Abstract
BACKGROUND: Poa annua is currently a lower-priority arable weed in Ireland; however, increasing target-site resistance to acetolactate synthase (ALS) inhibitors and declining availability of non-ALS herbicides highlight emerging control challenges. ALS-inhibiting co-formulations of foramsulfuron + thiencarbazone have been specifically developed for use in herbicide-tolerant beet varieties. While foramsulfuron can control grass weeds carrying specific target-site mutations, its efficacy within the co-formulation on ALS-resistant populations remains unknown. We addressed this knowledge gap using genetic characterisation, dose-response and interaction analyses, and ligand docking modelling in 12 independently evolved, ALS-resistant, Poa annua populations carrying Pro-197 or Trp-574 mutations. RESULTS: All Pro-197 variants (Thr, Gln or Ala) were thiencarbazone-resistant (resistance index (RI) = 20.9-33.3) but remained foramsulfuron-sensitive (2.5-4), while the co-formulation showed reduced efficacy (5.5-6.2). Interaction analyses showed no overall deviation from additivity for Pro-197 populations (P > 0.05), although antagonism occurred at reduced rates (P < 0.05). Structural modelling and ligand docking analyses indicated that most Pro-197 substitutions did not alter the ALS catalytic tunnel. Foramsulfuron retained extensive hydrogen-bonding and electrostatic interactions, with a more favourable binding free energy than thiencarbazone, which showed fewer interactions and a less favourable free energy. Trp-574-Leu variants exhibited cross-resistance to all treatments, consistent with disruption of key binding interactions. CONCLUSION: This study demonstrates that foramsulfuron + thiencarbazone efficacy on Pro-197-resistant Poa annua populations is poor, despite these populations being sensitive to foramsulfuron used alone. Consequently, this co-formulation, developed for ALS-tolerant beet, should be restricted to fields without ALS-resistant weeds to ensure effective in-crop control and avoid selection for resistance. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.
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Claims 11
Claim · model-written summary
Pro-197 variants of Poa annua were resistant to thiencarbazone (resistance index 20.9-33.3).
- Material
- Pro-197 variants (Thr, Gln or Ala)e1
- Treatment
- thiencarbazonee1
- Outcome
- thiencarbazone-resistante1
- Value
- 20.9-33.3e1
- Qualifier
- resistance index (RI)e1
Poa annuaOrganisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e1“All Pro-197 variants (Thr, Gln or Ala) were thiencarbazone-resistant (resistance index (RI) = 20.9-33.3) but remained foramsulfuron-sensitive (2.5-4), while the co-formulation showed reduced efficacy (5.5-6.2)”
Claim · model-written summary
Pro-197 variants of Poa annua remained sensitive to foramsulfuron alone (value 2.5-4).
- Material
- Pro-197 variants (Thr, Gln or Ala)e1
- Treatment
- foramsulfurone1
- Outcome
- foramsulfuron-sensitivee1
- Value
- 2.5-4e1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e1“All Pro-197 variants (Thr, Gln or Ala) were thiencarbazone-resistant (resistance index (RI) = 20.9-33.3) but remained foramsulfuron-sensitive (2.5-4), while the co-formulation showed reduced efficacy (5.5-6.2)”
Claim · model-written summary
The foramsulfuron + thiencarbazone co-formulation showed reduced efficacy on Pro-197 variants of Poa annua compared with foramsulfuron.
- Material
- Pro-197 variants (Thr, Gln or Ala)e1
- Treatment
- co-formulatione1
- Outcome
- efficacye1
- Direction
- reducede1
- Value
- 5.5-6.2e1
- Comparator
- foramsulfurone1
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e1“All Pro-197 variants (Thr, Gln or Ala) were thiencarbazone-resistant (resistance index (RI) = 20.9-33.3) but remained foramsulfuron-sensitive (2.5-4), while the co-formulation showed reduced efficacy (5.5-6.2)”
Claim · model-written summary
Pro-197 populations showed no overall deviation from additivity in the herbicide interaction analysis.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e2“Interaction analyses showed no overall deviation from additivity for Pro-197 populations (P > 0.05), although antagonism occurred at reduced rates (P < 0.05)”
Claim · model-written summary
Antagonism occurred in Pro-197 populations at reduced herbicide rates.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e2“Interaction analyses showed no overall deviation from additivity for Pro-197 populations (P > 0.05), although antagonism occurred at reduced rates (P < 0.05)”
Claim · model-written summary
In docking models, most Pro-197 substitutions did not alter the ALS catalytic tunnel.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e3“Structural modelling and ligand docking analyses indicated that most Pro-197 substitutions did not alter the ALS catalytic tunnel”
Claim · model-written summary
In docking models, foramsulfuron retained extensive hydrogen-bonding and electrostatic interactions.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e4“Foramsulfuron retained extensive hydrogen-bonding and electrostatic interactions, with a more favourable binding free energy than thiencarbazone, which showed fewer interactions and a less favourable free energy”
Claim · model-written summary
In docking models, foramsulfuron had a more favourable binding free energy than thiencarbazone.
- Compound
- Foramsulfurone4
- Outcome
- binding free energye4
- Direction
- more favourablee4
- Comparator
- thiencarbazonee4
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e4“Foramsulfuron retained extensive hydrogen-bonding and electrostatic interactions, with a more favourable binding free energy than thiencarbazone, which showed fewer interactions and a less favourable free energy”
Claim · model-written summary
In docking models, thiencarbazone showed fewer interactions and a less favourable binding free energy.
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e4“Foramsulfuron retained extensive hydrogen-bonding and electrostatic interactions, with a more favourable binding free energy than thiencarbazone, which showed fewer interactions and a less favourable free energy”
Claim · model-written summary
Trp-574-Leu variants of Poa annua showed cross-resistance to all treatments.
- Material
- Trp-574-Leu variantse5
- Outcome
- cross-resistance to all treatmentse5
- Qualifier
- consistent with disruption of key binding interactionse5
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e5“Trp-574-Leu variants exhibited cross-resistance to all treatments, consistent with disruption of key binding interactions”
Claim · model-written summary
The foramsulfuron + thiencarbazone co-formulation should be restricted to fields without ALS-resistant weeds.
- Outcome
- restricted to fields without ALS-resistant weedse6
Organisms pictured · Representative pictures of the matched species, not figures from the paper.
Verbatim
t0“Poa annua populations”
e6“this co-formulation, developed for ALS-tolerant beet, should be restricted to fields without ALS-resistant weeds to ensure effective in-crop control and avoid selection for resistance”