10 January 2025

Herbicide resistance update on 2024 harvest samples

As part of the Teagasc weed monitoring programme, grass weeds suspected of herbicide resistance or difficult to control were submitted by growers/advisors in 2024 for testing. Vijaya Bhaskar AV shares key findings and insights.

Highlights

Resistance in meadow grasses is arising due to our increased reliance on spring ALS herbicides and less use of autumn-applied residual herbicides.
Farms with ACCase (eg. Axial) resistant wild oats only have one herbicide type option available (ALS type: eg. 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.
Our work stresses the importance of resistance testing to determine which herbicide will work for a specific weed population.
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.

Annual meadow grass

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 (Figure 1).

Two populations (Wexford-1 and Wexford-2) 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 (ie. Dublin, Louth-1, Louth-2 and Meath-2) had Pro-197 ALS gene mutations while two (ie. Cork and Meath-1) 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.

Figure 1 Overhead view of test trays showing the symptoms of sensitive and resistant-suspect populations of annual meadow grass 30 days post-treatment

Figure 1: Overhead view of test trays showing the symptoms of sensitive and resistant-suspect populations of annual meadow grass 30 days post-treatment with ALS (Pacifica Plus, Broadway Star), ACCase (Centurion Max) and glyphosate herbicides. ALS-Broadway Star is not registered to control annual meadow grasses. Except for Pacifica Plus which was applied at half and full recommended label rates, all other herbicides were applied at full recommended label rates. Where plants are absent from a square full control was achieved.

Rough-stalk meadow grass

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 (Figure 2).

Both populations were resistant to ALS-Pacifica Plus.
The main resistance mechanism was target-site mutations, with plants of Louth-1 having double (Pro-197 and Trp-574) ALS gene mutations, and those of Louth-2 with Pro-197 ALS gene mutations. As a result, 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.

Figure 2 Overhead view of test trays showing the symptoms of sensitive and resistant populations of rough-stalk meadow grass 30 days post-treatment

Figure 2: Overhead view of test trays showing the symptoms of sensitive and resistant populations of rough-stalk meadow grass 30 days post-treatment with ALS-Pacifica Plus and Broadway Star, ACCase-Axial, Falcon, Stratos Ultra and Centurion Max and glyphosate all applied at the recommended rate. ALS-Broadway Star is not registered to control rough-stalk meadow grasses. Where treated-plants are absent from a square full control was achieved.

Spring wild oats

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 (Figure 3).

14 out of 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 (Figure 3A and 3B)

Figure 3A Overhead view of test trays showing the symptoms of suspected resistant populations of wild oats 28 days post-treatment

Figure 3B Overhead view of test trays showing the symptoms of suspected resistant populations of wild oats 28 days post-treatment

Figure 3C Overhead view of test trays showing the symptoms of suspected resistant populations of wild oats 28 days post-treatment

Figure 3: Overhead view of test trays showing the symptoms of suspected resistant populations of wild oats 28 days post-treatment with ACCase Axial, Falcon, Stratos Ultra and ALS Pacifica Plus and Broadway Star all applied at the recommended rate. Where treated-plants showed severe stunting, discoloration or yellowing and browning (death) of foliage full control was achieved.

Bromes

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 (Figure 4).

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

Figure 4 Overhead view of test trays showing the symptoms of suspected resistant populations of bromes 28 days post-treatment

Figure 4: Overhead view of test trays showing the symptoms of suspected resistant populations of bromes 28 days post-treatment with ACCase Falcon and Stratos Ultra and ALS Pacifica Plus and Broadway Star and glyphosate all applied at the recommended field rate. Where treated-plants showed severe stunting, discoloration or yellowing and browning (death) of foliage full control was achieved.