Brome: tales from the weed seedbank

Findings from a four-year investigation into the status of UK brome populations paint a concerning picture. Robert Saville, interim manager for weed research at AHDB, explains how some bromes are becoming less sensitive to certain herbicides and what this means for management.

Your field’s seedbank provides a unique evidence trail that, following a forensic analysis, can help shape management decisions. Central to any detective work is the assessment of the weed species present and, increasingly, their herbicide-resistance status. As this isn’t easy, we commissioned ADAS to conduct an evidence-based assessment of UK brome populations.

Conducted as part of the research, a UK-wide survey found that accurate identification of brome was generally poor. This is problematic because the best management approach depends on the species present – and with herbicide resistance concerns thrown into the mix, knowing which brome is in your field is going to become more important.

From a cultural-control perspective, for barren (sterile) brome and great brome (the Anisantha group), it is best to cover (with soil/chopped straw) the seeds as soon as possible after harvest. While for meadow brome, rye brome and soft brome (the Bromus group), it is best to leave the seeds to ripen on the soil surface for one month before cultivating.

The survey revealed that all brome species have increased, with the five main brome species found in all UK regions. Rye brome has increased considerably in the last 30 years (although the reason for this is unclear). The distribution of bromes has also changed, spreading from the field margins/headlands further into cropped areas. The evidence suggests this rise will continue too – especially as less intensive farming approaches are adopted, such as low/no-till cropping and more land is put down to margins/environmental areas.

The researchers used seed samples (collected during the survey) to grow brome in containers. These plants were used to assess the resistance risks associated with several modes of action. This identified resistance to ALS herbicides in the UK brome populations for the first time:

• Sterile brome – four populations (Lincolnshire, Worcestershire, Nottinghamshire, and Berwick)
• Rye brome – four populations (Shropshire x 2 and Bedfordshire x 2)
• Meadow brome – two populations (Yorkshire x 2)
• Great brome – one population (Shropshire)

The geographic distribution of these populations reveals that resistance is far from an isolated incident. However, on a positive note, the screens found no clear evidence of resistance to either of the two ACCase herbicides tested (propaquizafop, cycloxydim), even in populations showing resistance to ALS herbicides.

At present, good brome control is achievable, especially in non-cereal crops within the rotation. However, to sour that news, the researchers also detected reduced sensitivity to glyphosate in one sterile and one rye brome population – so it is essential to follow glyphosate resistance guidance.

When poor weed control is observed in the field, it is easy to jump to conclusions and assume that resistance is the cause. However, the survey found that although many people believed resistance was present in their fields, this could not always be backed up by hard evidence. Sub-optimal use of herbicides is the more likely reason – and tackling this is important to eliminate survivors of any treatment.

As spray timing is critical (as well as dose), the researchers investigated this aspect too in sterile (an Anisantha species) and rye brome (a Bromus species). The main findings are summarised in the table. However, irrespective of the herbicide or weed, a spray targeted at growth stages (GS) 21–23 (start of tillering) was consistently effective.

*Herbicide groups based on the Herbicide Resistance Action Committee (HRAC) Mode of Action Classification Map (2021). Group 1 = Inhibition of Acetyl CoA Carboxylase (ACCase). Group 2 = Inhibition of Acetolactate Synthase (ALS). Group 9 = Inhibition of Enolpyruvyl Shikimate Phosphate Synthase (EPSP synthase)

Note: the link between spray survivors and resistant risk is not as marked as in some other species, most probably due to the self-pollinating nature of bromes.

Integrated weed management is a knowledge-intensive process. We recognise this and have updated our brome pages. These include information about basic weed biology, identification and control, as well as tips on how to test for resistance.