02 May 2024

Continually using one herbicide type is causing more resistant meadow grasses

Resistance in meadow grasses could pose a threat to future weed control, writes Teagasc researcher 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.

In 2023, we 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.

The registered herbicides for controlling annual meadow grass include ACCase-Centurion (used in winter oilseed rape) and ALS-Pacifica and Monolith (used in winter wheat).
The registered herbicide 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.

Annual meadow grass is naturally tolerant to some ACCase herbicides

All populations of annual meadow grass possesses natural tolerance to the ACCase herbicides Axial, Falcon and Stratos, see Figure 1, and are not controlled by them. However the exception is the ACCase herbicide-Centurion which provides effective control (Figure 1).

Figure 1 Symptoms of annual meadow grass

Figure 1: Symptoms of annual meadow grass following application of ACCase-Axial, Falcon, Stratos and Centurion at dose rates from 0.25 to 8-times the recommended rates (in yellow). Where plants are absent from the square, full control was achieved.

ALS-resistant annual meadow grass

The broad-spectrum ALS-type grass-weed herbicides used to control meadow grasses, which were listed as susceptible on many of their labels, but our research indicates that this is changing.

Sensitive annual meadow grass was fully controlled by Pacifica and Monolith at a rate well below the recommended rate (Figure 2). But both herbicides were ineffective in all five resistant populations of annual meadow grass.

Although Pacifica and Monolith severely affected AMG-004 plants (Figure 2), 4 times the recommended rate was needed to achieve full control, which would not be allowed on the farm.

Figure 2 Symptoms of resistant and sensitive populations of annual meadow grass following application of ALS

Figure 2: Symptoms of resistant (AMG-003 to AMG-007) and sensitive populations of annual meadow grass following application of ALS Pacifica and Monolith herbicides. The populations were treated with dose rates of 0.25 to 8-times (for suspect resistant) or 0.0625 to 2-times (for sensitive) the recommended rates (in yellow). Where plants are absent from the pot, full control was achieved

Roughstalk meadow grass is sensitive to most ACCase herbicides

Roughstalk meadow grass control is usually not listed on product labels of the tested ACCase herbicides, but Axial, Falcon, Stratos and Centurion were all effective at recommended rates, see Figure 3.

Figure 3 Symptoms of roughstalk meadow grass each farm had two field samples denoted by alphabets following application of ACCase

Figure 3: Symptoms of roughstalk meadow grass (each farm had two field samples denoted by alphabets) following application of ACCase-Axial, Falcon, Stratos and Centurion at the recommended rate. Where plants are absent from the square, full control was achieved.

ALS-resistant roughstalk meadow grass

Pacifica and Monolith fully controlled sensitive roughstalk meadow grass at a rate lower than the recommended rate (Figure 4),

However, neither herbicide was effective on resistant populations RSMG-1A and RSMG-2A.

Figure 4 Symptoms of resistant and sensitive populations of roughstalk meadow grass

Figure 4: Symptoms of resistant (RSMG-1A and RSMG-2A) and sensitive populations of roughstalk meadow grass following application of ALS-Pacifica and Monolith. The populations were treated with dose rates of 0.25 to 8-times (resistant) or 0.0625 to 2-times (sensitive) the recommended rates (in yellow). Where plants are absent from the pot, full control was achieved.

In brief

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 on-going wet weather will affect this season’s spring herbicide strategies, so it is crucial to use the right products and rates to achieve maximum efficacy