Preliminary Study of Cross-Resistance Banyardgrass (Echinochloa crus galli) to Penoxsulam and Bispyribac Sodium in Malaysia

Abstract

The increasing failure of acetolactate synthase inhibiting herbicides, particularly penoxsulam, to control Echinochloa crus-galli in Malaysian rice fields has raised concerns of herbicide resistance. This study investigated the resistance to penoxsulam in E. crus-galli populations from Alor Setar, Kedah, and evaluated alternative herbicides with different modes of action. Seeds were collected from suspected resistant and known susceptible fields. After dormancy-breaking, seedling bioassays and dose-response trials were conducted under glasshouse conditions. Resistance screening showed 100% survival of the resistant (R) biotype with penoxsulam and bispyribac sodium, while the susceptible (S) biotype had complete mortality. The R biotype required 32 times (800 g a.i. ha⁻¹) the recommended rate of penoxsulam for control. GR₅₀ and LD₅₀ values indicated 480-fold and 535-fold resistance, confirming high resistance. To find alternatives, pretilachlor, quinclorac, and florpyrauxifen-benzyl were tested. All achieved 100% mortality in both R and S biotypes, indicating no multiple resistance. These results confirm ALS inhibitor resistance in E. crus-galli, while other herbicides remain effective. This study provides the first report of cross-resistance to penoxsulam and bispyribac sodium in Malaysia. The continued susceptibility of the resistant biotype to quinclorac, pretilachlor, and florpyrauxifen-benzyl offers opportunity for herbicide rotation and mixtures in direct-seeded rice systems.