The extent to which black-grass has become resistant to herbicides has raised concern that other grass-weeds, notably the brome species, could follow suit to create further problems for growers and agronomists.

Brome grasses are highly competitive.

Just five plants/sq.m of sterile brome, the most common UK species, can cut cereal yields by 5%.

A four-year AHDB-funded project* led by Dr Sarah Cook, ADAS senior research consult ant weed biology, has been trying to find ways to avoid a repeat of the black-grass resistance story in bromes.

Herbicide-resistant populations of bromes already occur in France and Germany, says Dr Cook.

We need to agree and communicate an integrated weed management system for bromes across the industry here to help prevent the evolution of herbicide resistance in brome species, she says.

Several areas have been researched to achieve this.

They included determining the range and resistance status of UK brome species.

Influenced We needed to find out whether the increasing presence of troublesome brome populations may have been influenced by the focus on black-grass control. Weve been assessing possibly resistant populations and investigating the processes that may lead to resistance in bromes.

We wanted to find out whether the bromes are naturally more herbicide tolerant than other grass-weed species, or if certain populations are responding to herbicide exposure and selection pressure and so becoming less susceptible, says Dr Cook.

We also wanted to see if bromes can be pushed towards resistance and identify herbicide modes of action that pose the most risk of resistance developing, she adds.

The final aspect of the work has been to find the best timing of herbicide treatments to maximise herbicide efficacy and potentially reduce or prevent resistance occurring.

The research involved a UK-wide farmer survey plus laboratory work and pot trials.

In the survey, 200 of the 206 respondents across 42 counties reported having brome

on-farm.

Interestingly, perhaps, of the 58 seed samples we received only 59% were correctly identified by the sender, says Dr Cook.

Of those with brome, 60% felt that the weed had increased in the past three years.

In screening work, an average of 33 brome samples from various sources were tested, in pots in a glasshouse applying full and half rate products each year, for herbicide resistance to ACCase herbicides, ALS inhibiting herbicides and glyphosate.

Results from part of this testing were published last year and confirmed resistance to ALS herbicides in two populations of sterile brome and one each of great brome and meadow brome.

Weve screened further populations in recent years and found more resistant populations. In 2019 we found one sterile brome, four rye brome and two meadow brome populations resistant to ALS herbicides, says Dr Cook.

Other experiments have been trying to push sterile and rye brome populations towards resistance by applying low doses of three herbicides cycloxydim (ACCase), pyroxsulam (ALS) and glyphosate to them over a three-year period.

Even at extremely low doses, plants treated with cycloxydim showed no signs of resistance developing, Dr Cook reports.

Selection For pyroxsulam, there was no resistance selection of any rye brome populations, but two sterile brome populations were less sensitive to the herbicide.'

No rye brome population showed any resistance to glyphosate and although one sterile brome population was less sensitive to the herbicide, the change was small.

The conclusion is low herbicide doses have little impact in the development of resistance in bromes, but the research shows there is potential for this to occur.

It may be that population size and plant size at the time of treatment may be more influential in resistance evolution than low doses, says Dr Cook.

Experiments in outdoor containers in 2018 and 2019 on three populations of sterile brome and three of rye brome, each involving phenotypes with different herbicide sensitivities, were also conducted.

The treatments included an untreated control, glyphosate, cycloxydim and pyroxsulam + cloquintocet-mexyl + florasulam at three growth stages (GS12- 13, 21-23 and 25).

For all the herbicides tested, the best control came from treatment before GS21.

Once the plants had tillered they were too large for effective control, says Dr Cook.

The brome research is timely and will help widen the focus on weed management in general, not just black-grass, says Bayer CropSciences Roger Bradbury.

The survey and container experiments are an important contribution to our knowledge of the sensitivity status and geographical spread of brome. The work has identified populations of several species which can resist specific herbicide classes. The good news is these are not widespread, so with proactive management it should be possible to limit the development of resistance on other farms, he says.

However, part of that management involves correctly identifying brome species to drive appropriate post-harvest cultivation decisions.

Sarah Hurry, Corteva area manager for Bedfordshire and Hertfordshire, urges the industry to work together to prevent herbicide resistance in bromes becoming the problem it is in black-grass.

Different species respond to different strategies, so its really important to ensure you have correctly identified your species before embarking on your cultural control method. Sterile and great brome germinate in the dark, so any cultivation should be carried out as soon as possible after harvest. Meadow, rye and soft brome ripen on the soils surface, so any cultivation should be delayed by a month to break seed dormancy. Cultivation will then encourage germination and allow successful use of a stale seedbed, she says.