Changes of weed community in lowland rice field[s] in Korea.
A nation-wide weed survey was conducted in lowland rice fields in Korea Republic in 1992 in order to determine variations in the weed communities and to identify the major dominant weed species and/or problem weeds. The proportion of broadleaved weeds, grasses, sedges and other weeds that occurred in rice were found to be 42.6, 9.0, 33.4 and 15.0%, resp. Annual weeds accounted for 33.4% and perennials for 66.6%. Weed occurrence was found to be affected by the rice planting method used: in hand-transplanted paddy fields, the predominant weed species were Sagittaria trifolia, Monochoria vaginalis and Aneilema japonica [japonicum]; in machine-transplanted fields of young rice seedlings, Eleocharis kuroguwai and S. trifolia were the predominant weed species; there was a high occurrence of M. vaginalis, Echinochloa crus-galli and Lee[r]sia japonica in water-sown crops; and E. crus-galli and Cyperus serotinus were the main weed species in dry-sown rice. Rice monoculture resulted in a high occurrence of E. kuroguwai, S. trifolia, M. vaginalis, E. crus-galli and Sagittaria pygmaea, and there was a high population density of S. trifolia, S. pygmaea, M. vaginalis, E. crus-galli and E. kuroguwai in double cropping systems based on rice culture. Different weeds also occurred as a result of different transplanting times: E. kuroguwai, S. trifolia, S. pygmaea, M. vaginalis and C. serotinus occurred in greater abundance when rice was transplanted on 25 May, S. trifolia, E. crus-galli, C. serotinus and M. vaginalis were predominant in rice transplanted on 10 June, and S. pygmaea, E. kuroguwai, M. vaginalis, S. trifolia and E. crus-galli in rice transplanted on 25 June. Autumn tillage resulted in greater populations of S. trifolia, E. kuroguwai, M. vaginalis and S. pygmaea, while spring tillage resulted in more E. kuroguwai, S. trifolia, E. crus-galli, M. vaginalis and S. pygmaea. In paddy fields situated on the plains there was higher occurrence of E. kuroguwai, S. trifolia, M. vaginalis, E. crus-galli and S. pygmaea, in semi-mountainous areas S. trifolia, E. kuroguwai, M. vaginalis, E. crus-galli and Ludwigia prostrata were important components of the weed flora, while in mountainous area S. trifolia, M. vaginalis, Potamogeton distinctus, E. kuroguwai and E. crus-galli predominated. In 1992, the top ten predominant weed species in rice field in Korea Republic, based on summed dominant ratios, were E. kuroguwai > S. trifolia > E. crus-galli > M. vaginalis > S. pygmaea > C. serotinus > L. prostrata > P. distinctus > A. japonica > Scirpus juncoides.