Spring is a crucial time for managing bromes or wild oats in winter wheat crops, but if easy-to-control weeds like annual meadow grass are not controlled early in the season (ie before the use of broad-spectrum ALS herbicides like Pacifica and Broadway Star), farms relying solely on ALS-Pacifica for grass-weed control are at high risk of resistance developing in annual meadow grass.

So far, Teagasc has confirmed ALS resistance in 14 of 16 suspected annual meadow grass populations. This 2025 harvest we have already received over eight new suspect samples for testing. The situation differs in the UK, where black grass is the primary focus and growers must use a robust pre-emergence programme, which also addresses meadow grass control.

Solution

The correct selection and application timing of residual-type, autumn-applied herbicides is a key part of the solution.

Annual meadow grass populations with different ALS resistance statuses were used to evaluate residual products in glasshouse trials during winter 2024/2025. Six common residual products were tested at label recommended rates:

  • Diflanil 500 SC containing diflufenican (DFF).
  • Avadex Factor containing tri-allate.
  • Stomp Aqua containing pendimethalin.
  • Firebird containing flufenacet + DFF
  • Defy containing prosulfocarb.
  • Tower containing pendimethalin + chlorotoluron + DFF.

    • The herbicides were applied at different growth stages (Figure 1). To ensure all seeds got the same exposure to herbicide, the pre-emergence treatment applied the herbicide directly onto the seeds placed on moist soil (9% organic matter) and were covered immediately with soil.

    In other treatments, residual herbicides were applied at the 1-leaf (GS 11), 2-4 leaf (GS 12-14) or tillering stage (>GS 21). Assessments were carried out 30 days post-treatment.

    Residual herbicides provide broad-spectrum weed control, but the results below are specific to annual meadow grass and cannot be used to predict the response in other weeds.

  • Our results clearly demonstrate that residual herbicides, which are less prone to rapid resistance development, by controlling annual meadow grass early, play a crucial role in delaying herbicide resistance evolution.
  • The population response to residual products was similar and the individual sensitivity status to ALS herbicides did not affect the efficacy of the residual products.
  • Generally pre-emergence timing is best, but in these glasshouse conditions, later application still checked/suppressed growth.
  • Weed control differences between Tower, Defy or Firebird were small when applied at either true pre-emergence timing (ie before germination), one-leaf or two- to four-leaf stages (Figure 1 A-C). But there were visible differences in product performance when applied on stronger plants at early tillering (Figure 1 D).
  • Tower with multiple actives delivered the greatest efficacy on annual meadow grass when applied at true pre-emergence and one-leaf stages (100% mortality) followed by Defy and Firebird (>90%). These products also delivered control or suppression when applied at two to four leaves. All of these products had growth checking effects when applied during early tillering with Tower having the most significant effect.
  • There were variations within and between the annual meadow grass populations in response to other residual herbicides. Early application of DFF alone or Stomp or Avadex may result in some control, but their performance drops significantly when applied late or after weed emergence (GS >11).
  • Pendimethalin when used as a co-formulation (eg Tower) was more effective than stand-alone pendimenthalin (such as Stomp Aqua).
  • DFF mainly provides broadleaved weed control with some annual meadow grass activity; so it must be used in combination with other actives to get a significant effect on annual meadow grass.

    • Future of herbicides

    Overall, while weed dormancy, appropriate timing and soil conditions influence the actual field performance of residual herbicides, our glasshouse trials reiterate that the use of residual herbicides (based on flufenacet, prosulfocarb or pendimethalin), especially products containing actives from multiple modes of action (eg Tower), at the recommended label rate is critical for annual meadow grass control, including those of ALS-resistant populations.

    The best efficacy would come from pre-emergence timing (ie 24-48 hours after drilling). However, if pre-emergence is not possible or is missed, products like Tower can still have an impact at later application timings. The future of autumn-applied herbicides is under increasing pressure, with significant changes on the horizon. Metribuzin-based products (such as Firebird-Met) are set to expire by the end of 2025 and flufenacet is expected to be withdrawn from the market in 2026. Additionally, key actives like pendimethalin, chlortoluron and DFF, are currently listed as candidates for substitution under EU regulations. These developments threaten the availability of crucial residual herbicides.

    As a result, there will likely be greater reliance on post-emergence options such as ACCase and ALS herbicides, along with glyphosate. This increased reliance could accelerate resistance development, further complicating grass-weed control in cereal crops.