Smallflower umbrella sedge is one of the most problematic weeds in direct-seeded rice in India. Bispyribac-sodium (acetolactate synthase [ALS]-inhibiting herbicide) is commonly used in rice, but growers have recently reported lack of smallflower umbrella sedge control with this herbicide. An extensive survey was carried out in two rice-growing states, Chhattisgarh and Kerala, where 53 putative bispyribac-sodium-resistant (BR) biotypes were collected. Studies were conducted to confirm resistance to bispyribac-sodium and to test the efficacy of the newly developed synthetic auxin herbicide florpyrauxifen-benzyl on putative BR biotypes. A whole-plant bioassay revealed that bispyribac-sodium is no longer effective. Of 53 putative BR biotypes, 17 biotypes survived the recommended label rate of 25 g ai ha^–1. The effective bispyribac-sodium rate required to control 50% of the plants in most of the BR biotypes (ED₅₀) ranged from 19 to 96 g ha^–1, whereas it was 10 g ha^–1 in a susceptible biotype. In two highly resistant biotypes, the ED₅₀ was beyond the maximum tested rate, 200 g ha^–1. This suggests 2- to >20-fold resistance in BR biotypes. An ALS enzyme activity assay suggests an altered target site as mechanism of resistance to bispyribac-sodium. This study confirms the first case of evolved resistance to bispyribac-sodium in smallflower umbrella sedge in India. However, the newly developed synthetic auxin florpyrauxifen-benzyl effectively controlled all BR biotypes at the field use rate of 31.25 g ai ha^–1.

Nomenclature: Bispyribac-sodium; florpyrauxifen-benzyl; smallflower umbrella sedge, Cyperus difformis L.; rice, Oryza sativa L.