Effectiveness of pre and post-emergence herbicides for knotgrass (Paspalum distichum) control in paddy rice

Knotgrass)  Paspalum distichum L.) is a rhizomatous perennial narrow-leaf weed belonging to the Poaceae family. Widespread use of herbicides to manage paddy fields’ weeds and their ineffectiveness in knotgrass control, this weed is now considered the main constraint to producing rice in tens of thousands of hectares of paddy fields. Two open-air pot studies were conducted to evaluate the knotgrass response to paddy field selective herbicides. The results of the first experiment showed that 8 out of 16 rice selective pre-emergence herbicides were less than 44% effective in reducing the biomass of knotgrass. Molinate, 58%, pretilachlor + pyrazosulforoun-ethyl, 62%, oxadiargyl, 64%, penoxsulam, 73%, clomazone + pendimethalin, 87%, metazosulfuron, 92%, and triafamone+ethoxysulfuron, 96% reduced the biomass of knotgrass and halosulfuron increased the biomass of this weed by 20%. The results of the second experiment indicated that 9 out of 13 foliar applied herbicides, reduced the shoot biomass of knotgrass by less than 50%. Pre-packaged herbicides propanil + pretilachlor, 52%, triafamone + ethoxysulfuron, 71%, cyhalofop + penoxsulam, 71%, and glyphosate caused 93% reduction in its shoot biomass. According to the results of this study, all common herbicides of recent decades lacked efficiency to control knotgrass in paddy fields of Iran, but the new herbicides are more effective in this regard. Further field studies are necessary to provide practical recommendations for the chemical management of knotgrass.