How to minimise glyphosate resistance risks in grass weeds

Learn about the proactive measures to minimise glyphosate resistance risks in weeds, as well as the reactive measures to monitor for and deal with weeds that survive a glyphosate application.

An over-reliance on a limited group of herbicide modes of action has accelerated the development of herbicide-resistant grass weeds, particularly black-grass.

The herbicide glyphosate has been commercially available for over 50 years.

It is one of the most frequently used herbicides in the UK, including in annual arable and perennial crops and non-cropped areas.

The Weed Resistance Action Group (WRAG) confirmed the first case of resistance to glyphosate in a UK weed species on 10 January 2025.

Resistance at the maximum label rate of glyphosate was identified in tests on a single field population of Italian rye-grass (Lolium multiflorum) from a site (in Kent) considered to have an elevated risk of developing glyphosate-resistant weeds.

More potential cases were identified during 2025.

In addition, there is evidence of varying levels of sensitivity to the herbicide in other weed species.

Risk is high in annual arable crops, especially when glyphosate is used to control grass weeds that exhibit resistance to many selective herbicides.

Key weed species at risk:

• Black-grass
• Italian rye-grass

Maximise efficacy (to reduce the number of survivors): use an appropriate dose for the target weed, at the right growth stage, in the right conditions.

• Use the manufacturer’s recommended dose rate for the weed sizes present
• Reduced doses increase the risk of lower efficacy
• Typically, annual grasses require a minimum of 540 g a.i./ha for seedlings up to 3 leaves, 720 g a.i./ha when tillering* and 1,080 g a.i./ha when flowering
• The choice of nozzles and water volume can be influential
• Since hard water can reduce glyphosate effectiveness, the addition of water conditioner is recommended, where appropriate
• Use a forward speed of no more than 12 km/hour

*The 720 g a.i./ha dose was added in 2021, following AHDB research findings.

• Apply to actively growing plants in warm conditions (15–25°C), with at least 6 hours before any rainfall
• Ideally, spray when plants have at least 5 cm of growth but before the start of rapid stem extension
• Apply prior to ‘shading’ from other plants
• Do not apply to plants under stress, such as waterlogging or drought

Application of glyphosate within crops, such as between rows with shrouded sprayers, will increase the risk of resistance developing in survivors.

Adjuvant use is often associated with use of reduced rates of glyphosate products. Unless recommended on glyphosate product labels, extreme caution should be taken – any reduction in efficacy could increase the risk of resistance development.

• Use multiple and diverse approaches, including sufficient cultivations and other non-chemical techniques to control weeds
• Avoid using more than two glyphosate applications*
• Do not make repeat applications to the same (surviving) plants
• Use other herbicides (pre- and post-emergence) in sequence or in mixture (if recommended)

*A single pre-harvest application of glyphosate per crop should not increase the risk of resistance developing, unless there are survivors present from prior glyphosate applications.

• Investigate any unexpected survivors
• Quickly remove surviving individuals (e.g. via cultivation)
• Report survivors to your crop protection adviser and/or the product manufacturer
• Send samples (whole plants or seeds) for resistance testing to help confirm the cause (there are many reasons for poor control)

Two applications of glyphosate (with sufficient cultivation to kill survivors and effective subsequent herbicide use) is likely to be a manageable risk. In the absence of sufficient cultivation, avoid multiple applications.

When applying glyphosate in these situations, always aim to:

• Avoid over-reliance on a single herbicide (use mixtures of herbicides, whenever possible)
• Use non-chemical control measures (e.g. hand weeding, cutting, flaming, steam treatments) as a supplement or alternative to herbicide treatment
• Monitor and assess herbicide efficacy after spraying to detect any loss of control
• If resistance is suspected, act quickly to prevent it spreading – use alternative herbicides and non-chemical methods and continue to monitor
• Consider collecting and testing seed samples to confirm resistance
• Good record-keeping and assessment after spraying is essential in the early detection of herbicide resistance