In a experimental area of Universidade Federal Rural do Rio de Janeiro, on hidromorphic soil, it was carried a field experiment with upland rice. In a greenhouse it was carried out two essays and one experiment was carried out under laboratory condition. The field assay consisted of 18 treatments combining three sowing densities (100, 150 and 200 seeds/m), two row spacing (30 and 50 cm) and three weed control methods (post emergence herbicide, weedy and weed free checks). It was taken samples of Cyperus rotundus at 20 cm depth on 0,25 m2 area to obtain the number and dry matter rhizomes and aerial culms at 35 days after sowing. In greehouse it was used sifted soil in 1,5 kg pot with 16 treatments placed in 4 x 4 factorial, settled 0, 4, 8 and 16 purple nutsedge with 0, 8, 16 and 32 rice seeds. In the laboratory, it was used aqueous extract of C. rotundus on dilutions 1:1, 1:4 and 1:8 placed in petri plate, together rice seeds to observ germination. In the field it was concluded that number and dry matter rhizomes and dry matter C. rotundus culms did not experience significant reduction by rice spacing and sowing density although dry matter of aerial culms was significantly reduced on treatments with herbicide and hand weeding. In rice, the number of panicles was significantly reduced on 50 cm spacing but 100 seeds weight increased in this condition. Competition of purple nutsedgeg reduced grain yield in 75% and number of panicles in 69%. The increase in density and spacing reduction were not enough to control the great competition due to C. rotundus. In greenhouse the number of rhizomes did not decrease rice germination, but at 55 days there was rice culm reduction with 4 rhizomes per pot. The aerial dry matter of C. rotundus was also reduced in the higher rice density. The aqueous extracts of C. rotundus did not reduce rice germination.
Alelopathie; Oryza sativa; competition