This work was carried out to evaluate weed interference periods in upland rice during two growth seasons (2003/2004 and 2004/2005) in an experimental area owned by Universidade de São Paulo - UNESP, Jaboticabal/SP located at 21º 15' 22" South latitude and 48º 18' 58" West longitude, using the rice cultivar IAC 202, sown on November18, 2003 and November, 24, 2004. The experimental design was randomized blocks, and the treatments consisted of increasing periods of control or coexistence with the culture. The initial periods of control or weed coexistence after culture emergence were: 0-10; 0-20; 0-30; 0-40; 050; 0-60; 070; 0-harvest. The main weeds in 2003/04 were C. rotundus, C. echinatus, Digitaria spp., E. crusgalli and B. decumbens. In 2004/2005, Digitaria spp., C. echinatus, R. raphanistrum and A. tenella were identified. Considering 5% of tolerance in rice yield reduction during the two growth seasons (2003/2004 and 2004/2005), it was concluded that the periods prior to interference were 12 and 26 days after emergence (DAE); total periods of interference prevention, were 40 and 42 DAE and critical periods of interference prevention, between 12 and 40 DAE and 26 and 42 DAE, respectively.
Oryza sativa; competition; critical period
