Weeds are one of the major issue in the European rice ecosystems, as they may severely affect crop yields. All rice management practices may have a significant influence on weed composition and then on the competitive relationships between weeds and rice. This study, conducted in 2003-2006 at Vercelli (north-west of Italy,) was aimed at assessing the effect over the time of the following management systems (treatments) on the weeds: straw and liquid manure incorporated in autumn and rice seeded in flooded field (SMAF), straw incorporated in autumn and rice seeded in flooded field (SAF), straw incorporated in autumn and rice seeded in dry field (SAD), straw incorporated in spring and rice seeded in flooded field (SPF), straw burned in autumn and rice seeded in flooded field (SBF), straw incorporated in spring and rice seeded in flooded field in rotation with maize (SPFR). In SAD system field was conventionally flooded starting from rice tillering stage. Weed assessments were done in 3 untreated areas of each plot, were regular chemical weed control was carried out. A significant influence of the management system on weed cover and density started to be noticeable from the second year of the experiment. Heteranthera reniformis, Schoenoplectus mucronatus, Ammania coccinea and Lindernia dubia were the main weeds in the water seeding system, while Echinochloa crus-galli was the major weed problem in the dry seeding rice. A significant increase of weed density over the seasons was observed in dry-seeded rice treatment (SAD), mainly because of the higher presence of E. crus-galli. In general, species diversity tended to be higher in the rotational system. Weed biomass assessed at panicle filling in all water seeded treatments was on average much lower (95 gDMm-2) than in the dry seeded one (650 gDMm-2). From 2003 to 2005, weed seed bank in the 0-10 cm layer roughly doubled in SBF and SMAF, while remained quite unchanged in the other conditions.
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