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The objective of this research was to identify the weed species and to determine the critical periods of weed interference in upland rice in Maranhao State, northeastern Brazil. The experiment was laid out in a randomized complete block design with 16 treatments and four replications in two growing seasons. Treatments were increasing periods of control and coexistence of the crop with weeds, for every 10 days. Weeds were sampled with a 0.5 × 0.3 m metal rectangle which was randomly thrown four times in each plot. Phytosociological parameters computed were Density, Frequency, Dominance and Importance Value Index of each species. The more important families in both growing seasons were Poaceae and Cyperaceae. Cyperus spp., Phyllanthus niruri L., Alternanthera tenella Colla and Digitaria spp. reached higher values of relative importance than the other species. Coexistence of weeds with rice during the whole crop cycle in both growing seasons decreased rice grain yield by 83.4 and 72.0%, respectively. Taking into account 5% tolerance in yield reduction in cropping seasons 2010/2011 and 2011/2012, the periods before interference (PBI) were 15 and 13 days after emergence (DAE), the total periods of interference prevention (TPIP) were 25 and 45 DAE and the critical periods of interference prevention (CPIP) were from 15 to 25 and from 13 to 45 DAE, respectively. It was concluded that the weed control in upland rice must be carried out from 13 till 45 DAE to promote weed free development since the crop has low natural competitive capacity.       Key words: Competition, critical period, Oryza sativa L., weed community.

Phytosociology and interference of weeds in upland rice in Maranho State, northeastern Brazil