Rat’s tail fescue - an emerging threat

Rat's tail fescue can become dominant where ploughing is not practiced due to limited effective chemical options, writes Vijaya Bhaskar of Teagasc Oak Park.

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 (eg. min-till, direct drill, etc.) 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: 2-4 leaf (GS 12-14) stage and a tillering (>GS 22) stage. At the early stage, rat's tail fescue had leaves of 5 cm length. Assessments were carried out 30 days post-spraying.   

Two generics (Monsanto-Bayer and other manufacturer) with 360 g/L glyphosate and three Roundup (Monsanto-Bayer) branded products with 360 g/L, 480 g/L or 720 g/kg concentrations were tested on smaller plants, and two generic products (360g/L)  and two Roundup products (360 g/L or 480 g/L) were tested on larger plants.

• On smaller plants, glyphosate rates up to the maximum (1440 g/ha), whether using Monsanto-Bayer or generic products, did not achieve 100% control (Figure 1). The plants were able to recover despite the damage/severe damage caused by the higher glyphosate rates.

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 larger tillering plants, only when higher glyphosate rates of 1080 g/ha (Roundup) or 1440 g/ha (generics or Roundup) were used, was 100% control achieved (Figure 2).

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).

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 2-4 leaf (GS 12-14) stage and assessments were carried out 30 days post-spraying

• Rat’s tail fescue is not controlled by ACCase herbicides Axial, Falcon, Stratos Ultra and Centurion (Figures 3 and 4). Subsequent genetic studies confirmed that all populations of rat’s tail fescue inherit homozygous ACCase (Ile-1781-Leu) mutation, which contribute to their natural tolerance.
• The results with ALS (Pacifica, Monolith, Broadway) herbicides were not very clear.
  • In one population, all ALS herbicides at the label rate were effective (Figure 3) but in others, total control with ALS (Pacifica, Broadway) was not achieved (Figure 4).
  • Much more testing will be required to determine if this is a tolerance or 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.

Figure 3: Symptoms of rat’s tail fescue following application of half and full recommended rates of ACCase (Axial, Falcon, Stratos) and ALS (Pacifica, Monolith, Broadway) herbicides. Where plants are absent from a square, effective control was achieved. The full recommended rates of ACCase and ALS herbicides used were: Axial @ 0.55 l/ha, Falcon @ 1.5 l/ha, Stratos @ 1.5 l/ha, Pacifica @ 0.5 kg/ha, Monolith @ 0.33 kg/ha and Broadway @ 265 g/ha.

Figure 4: Symptoms of populations (RTF-001 and RTF-002) of rat’s tail fescue following application of full recommended rates of ALS-Pacifica and Broadway and ACCase-Centurion Max herbicides. Effective control was not achieved where surviving green plants (whether alive or damaged/severely damaged but not dead) are present from a square.

In short

Based on preliminary glasshouse research

For 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 (1440 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 Axial, Falcon, Stratos and Centurion herbicides
• The efficacy of ALS (Pacifica, Broadway) 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 suggest 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.

Weed Watch-2024

Teagasc has initiated a Weed Watch-2024 campaign with the Irish Farmers Journal.

If you have weed control difficulties or suspect resistance, it might be worth getting your populations tested now. This unique service from Oak Park is available for free for a limited time, as part of the DAFM-funded EVOLVE research project. Avail of free resistance testing to know best control options to control your suspect weed (grass or broad-leaved) in your tillage crops. 

Contact your advisor, or click here for further information on the Weed Watch campaign.