Competitive relationships of watergrass and barnyardgrass with rice in a replacement series study

Competitive relationships of watergrass (Echinochloa oryzoides) and barnyardgrass (Echinochloa crusgalli) with rice in different plant proportions of 0:4, 1:3, 2:2, 3:1, and 4:0, weed:rice in each hill were investigated in a field experiment. Weed biological and grain yield (both Echinochloa species) were higher than rice; however, the harvest index was higher for rice compared to weed species. Relative yields (based on number of tillers, biological yield, and grain yield), replacement series diagrams, rice aggressivity index, and relative crowding coefficient exhibited higher competitiveness for weed than rice. Rising in the ratio of each species at the planting proportion increased the biological, grain and relative yield and harvest index in both rice and weed. Furthermore, it caused an increase in rice aggressivity index. In all tested plant proportions, weeds were superior to rice regarding biological and grain yield, except for 1:3 weed:crop in which both species were similar for grain yield. Comparing two Echinochloa species, watergrass, had lower biological yield, but higher harvest index. Grain and relative yields, and relative crowding coefficient were not significantly different for two weed species; however watergrass led to more reduction in grain yield, harvest index, relative yield and aggressivity index of rice.