Use brome maps to target weed control

Monday, 9 May 2022

Where are bromes located in your fields? Which species are present? Is herbicide resistance an issue? Here’s why weed maps need to become standard in your integrated pest management (IPM) plans.

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How to manage brome weeds in cereals

The integrated strategy

To prevent, minimise and reduce brome pressure, it is important to build an integrated strategy around non-chemical control. This strategy is formed around six key components:

  1. Understand the weed population in your field.
  2. Prevent weed introduction and minimise spread.
  3. Exploit the whole rotation to reduce weed pressures.
  4. Make best use of carefully timed and chosen cultivations.
  5. Consider drilling later and increasing seed rate.
  6. Use chemistry carefully to maximise control and protect efficacy.

The importance of each one of these components is elevated when the threat of herbicide resistance is considered.

Herbicide resistance

Last year, AHDB reported findings from ADAS-led research. The researchers gathered UK seed samples to grow brome in containers for herbicide screens.

Through the screens, they identified resistance to ALS herbicides in UK brome populations for the first time – with resistance found in some sterile brome, rye brome, meadow brome and great brome populations.

The researchers also detected reduced sensitivity to glyphosate in one sterile and one rye brome population, but, thankfully, found no clear evidence of resistance to the ACCase herbicides tested.

As the seed for the ALS-resistant populations was derived from several English counties, it showed that resistance is far from an isolated incident.

It is critical that steps are taken before the resistance situation spirals out of control – lessons from other resistant weed species emphasise the importance of this statement.

The Weed Resistance Action Group (WRAG) document, ‘Ten facts everyone should know about herbicide resistance’, states “It is much cheaper to stop resistance developing, so spotting the early signs is vital.”

Weed maps

With clear evidence of herbicide resistance in bromes on our shores, it is essential to watch weed populations like hawks – and this involves mapping brome patches in the summer (usually, June or July).

Species-level maps are needed. This is because the type of brome present determines the most appropriate management response.

As such assessments require careful observations of flowering structures, AHDB has published guidance to help you tell your barren bromes from your great bromes.

Which brome species is in your field?

During the same period, it is a good idea to harvest ripe weed seeds for herbicide resistance tests – especially, if you have observed any difficulty controlling bromes with herbicides.

A mugful of seed provides a good sample size. If taking multiple samples, be sure to have a system in place to help you trace back samples to weed patches.

Sterile brome: collect ripe seed from early June to the end of July.

Meadow brome and rye brome: collect from mid-late July to the end of August.

WRAG guidance provides in-depth information on resistance testing

No survivors

When poor weed control is observed in the field, it is easy to jump to conclusions and assume that resistance is the cause.

A survey, conducted as part of the ADAS-led research, found that many people believed resistance was present in their fields. However, this could not always be backed up by hard evidence – such as the results of resistance testing.

Sub-optimal use of herbicides is the more likely reason. This is far from ideal because it helps some weeds survive treatment. And survivors will quite happily spread their genetic defences to offspring.

Once resistance is present, it doesn’t go away. To the contrary, without appropriate intervention, herbicide resistant weeds will build and spread – readily and rapidly.

At present, good brome control is achievable, especially in non-cereal crops within the rotation. The use of cultural and chemical control is critical to success. With sprays, it is important to maximise efficacy – with optimised herbicide programmes, timings and doses – to ensure weed kill.

The five main brome species are increasing and are now found in all UK regions. Collectively, we need to up our game to keep a lid on resistance threat. However, there are many factors that drive spread, and the finger should not be pointed at herbicide resistance without hard evidence to hand.

The adoption of less intensive farming practices, including increases in land put down to low/no-till production and margins/environmental areas, will help to tip the balance in the favour of brome. Every ounce of advantage gained by brome needs to be countered by an equivalent mass of weed management strategy.

If management remains static, then the weeds will win. If poor choices are made, then the weeds will win.

By understanding the weed population in your field, you will be able tailor management and retain the upper hand. It is why weed maps need to be a field standard.

Find out how to combine control options for arable weeds