Herbicide resistant meadow grasses could pose a threat to future weed control, according to Teagasc research scientist, Vijaya Bhaskar.
In a normal growing season, meadow grasses can be easily controlled in winter cereals with residual herbicides (eg., flufenacet or pendimethalin-based products).
And they have no significant impact on cereal yields, compared to other grass weeds.
But in recent years, some farms that have only used spring herbicide, or a one-product approach for grass-weed control in winter cereals have recorded instances of ineffective control of meadow grass.
Weed control
In 2023, Teagasc received resistance-suspect samples of five annual meadow grass and two roughstalk meadow grass populations from growers/advisors for testing.
Plants at the 2-4 leaf stage were subjected to detailed dose-response experiments with ACCase (Axial, Falcon, Stratos, Centurion) and ALS (Pacifica, Monolith) herbicides.
Currently, the registered herbicides for controlling annual meadow grass according to Teagasc, include ACCase-Centurion (used in winter oilseed rape) and ALS-Pacifica and Monolith (used in winter wheat).
The registered herbicides for controlling roughstalk meadow grass include ACCase-Foxtrot (used in wheat and barley) and ALS-Pacifica.
Other ACCase/ALS herbicide products do not claim meadow grass control or lack effectiveness as per their labels.
Meadow grasses
According to Bhaskar, annual meadow grass is naturally tolerant to some ACCase herbicides.
Teagasc has confirmed that all populations of annual meadow grass possess natural tolerance to the ACCase herbicides Axial, Falcon and Stratos. As a result, they are not controlled by them.
However the exception is the ACCase herbicide-Centurion, which provides effective control.
Teagasc trials have also looked at the impact of ALS-type grass-weed herbicides that are used to control meadow grasses.
Significantly, this work is showing that sensitive annual meadow grass was fully controlled by Pacifica and Monolith at a rate well below the recommended rate.
But both herbicides were ineffective in all five resistant populations of annual meadow grass.
In light of the recent meadow grass trial work, Teagasc has issued guidance.
Annual meadow grass is naturally tolerant to most ACCase (Axial, Falcon, Stratos) herbicides, except Centurion, while roughstalk meadow grass is sensitive to all ACCase herbicides.
In winter crops, it is not recommended to rely on spring applications of grass-weed products, typically aimed at bromes, for meadow grass control.
Residual herbicides applied pre-emergence or early post-emergence should be used to reduce the pressure on resistance development.
Meadow grasses can succumb to resistance when single type high-risk herbicides are used as the sole method of weed control.
Fewer pre-emergence spray opportunities, and the ongoing wet weather will affect this season’s spring herbicide strategies, so Teagasc has said that it is crucial to use the right products and rates to achieve maximum efficacy.