have been an issue.

Glyphosate performance depends on translocation throughout the weed to get effective kill with no risk of regrowth. Weeds sat in wet anaerobic conditions (waterlogged ground) were possibly stressed and not translocating effectively leading to poorer control. Once conditions improved, farmers may have only had the time and the opportunity to spray once stem extension was underway. Stewardship guidelines have always advised to avoid applications at this time because the glyphosate is translocated to the growing tip but not to the roots leading to potential regrowth.

9. How likely is it to spread / appear on other farms?

Unlike insects and pathogen spores, weeds are not highly mobile, so resistance does not spread quickly. Hence, simply because there are cases of herbicide resistance, it doesn’t mean it will soon affect every farm. On the handful of farms with suspected resistance, robust measures are needed to stop it spreading more widely.

Field management practices and weed spectrum determine the risk the resistance developing independently on farm. Good stewardship measures can greatly reduce the risk. Anticipating the danger, the Weed Resistance Action Group (WRAG) published revised guidelines for preventing glyphosate resistance in 2021. The main risk factors for the development of glyphosate resistance are outlined below:

10. What are the risk factors for the development of glyphosate resistance?

Lack of diversity in control methods Reliance on few or only one method of control greatly increases the risk of resistance developing. This applies to chemical and cultural control methods, the more diversity across the cropping cycle, the better.

Repeat applications to survivors 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 any case, take no chances, ensure survivors are controlled with another chemical mode of action or cultural method. Repeat applications of glyphosate to surviving weeds is a major resistance risk. Stale seedbeds used to manage grass-weeds are an important case in point. The guidelines suggest a maximum of two glyphosate applications with a cultivation in between is the optimum balance between control and stewardship.

Low rates Insufficient application rate for the target weed and poor application timing means too many weeds survive the application. These surviving weeds are a potential resistance risk and an immediate problem for farmers aiming for clean, high-yielding crops.

Poor application technique Incorrect sprayer set up and operation can result in under or overdosing the product, affecting performance in the field. In the long term, this can contribute to resistance development. Examples of poor application technique include:

• Poorly maintained machinery which is not routinely calibrated and serviced

• Incorrect selection of nozzles for the job at hand and/or not replaced when worn out

• Operating at the wrong boom height and travelling too fast (>12km/hr)

Additionally physical losses from drift, run-off and bounce, and chemical losses due to pH, hard water can all reduce the quantity of active reaching the target causing sub-optimal control.

Incorrect timing Adverse weather conditions and weed growth stage can both impair glyphosate performance. Rain at the time of application, wind and very cold conditions (<5°C) all reduce efficacy.

Weeds should ideally be actively growing and not under stress such as waterlogging and drought, or during winter dormancy. 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.

Population dynamics High weed populations tend to increase the risk of resistance development. Reduce weed numbers by other means to reduce the risk.

Learn more from the 2021 WRAG Guidelines and the Bayer 2022 Stewardship leaflet.

11. How can I prevent resistance on farm?

The only way of truly preventing development of resistance is to avoid use of the herbicide, but a planned programme utilising a range of weed management tools judiciously through the rotation can help to reduce the risk of resistance development. Take a pro-active approach to stewardship, irrespective of your farming system and target weed spectrum.

• Apply at the correct timing with well-maintained equipment and good application technique.

• Make sure rates are correct for the target species and the application conditions.

• Avoid applications to stressed plants (waterlogging, drought) during very cold weather (<5°C) and during stem extension.

• Ensure there are diverse cultural and chemical control methods used across the rotation.

• Do not make repeat glyphosate applications to surviving weeds.

• Consult your agronomist if you notice poor control.

12. What should I do if I suspect resistance?

If your initial assessment rules out the other potential causes of poor control, do the following:

1. Discuss the situation with a BASIS-qualified agronomist

2. DO NOT apply a further dose of glyphosate

3. DO NOT crop the field

4. Collect some plants from the field and get them tested as a priority (e.g. ADAS offer resistance testing services)

5. DO NOT allow the plants to set viable seed and shed; destroy the plants using a cultivation (e.g. Plough them in)

6. Good machine hygiene is essential to avoid spreading seed into uninfested fields from cultivation or harvest equipment

7. Consider removing individual fields from arable production and manage the land to degrade the viability and numbers of any shed seed

13. How can I stop resistant seed from spreading?

In the event plants appear to survive glyphosate treatment and this cannot be explained by other factors, act decisively to destroy plants by mechanical means before viable seed shed.

