Pondweed is a rhizomatous perennial weed of aquatic habitats that recently adapted to rice ecosystems in northern Iran. Two field experiments were conducted at the Rice Research Institute of Iran to determine the impact of pondweed on rice yield and identify effective herbicides for pondweed control. The focus of the first study was to evaluate the herbicides commonly used in Iranian rice, including butachlor, pretilachlor, oxadiargyl, pendimethalin, thiobencarb, and bensulfuron-methyl. None of these herbicides effectively controlled pondweed, except bensulfuron, which reduced pondweed biomass by ≥95% and produced 26% higher rough rice grain yield than the nontreated plots. The second experiment evaluated the performance of acetolactate synthase–inhibiting herbicides on pondweed control, rough rice yield, and pondweed regrowth. Herbicide efficacy on pondweed varied from 36% to 100%. Five preemergence herbicides, bensulfuron at 45 g ai ha^–1, flucetosulfuron at 30 g ai ha^–1, triafamone plus ethoxysulfuron at 40 g ai ha^–1, and metsulfuron-methyl at 15 g ai ha^–1, provided ≥98% control of pondweed. Use of postemergence herbicides penoxsulam at 35 g ai ha^–1, bispyribac-sodium at 30 g ai ha^–1, and pyribenzoxim at 35 g ai ha^–1 provided 36%, 89%, and 93% pondweed control, respectively. Rough rice yields ranged from 107% to 124% in herbicide-treated plots compared with the nontreated plots. Soil-applied herbicide treatments produced higher (≥119%) yield than the hand-weeded control or foliar-applied herbicides. Pondweed regrowth was affected by herbicides and was variable. Soil-applied residual herbicides metazosulfuron, flucetosulfuron, and metsulfuron provided complete control of pondweed and prevented regrowth. In contrast, pondweed regrowth in other soil- and foliar-applied herbicide treatments occurred, indicating their lesser translocation to underground vegetative rhizomes. This study shows that although most sulfonylurea herbicides can control pondweed effectively to achieve high rough rice yield, only a few soil-applied herbicides were able to prevent pondweed regrowth.

Nomenclature: Bensulfuron-methyl; bispyribac-sodium; butachlor; flucetosulfuron; metazosulfuron; metsulfuron-methyl; oxadiargyl; pendimethalin; penoxsulam; pretilachlor; pyribenzoxim; thiobencarb; triafamone plus ethoxysulfuron; pondweed, Potamogeton nodosus Poir. ‘PTMNO’; rice, Oryza sativa L.