Rice production is the most dominant economical, social and agricultural activity in northern part of Iran. Weeds are the main constraint in rice production that could cause up to 90% yield loss in transplanted (TR) rice. Potamogeton nodosus (Pondweed) has emerged as an invasive weed in water logging rice fields because of its tolerance to flooding, herbicide and reemergence after hand weeding. This study was laid out in 2012 in order to investigate the efficacy of some herbicides on chemical control of P. nodosus. Treatments included butachlor (Buta), thiobencarb (TB), oxadiargyl (Oxa), bensulfuron-methyl (BSM), applied at 3 day after transplanting (DAT) and the mixture of BUTA, TB and OXA with BSM applied at 3 and 10 DAT. Experimental design was a RCBD with four replications. Full season hand weeded and infested controls were also included. In herbicide treated plots all weeds were hand weeded up to six week after TR but Pondweed. Results showed that none of the treatments caused significant toxicity to rice based on visual evaluation 3 and 6 week after TR. Buta and TB had no significant effect on P. nodosus, Oxa reduced the biomass of pondweed 42%. Application of BSM alone or in mixture with other herbicides applied at 3 DAT reduced P. nodosus dry weight around 75%. Application of BSM or mixture treatments at 10 DAT controlled pondweed 95% or more. Based on the study it is recommended to apply herbicides intermittent, first graminicides and then sulfuonyl urea in order to increase their efficacy on delayed germinated weeds like P. nodosus.
