How to minimise glyphosate resistance risks in grass weeds

The herbicide glyphosate has been commercially available for over 40 years. There are no known cases of glyphosate resistance in the UK. However, globally, resistance to glyphosate has evolved and there is evidence of some UK weed populations with varying levels of sensitivity. It is essential to retain the value and efficacy of glyphosate by minimising resistance risk.

How serious is the glyphosate-resistance threat in the UK?

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

Glyphosate is one of the most frequently used herbicides in the UK, including in annual and perennial crops and non-cropped areas. Evidence of some weed populations with varying levels of sensitivity is a concern with the main threats in:

Annual arable crops, especially when used to control grass weeds that exhibit resistance to many selective herbicides
Perennial crops and amenity use, especially when used to control annual and perennial weeds where few alternative modes of action are available

How to manage glyphosate-resistance risks

Factors that affect glyphosate resistance risks

Agronomic factor	Higher risk	Lower risk
Cropping system	Continuous monoculture or perennial crops	Varied rotation – winter and spring cropping
Cultivation	None or insufficient to kill weeds	Thorough disturbance to kill weeds
Weed infestation level	High	Low
Control method	Glyphosate only	Mixed use of glyphosate with effective use of other modes of action and cultural control
Number of glyphosate applications pre-drilling	More than two applications and no cultivation	Fewer than two applications and sufficient cultivation
Target weed size for glyphosate rate	Weeds too large for dose rate; reduced or less effective dose rates used	Weeds at correct growth stage; full and effective dose rates used

Always aim to maximise efficacy

It is essential to maximise the efficacy of any spray required. It is important to use the right dose for the target weed, at the right growth stage, in the right conditions.

Annual grasses in annual crops

Key species at risk: black-grass and Italian rye-grass

When applying glyphosate prior to drilling a crop, always aim to:

Maximise efficacy (to reduce the number of survivors)

Use the manufacturer’s recommended dose rate for the weed sizes present*
Apply prior to ‘shading’ from other plants
Apply to actively growing plants in warm conditions (15–25°C), with at least 6 hours before any rainfall.

*Ideally, spray when plants are at least 5 cm but before the start of rapid stem extension. Reduced doses increase the risk of lower efficacy. Typically, annual grasses require a minimum of 540g a.i./ha for seedlings up to 2-3 leaves, 720g a.i./ha when tillering and 1,080g a.i./ha when flowering.

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

Managing the resistance risk to retain long-term effectiveness of glyphosate for grass-weed control in UK crop rotations (research)

Use alternatives

Use non-chemical options, as part of an integrated approach to control
Supplement glyphosate use with sufficient cultivation to kill survivors
Make best use of effective subsequent herbicides (pre- and post-emergence)

Prevent survivors

Remove survivors to reduce seed spread
Report suspected survivors to your crop protection adviser and/or the product manufacturer
Avoid repeat applications of glyphosate to the same (surviving) weeds

Monitor success

Monitor the success of control strategies carefully and frequently
Test seed samples of survivors
Discuss resistance with your agronomist or supplier

Based on current understanding, 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.

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.

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.

Choice of nozzles, water volume and the addition of water conditioner can also be influential.

Perennial crops and amenity areas

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