On any farm with suspected cases of herbicide resistance (glyphosate or otherwise), preventing the direct spread of resistant weed seeds is a priority, tactics include:

• Good machinery hygiene, blow down harvest machinery before moving from field-to-field to not spread contaminated seed

• Seed destruction at harvest.

• Whole cropping before seed shed.

• Ploughing then long-term grass leys to deplete resistant seed in the seed bank

• Hand rogueing fields.

14. How do I prevent resistance in no-till systems?

Mechanical weed control through uprooting seedlings and burial of seed is an important alternative to chemical control. Low and zero-tillage farming has economic, environmental and agronomic benefits for many farmers, but removing mechanical weed control from the toolbox can put more pressure on glyphosate. Avoid making repeat applications to stubbles prior to drilling winter or spring crops. Rotations need to provide cultural grass-weed control to avoid over-reliance on glyphosate.

15. Is it safe to control cover crops with glyphosate and then make a second application to the stubble before drilling spring crops?

Cover crops, dependent on the composition and diversity of species, can present a challenging canopy architecture to effectively manage with a glyphosate application; there is a risk of shading lower growing plants in the base of a tall, large biomass cover crop. Alongside choice of the correct dose rate and a suitable timing, application practice is an important consideration to maximise efficacy. Nozzle choice and water volumes can also be influential.

The current edition of the WRAG guidelines ‘Minimising the risk of glyphosate resistance in UK’ acknowledges that a maximum of 2 applications of glyphosate is likely to represent a manageable risk in the field long-term provided this is supported by cultivation and an effective subsequent selective herbicide regime

The key is to avoid treating the same surviving plants with a second dose of glyphosate which risk escaping other control measures and potentially set seed. Where this is identified to be a likely risk, then ensure an alternative control (e.g. cultivation) is employed between successive glyphosate treatments

16. Are more farms likely to report resistance?

Apart from those already mentioned, no other cases currently being investigated but the news of resistance will most likely increase vigilance and there may be further suspected cases; however, there are other reasons why glyphosate doesn’t perform. Timing, rate and application conditions can all cause poor control. These need to be ruled out before investigating resistance.

17. Italian ryegrass isn’t a serious problem on my farm, do I need to worry about stewardship?

Yes, because resistance could appear in other weeds too. Once there is resistance it is very difficult to get rid of so prevention is much better and more achievable than a cure. Monitor weeds and investigate instances of poor control with a BASIS-qualified advisor.

18. Am I at risk if I plough regularly?

Ploughing within an integrated programme is a good method of reducing risk glyphosate resistance but stick to all good stewardship principles around timing, rate and number of applications to reduce the risk further.

19. What is the difference between resistance and a sensitivity shift?

Sensitivity shifts are detected scientifically from dose response experiments conducted in the glasshouse comparing the response of a field population relative to a sensitive standard and / or an average response across a broad selection of local field populations. Populations that exhibit shifts in sensitivity are usually still well controlled by label dose rates. Such changes are not visible in the field until a more dramatic change in sensitivity is observed. Changes in sensitivity detected in such experiments serve as an early warning indicator that resistance may be starting to develop.

This is distinct from resistance which is a heritable characteristic conferring an ability on plants to tolerate a dose rate of herbicide which would ordinarily control a wild type of the same species.

20. Is glyphosate resistance development purely a problem in agricultural systems or do other settings pose a risk?

Weed control in the amenity sector is also a potential source of risk for resistance development if glyphosate is relied upon as the sole source of weed control in the same space year after year.

As such amenity weed control is covered with some guidance in the published WRAG guidelines ‘Minimising the risk of glyphosate resistance in UK.” Loss of glyphosate to resistance would make weed control less convenient and more expensive

To minimise the risk avoid over-reliance on a single mode of action. Use in sequence with chemistry from alternative HRAC mode of action groups and alternative weed control measures where practicable.