Evaluation of the novel herbicide (TVE29) for penoxsulam-resistant Echinochloa crus-galli control in direct-seeded rice

Herbicide-resistant Echinochloa crus-galli has become a major constraint in Malaysian rice production, particularly in direct-seeded systems that rely heavily on chemical control. Intensive use of the ALS-inhibiting herbicide penoxsulam has accelerated the evolution of resistant populations, resulting in control failures. This study aimed to (i) confirm and determine the penoxsulam resistance level in Echinochloa crus-galli, (ii) investigate the role of enhanced metabolic mechanisms in conferring resistance to penoxsulam, and (iii) identify the optimal rate and timing of TVE29, a new herbicide with a novel mode of action, for controlling penoxsulam-resistant (R) biotypes in rice. Freshly matured resistant and susceptible E. crus-galli seeds collected from rice fields in Alor Setar, Kedah, were used in a series of glasshouse and field experiments. Herbicide resistance was characterised through screening and dose–response assays, cytochrome P450-mediated metabolic resistance was investigated using malathion inhibition bioassays, and the efficacy and crop safety of TVE29 were evaluated at different application rates and timings under glasshouse and field conditions. Data were analysed using chi-square analysis, log-logistic regression, and analysis of variance (ANOVA). Dose–response experiments showed that the dosages required to cause 50% survival and 50% growth inhibition for the R biotype were 480 to 535-fold higher than those of the susceptible (S) biotype, confirming severe resistance to penoxsulam along with cross-resistance to bispyribac-sodium. In contrast, pretilachlor, quinclorac, and florpyrauxifen-benzyl provided complete control of both biotypes, indicating that resistance is specific to ALS-inhibiting herbicides. To assess non-target-site resistance, malathion was used as a cytochrome P450 inhibitor. Malathion alone had no effect on either biotype. Penoxsulam and bispyribac-sodium completely inhibited the S biotype, but the R biotype maintained high survival (72–88%) and growth (52–71%). However, malathion pre-treatment followed by ALS inhibitors significantly reduced survival and shoot biomass of the R biotype to near zero, demonstrating that resistance is largely mediated by enhanced metabolic detoxification involving cytochrome P450 enzymes. Glasshouse evaluations showed that TVE29 at 100 g a.i. ha⁻¹ was safe for rice across pre-plant, pre-emergence, and early post-emergence applications, whereas higher rates caused early-stage phytotoxicity. TVE29 provided complete control of the R biotype, with improved efficacy under prolonged standing water. Preliminary field evaluations confirmed that pre-plant TVE29 at 100 g a.i. ha⁻¹ achieved 93–100% significantly higher control of major rice weeds including Monochoria vaginalis, Cyperus iria, and Fimbristylis miliacea, and outperformed hand weeding, particularly against the R biotype, while maintaining excellent rice selectivity. This study confirms that enhanced metabolism mediated by cytochrome P450 enzymes is the primary mechanism responsible for ALS-inhibiting herbicide failure in E. crus-galli and demonstrates that TVE29 offers an effective, paddy-safe alternative for managing the R biotype.