A field experiment was established to evaluate quantitative changes in weed species prevailing in a crop rotation and a 30-year cereal monoculture. The subject of the study included cropping systems: a) crop rotation, and b) cereal monoculture. First (1989–1992) and second (1993–1996) rotation focused on weed infestation of winter triticale in crop rotation and monoculture; third (1997–2000) and fourth (2001–2004) focused on weed infestation of winter wheat; fifth (2005–2008), sixth (2009–2012), seventh (2013–2015) and eighth (2016–2018) focused on weed infestation of spring wheat. The prevailing weed species included: Apera spica-venti, Avena fatua, Galium aparine, Fallopia convolvulus, Stellaria media, Viola arvensis, Anthemis arvensis, Veronica persica, Consolida regalis, and Papaver rhoeas. Among these, A. spica-venti predominated in the winter cereals whereas A. fatua in the spring ones. Numbers of A. spica-venti and A. fatua plants were several times higher in the monoculture than in the crop rotation. The substitution of winter cereals with spring ones in cultivation decreased A. spica-venti population in the spring cereals, but increased the population of A. fatua, particularly in the monoculture.