267 Scientific Papers, USAMV Bucharest, Series A, Vol. LII, 2009, ISSN 1222-5339 INFLUENCE OF CROP ROTATION UPON WEED DEVELOPMENT ON CORN, WHEAT AND SOYBEAN CROPS A.I. POP, P. GUŞ, T. RUSU, ILEANA BOGDAN, PAULA MORARU, LAVINIA POP University of Agricultural Science and Veterinary Medicine of Cluj-Napoca Keywords: crop rotation, weed suppression, herbicides Abstract In every type of culture or group of cultures we can observe specific weeds. These weeds are adapted to the particular biological characteristics of the cultivated plant and at the specific technological issues. Monoculture or short crop rotations fulfill this mentioned above conditions for a longer period of time infesting the soil with weed seeds. On the other hand, long crop rotation leads to a proper development of technological changes for each type of crops. This aspect is obvious at corn and soy bean where the crop rotation for two years leads to a weed development of 110-116% compared with the four year crop rotation. The differences are reduced at the three year crop rotation, with a weed development level of 13-64% higher compared with the four year crop rotation. In the four year crop rotation compared with the two year one, the structure of weed development on the wheat culture involves significant modifications. The frequency of the following weeds was reduced: Centaurea, Adonis, Thlaspi, Sinapis, Vicia, Viola, Matricaria, Galeopsis şi Raphanus. On the other hand, the intensity of appearance with perennial weeds is: Agropyron repens, Cirsium arvense, Convolvulus arvensis and Symphitium officinale. INTRODUCTION Crop rotation is considered one of the most important technological measures for increasing soil fertility, suppress weeds, and fight against weed development, pest and diseases. Nowadays crop rotations are considered crucial for sustainable agriculture, seen as prime objective for accomplishing optimum yields without negative effects upon the ecological balance and environment stability. During 1985-2003 [1, 2, 3], in the experimental plots focused on crop rotation on different types of soils and different structures of crops the agrotehnical protocols where followed for the improvment of technological aspects. Good results where obtained by conducting a four year crop rotation (soy bean-autumn wheat-potato-corn) for soil tillage, fertilizers usage, herbicides usage, with optimum conditions for suppressing perennial weeds.