The prediction models for rice yield, as affected by competition according to weed species, were as follows: Y = 537.4/1+0.03479x in C. serotinus, Y = 560.2/1+ 0.00417x in E. kuroguwai and Y = 556.0/1+0.002776x in P. distinctus. The weed densities to cause a 50% yield loss were observed as follows: 25.3 plants/m2 in C. serotinus, 240 plants/m2 in E. kuroguwai, and 360 plants /m2 in P. distinctus.