United Kingdom
February 13, 2025
by Jason Pole, Technical Content Manager – Cereals & Oilseeds, AHDB
When the UK Weed Resistance Action Group (WRAG) identified a population of Italian rye-grass (Lolium multiflorum) in Kent with clear resistance to glyphosate, few were shocked. In fact, many were surprised by how long it took to reach our shores.
Earlier this week, WRAG issued an update based on three more Italian rye-grass populations (from Somerset, Essex and North Yorkshire). Two of these samples demonstrated significantly reduced control from appropriate doses of glyphosate, with the third showing decreased glyphosate sensitivity. All cases were associated with situations that were very high risk for resistance.
Drivers of glyphosate resistance
- Continuous monoculture
- High weed infestations
- Insufficient cultivation
- Over two applications
- Insufficient dose
Encouragingly, screens of over 300 high-resistance-risk seed samples, by ADAS and NIAB (since 2019), found no other indication of glyphosate resistance in the UK – although tests did discover about a fivefold difference in glyphosate sensitivity.
The battle is not lost, but a new chapter to stop resistance spreading has started.
Resistance management
The use of glyphosate is ubiquitous, and resistance to it is already present in many weed species and in many countries.
In 2015, AHDB and WRAG flagged black-grass and Italian rye-grass as weeds to watch in the first iteration of glyphosate resistance guidance for the UK.
Since then, black-grass has hogged the headlines, because of the sheer number of farms affected by resistance to other herbicides.
Because Italian rye-grass shook off glyphosate at the maximum label rate, it now finds itself in the spotlight.
Of course, Italian rye-grass can resist other herbicides too, with the first cases documented in the 1990s. As with black-grass, resistance spread to most of England, highlighting why resistance needs to be nipped in the bud.
The positive news is that glyphosate resistance cases in mainland Europe (2006 in Spain and 2012 in Italy) did not become widespread, due to a combination of proactive and reactive measures. Following Europe’s lead is wise.
Proactive measures
Integrated weed management needs a solid understanding of the weeds (and resistance) present. It also requires a greater diversity in control methods (non-chemical and chemical), especially to tackle the most challenging weeds.
Italian rye-grass is such a weed. It needs very high control levels, especially as it produces more seeds per plant than black-grass, which remain viable in the soil for longer (over five years).
Most seeds (on or near the surface) germinate by November, which means autumn is particularly critical for management.
Seedlings will continue to emerge beyond autumn, and it may tempt the use of glyphosate multiple times ahead of spring cereals (especially when drilled late). However, this is a major driver of resistance.
The use of targeted cultivations can help mitigate the risk. For example, creating a stale seedbed straight after harvest will promote a flush of weeds that can be killed (by cultivations or herbicide).
Although resistance concerns are unlikely to determine your primary cultivation system, it is critical to monitor for survivors and be prepared to use metal to uproot them.
Glyphosate dose is also key. Based on AHDB research, annual grasses typically require a minimum of 540 g a.i./ha for seedlings up to 2–3 leaves, 720 g a.i./ha when tillering and 1,080 g a.i./ha when flowering.
‘Minimum’ is the key word, with higher doses needed in suboptimal conditions (if permitted on product labels).
As the herbicide favours actively growing weeds (with at least 5 cm of growth), it should be used in warm conditions (optimum 15–25°C, but not under 5°C) and not applied to stressed plants (waterlogged or drought conditions).
It is also important to apply it before the start of rapid stem extension (as less glyphosate gets to the roots after this stage) and not to apply it when rain is expected (within a few hours).
Water pH and hardness (high levels of metal cations) can also reduce efficacy. Using a water conditioner and low water volume can help. The maintenance of sprayers, appropriate nozzle (angled) choice and low forward speeds (no more than 12 km/hour) will also help target glyphosate.
Reactive measures
Spotting resistance early will help stop a situation spiralling out of control. Look out for:
- A gradual decline in control (over several years)
- Healthy plants (patches) beside dead plants of the same species
- Poor control of one susceptible species when other susceptible species are well controlled
To destroy survivors, use an alternative method (not glyphosate), such as cultivating or hand-roguing. If resistance is suspected, get plant or seed samples tested (the cause may not be resistance).
Finally, good farm hygiene is essential. Clean equipment and plan straw movements to avoid spreading seeds (across fields and farms). In very-high-risk situations, it may be necessary to remove and incinerate straw (and ensure any farm-saved seed is clean) or wholecrop before seeds are shed.
Resistance testing
To help monitor resistance across the UK, ADAS is screening live Italian rye-grass plant samples that survive glyphosate applications (prior to drilling commercial spring crops).
The best time to collect Italian rye-grass seed samples for resistance testing is in June and July.
For information on the free resistance testing (live plants) and glyphosate resistance guidance, visit ahdb.org.uk/glyphosate
Glyphosate renewal
In 2023, the European Union renewed the approval of glyphosate until 2033. Although the UK expiry date is December 2025, it is anticipated that the deadline will be extended for this strategically important herbicide.
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