Community Succession And The Controlling Of Weeds In Rice Fields Of Qingpu In Shanghai

The species, architecture of community and harm of weeds occurred in rice fields of Qingpu in Shanghai were investigated from the rice late tillering stage to booting stage by quincunx sampling and five-ranking visual observation between 2003 and 2004. The relative density, relative uniformity, relative occurrence frequency, relative abundance and harm index were adopted in the data analysis. Compared with the investigation results of 1980s, conclusions were obtained as follows:28 weed species belonging to 13 families were found, which were greated decreased as the corresponding numbers were 80 and 27 in 1980s. In all the weeds, there were 5 of Gramineae (17.86%),8 of Cyperaceae (28.57%) and 15 of broadleaf weed (53.37%), among which 12 weed species could affect the rice production.It was found that the predominant weed species in rice fields had been evolved during the past 20 years. The following weeds were listed by their predominance:Rotala indica (willd.) Koehne., Lindernia procumbens (Krock.)Philcox,Sagittaria pygmaea Mig.,Echinochloa crusgalli (L.) Beauv., Monochoria vaginalis (Burm.f.) Presl ex. Kunth, Leptochloa chinensis (L.) Nees., Eclipta prostrata L., Ammannia baccifera L., Alternanthera philoxeroides (Mart.) Griseb., and Cyperus difformis L. The constitution of weed community in rice fields has been changed from gramineous weed+cyperaceous weed+broadleaf weed to broadleaf weed+gramineous weed+cyperaceous weed; and from the annual weeds to perennial weed by their predominance; and the harm of weeds caused to rice was controlled under a low level by chemical control. In this paper, the factors leading to the evolution of weed community in rice fields were investigated, in which the primary factors were the improper application of herbicides, change of cropping system and irrational cultivation system. Also the weed controlling efficiency of pretilachlor and its mixtures were studied. By field trial, the proper application time, dose, frequency and combination of herbicides were determined, which could retard the weed community evolution and ecology environment depravation caused by purposelessly application of herbicides.