Studies On The Succession Of Weed Communities In Paddy Field And Their Control Techniques

A field survey of weed species, community structure and damage level was carried out in paddy rice field at rice heading stage during 1998-1999. The parameters such as relative density, relative uniformity, relative frequency, relative abundance and damage index were used to analyze the survey data. Compared to the data surveyed in 1980s (80 weed species belonging to 27 families), the number of weed species decreased to 28 species belonging to 13 families. Among them, there were 5 species in Gramineae (17.86%) and 8 species in Cyperaceae (28.57%). There were 15 species of broad-leaved weed (53.57%). Twelve weed species had relatively large effect on the rice yield.Dominant species changed along with the frequent use of rather limited herbicides, in the order of Sagittaria pygmaea Mig., Monochoria vaginalis (Burm.f.) Presl ex. Kunth, Lindernia procumbens (Krock.) Philcox, Echinochloa crusgalli (L.) Beauv., Eclipta prostrata L., Ammannia baccifera L., Rotala indica (willd.) Koehne., Cyperus difformis L., Alternanthera philoxeroides (Mart.) Griseb., Leptochloa chinensis (L.) Nees. The structure of weed communities changed from "Gramineae weeds, Cyperaceae weeds and broad-leaved weeds" to "broad-leaved weeds, Cyperaceae weeds and Gramineae weeds". The community of annual less-damage weeds was gradually succeeded to that of perennial virulent weeds. Through the application of chemical treatments, the weed damage degree could be reduced to less than level one.The present paper discussed the factors causing the succession of weed communities such as virulent weeds, including non-rational use of herbicides, change of farming system and cropping system. The combination of agricultural and mechanical control practices and chemical (herbicide) treatment was attempted in this study. Based on the results of laboratory and field experiments, the suitable concentration, application time and number, and their combination of herbicides were proposed. The integrated control system combining agricultural, mechanical and chemical approaches could alleviate the succession of weed communities and reduce the deterioration of ecological environment.