Teagasc has published an overview of the herbicide resistance status of grass weed populations within tillage systems during 2024.

Significantly, the work relates to tests carried out on suspect plant samples submitted by commercial farmers.

This work has confirmed that resistance in meadow grasses is arising due to the increased reliance on spring ALS herbicides and less use of autumn-applied residual herbicides.

Farms with ACCase (e.g.,l Axial) resistant wild oats only have one herbicide type option available (ALS type: e.g., Pacifica Plus or Broadway Star) which can only be used on wheat.

Although no fully herbicide-resistant brome has been detected to date, our previous studies have indicated that loss of sensitivity in brome populations is widespread when herbicide rates are reduced.

Teagasc research scientists are now stressing the importance of resistance testing to determine which herbicide will work for a specific weed population.

Moreover, there is an immediate need to develop robust integrated weed management practices to reduce reliance on herbicides, which are becoming more restricted and much less effective.

Eight annual meadow grass populations, grown along with a sensitive population were sprayed at the 2-to-4 leaf stage with ALS, ACCase and glyphosate herbicides.

Two populations were susceptible to all herbicides tested with six resistant to the ALS-Pacifica Plus.

Target-site mutations were the main mechanisms that conferred resistance in these six ALS-resistant populations.

Four of the populations had Pro-197 ALS gene mutations, while two had Trp-574 ALS gene mutations.

All six populations that were resistant to Pacifica Plus showed cross-resistance to Broadway Star (not labelled to control meadow grasses).

ACCase-Centurion Max (used in winter oilseed rape) and glyphosate herbicides were found effective on all eight populations.

Two rough-stalk meadow grass populations grown alongside a sensitive population were sprayed at the 2-to-4 leaf stage with label rates of ACCase (Axial, Falcon, Stratos Ultra, Centurion Max), ALS (Pacifica Plus, Broadway Star) and glyphosate (Roundup Flex) herbicides

Both populations were resistant to ALS-Pacifica Plus.

The main resistance mechanism was target-site mutations.

The two resistant populations showed cross-resistance to Broadway Star, even though Broadway Star is not labelled to control meadow grasses.

Although, rough-stalk meadow grass control is not listed on most ACCase product labels, widely-used cereal (Axial) and graminicide (Falcon, Stratos Ultra or Centurion Max) herbicides and pre-sowing glyphosate were all effective at recommended label rates.

A total of 20 different wild oat populations were sprayed at the 3-to-4 leaf stage with label rates of ACCase (Axial, Falcon, Stratos Ultra) and ALS (Pacifica Plus and Broadway Star) herbicides. 

Trials conformed that 14 out of the 20 populations were ACCase-resistant, with none resistant to ALS type herbicides at this point.

All 14 populations were resistant to Axial and Falcon, with 7 of those resistant to Stratos Ultra also.

A total of 15 different bromes (including sterile, great and soft) were sprayed at the 2-to-4 leaf stage with label rates of ACCase (Falcon, Stratos Ultra), ALS (Pacifica Plus, Broadway) and glyphosate (Roundup Flex) herbicides.

ACCase, ALS and glyphosate herbicides were effective on all populations.