The rise of resistance: Lessons for herbicide management

Ellie Dearlove provides key lessons on herbicide management from our Resistance Roadshow events, including the resistance status of UK weeds and the emerging risks.

Our Resistance Roadshow events tackled the management of pesticide resistance in insects, weeds and diseases in arable rotations.

In this blog, I will look at herbicide resistance, using insights from ADAS weed specialists, Lynn Tatnell, Sarah Cook and John Cussans, who presented at the roadshow.

The main takeaway messages are sobering and practical:

• Resistance is widespread in several species and emerging in others
• Resistance needs to be managed long before it occurs

Cases of herbicide resistance have been confirmed in several UK grass-weed species, which impact several herbicide mode-of-action (MoA) groups (see Table 1).

However, the scale and spread of the resistance vary considerably, depending on the MoA and weed species.

Table 1. Mode of action groups and chemical families associated with herbicide resistance in UK grass weeds

The most problematic grass-weed species are black-grass and Italian rye-grass.

These show widespread resistance to post-emergence group 1 and 2 herbicides and increasing resistance to chemistry in pre-emergence groups 3 and 15, which has significantly reduced control flexibility in winter cereals.

The confirmation of a small number of cases of glyphosate resistance (group 9) in UK Italian rye-grass populations adds to weed-control concerns.

Bayer Crop Science will continue to fund targeted testing of Italian rye-grass this year (by ADAS) to help reveal the extent of the problem.

For wild oats and brome species, the picture is more mixed.

The confirmed cases of resistance to post-emergence group 1 and 2 herbicides are a particular concern, especially the rapid development of ALS resistance in brome.

Recent Niab-led screens of populations of five brome species classified more than 24% of meadow brome and 28% of rye brome samples as resistant to this herbicide group.

Their work also identified distinct clusters of resistant populations across the UK, which highlights the importance of biosecurity to slow down resistance spread.

Although not currently significant, pre-emergence herbicide risks still need to be managed in these species.

There has been a marked rise in resistance in annual meadow-grass outside the UK in recent years, so this threat also needs to be considered.

This species can germinate throughout the year, creating multiple flushes that can escape treatment and quickly increase its presence in the seedbank.

It is also naturally tolerant to most group 1 herbicides (except for clethodim and propaquizafop).

The withdrawal of key residual chemistry, such as isoproturon (IPU) and flupyrsulfuron (FPU), means options are becoming limited.

This has increased intensive use of group 2 sulfonylureas, which is exerting a strong selection pressure, especially when applied at reduced rates.

There are already multiple populations with target-site mutations that confer resistance to all ALS herbicides in Ireland.

Finally, rat's-tail fescue is increasing on UK farms.

Although there are no known UK cases of resistance in this weed, management needs to reduce the risks, with resistance to ALS herbicides and glyphosate of particular concern.

Resistance in broad-leaved weeds is also well established.

Globally, group 2 has the highest number of resistant weed species (about 175 species), which is driven by the widespread use of this chemistry and the relative ease with which resistance mutations arise.

In the UK, cases have been confirmed that affect group 2, including sulfonylureas, such as metsulfuron, and triazolopyrimidines, such as florasulam.

Impacted weed species include common poppy, chickweed, scentless mayweed and prickly sow-thistle.

Resistance to group 5 triazinones, including metribuzin, has also been documented, such as in sugar beet, potato and asparagus crops.

Reliance on a limited range of herbicides will drive further cases of resistance.

High-risk species in the UK include fat hen (with resistance to triazones already recorded in mainland Europe), charlock, bur chervil and wild radish.

Although there have been historic reports of resistance in chickweed to mecoprop (group 4), the extent of this resistance is unclear.

Resistance to glyphosate is also a threat in broad-leaved weeds, with resistance cases document internationally.

As you plan your herbicide strategies, consider how to minimise resistance risks.

Once resistance has developed, it will not go away.

Diversity in non-chemical and chemical (MoA) approaches is the cornerstone of resistance management.

Non-chemical approaches include diverse rotations (including leys and competitive crops), cultivation strategies and delayed drilling/stale seedbeds.

Chemical diversity should be deployed across the whole rotation, especially to deal with limited MoA in some cropping situations.

In herbicide programmes, mix, sequence and rotate MoA as much as possible.

For broad-leaved weeds, the following MoA can support the use of group 2 and 5 herbicides:

• Chickweed: fluroxypyr (group 4)
• Mayweed: clopyralid (group 4) or bromoxynil (group 6)
• Poppies: pendimethalin (group 3)

For annual meadow-grass, focus on robust, broad-spectrum strategies in the autumn, rather than relying too much on spring ALS-based treatments.

Practical in-field monitoring options are limited, but as Lyn Tatnell said:

“If you see live plants next to dead plants after a herbicide application or patches or clusters across an otherwise dead field, that’s a key resistance sign”.

It is important to spot survivors and remove them to stop them setting seed.

Resistance tests will provide clarity on which products are no longer effective.