Rat’s tail fescue is an invasive grass found on a small number of our tillage farms, where early-sown winter cereals are established with non-inversion tillage methods and growers are finding it extremely difficult to control.

Control of rat’s tail fescue is not claimed on most grass-weed herbicide labels, which is a major concern.

The control efficacy of glyphosate, ACCase and ALS herbicides, using various rates and application timings, was evaluated in a preliminary series of glasshouse trials at Oak Park recently.

Glyphosate efficacy

Rat’s tail fescue plants were sprayed at two separate timings: two to four leaf (GS 12-14) stage and at tillering (>GS 22) stage.

At the early stage, rat's tail fescue had leaves of 5cm length. Assessments were carried out 30 days post-spraying.

Two generics - in this case, products not marketed under the original manufacturer's brand name - with 360g/l glyphosate and three Roundup (Monsanto-Bayer) branded products with 360g/l, 480g/l or 720g/kg concentrations were tested on smaller plants and two generic products (360g/l) and two Roundup products (360g/l or 480g/l) were tested on larger plants.

Figure 1. Rat’s tail fescue appearance at 30 days following application of glyphosate products at the 2-4 leaf stage. Surviving green plants (some severely damaged) within all pots were able to recover.

  • On smaller plants, glyphosate rates up to the maximum (1,440g/ha), whether using Monsanto-Bayer or generic products, did not achieve 100% control (Figure 1). The plants were able to recover despite the damage caused by the higher glyphosate rates.

    • Figure 2. Rat’s tail fescue appearance at 30 days following application of glyphosate products to larger tillering plants. Effective control was only achieved at higher rates (ie. from 1080 g/ha for Roundup and only at 1440 g/ha for generics-1 and 2).

  • On larger tillering plants, only when higher glyphosate rates of 1,080g/ha (Roundup) or 1,440g/ha (generics or Roundup) were used was 100% control achieved (Figure 2).

    • ACCase/ALS herbicide efficacy

    Different populations of rat's tail fescue were evaluated with ACCase (Axial, Falcon, Stratos Ultra, Centurion) and ALS (Pacifica, Monolith, Broadway) herbicides.

    As these herbicides do not claim to control rat's tail fescue, the label rates recommended for controlling key grass weeds, such as Italian ryegrass, were used.

    Plants were sprayed at the two- to four-leaf (GS12-14) stage and assessments were carried out 30 days post-spraying

  • Rat’s tail fescue is not controlled by ACCase herbicides. Subsequent genetic studies confirmed that all populations of rat’s tail fescue inherit ACCase mutation, which contribute to their natural tolerance.
  • The results with ALS herbicides were not very clear.

    • - In one population, all ALS herbicides at the label rate were effective, but, in others, total control with ALS (Pacifica, Broadway) was not achieved.

    - Much more testing will be required to determine if this is a resistance issue that has already evolved in fields where ALS products have been used previously or whether other factors, such as poor uptake or effectiveness at early growth stages, are involved.

    Preliminary glasshouse research by Teagasc has shown that rat’s tail fescue is poorly controlled by many different herbicides.

    Control with glyphosate

    Rat's tail fescue is not easily controlled by glyphosate.

    Targeting smaller plants is normally crucial for all grass weeds, but, with rat’s tail fescue, even with the maximum rate, it may not lead to total control.

    Whether this is due to the tiny leaf blades' inability to take up sufficient glyphosate or some other factor is unknown.

    Targeting larger tillering plants and canopies of greater density with a maximum rate (1,440 g/ha) was more effective in these glasshouse trials, but field conditions are different. A field canopy will include plants at different growth stages.

    Further research is needed to understand the tolerance mechanism, the effects of sequential application and control efficacy at different stages of tiller development.

    For ACCase/ALS herbicides

    Rat's tail fescue is totally resistant to all ACCase herbicides such as Axial, Falcon, Stratos Ultra and Centurion herbicides.

    The efficacy of ALS herbicides such as Pacifica and Broadway Star herbicides could not be clearly determined in these limited trials, as there may be populations with different sensitivity levels to these products.

    Further research is needed, including the evaluation of ALS products on larger tillering plants.

    Cultural and integrated options

    Clearly, cultural control options must be to the fore in preventing rat’s tail fescue becoming a problem.

    Research in the UK and Denmark suggests that spring cropping, delayed autumn drilling and deep ploughing are all effective in reducing rat’s tail fescue populations.

    Residual herbicides (flufenacet- and pendimethalin-based products) are also effective, making integrated practices combining cultural methods and residual herbicides a sensible approach.