Noxious Weed Species Monochoria vaginalis Resistant to Synthetic Auxin and Acetolactate Synthase Inhibitor Herbicides

<em>Monochoria vaginalis</em> (Burm. f.) C. Presl is a dominant weed in paddy rice cultivation, significantly threatening productivity and farming sustainability. In Subang, Indonesia, synthetic auxin and acetolactate synthase (ALS) inhibitors are commonly used to control <em>M. vaginalis</em>. However, farmers have currently reported declining efficacy of these herbicides. Therefore, this study aimed to confirm the resistance of <em>M. vaginalis</em> to the early post-emergence application of synthetic auxin and ALS inhibitors. Plant bioassays were conducted using the pot test method to determine resistance level. Herbicides were applied 2 weeks after planting <em>M. vaginalis</em> at 0.25, 0.5, 1.0, 2.0, 4.0, and 8.0 times the recommended dose, and untreated plants as control. Herbicides tested were 2,4-D, bensulfuron-methyl, bispyribac sodium, penoxulam, and sulfentrazone. The results obtained based on resistance index values (RI) showed that <em>M. vaginalis</em> populations from Kediri and Gandasari, Subang, had moderately to high resistance to 2,4-D (RI: 8.15 and 13.0) and bensulfuron-methyl<em> </em>(RI: 205.61 and 21.80). Weed was moderately resistant to bispyribac sodium<em> </em>(RI of Kediri biotype: 8.79) and had low to moderate resistance to penoxulam herbicides (RI: 8.94 and 3.56). In contrast, <em>M. vaginalis-</em>resistant biotypes remained susceptible to sulfentrazone (protoporphyrinogen oxidase/PPO inhibitors) herbicide. A significant increase in GR50 was observed between 28 and 56 days after herbicide application, signifying enhanced regrowth. The resistance of <em>M. vaginalis</em> to synthetic auxin and ALS inhibitors presents the need for farmers to consider alternative herbicides, such as PPO inhibitors, to prevent the development of resistant weeds.