The phytosociological method helps evaluate the vegetation composition. The objective of this study was to identify and quantify the main weed species present in irrigated rice. The phytosociological survey was carried out in three mainly rice monoculture, Clearfield® technology using properties in Itaqui-RS, between 2013 and 2014. For each property and period, a ½ ha representative area was selected and 10 random samples were collected, using a quadrat square of 1 m2. The sample colletion was conducted in two periods of time: during the irrigated rice crop initial growth and during the cereal’s pre-harvesting phase. All collected plants were identified and counted, allowing for their frequency, relative frequency, density, relative density, abundance, relative abundance, relative importance index and similarity index to be calculated. Eleven weed species belonging to five families were identified. Poaceae and Cyperaceae occurred more frequently. During the initial growth of rice, the Poaceae family presented the largest index of relative importance, mainly to Echinochloa crus-galli, Echinochloa colona and Digitaria horizontalis. Overall, there was a reduction in plant density in the pre-harvest period. However, Echinochloa colona and Oryza sativa occurred in density levels that may interfere with the cereal yield. Aeschynomene denticulata, Cyperus iria and Oryza sativa are not controlled efficiently, indicating possible cases of herbicide resistance. Our conclusions emphasize the importance of correct weed species identification for the management of Echinochloa colona and Cyperus ferax. Integrated management practices are necessary for efficient weed control, avoiding productivity loss in the region’s flooded rice fields.
phytosociology; Oryza sativa; identification