The paper presents results of research on weed infestation of inter-variety mixtures of spring barley in crop rotation and 7-year monoculture in the loessial soil of the central Lubelszczyzna. It has been proved thad the years of monoculture had on essential influence on the increase of number and weight of weeds in spring barley canopy as compared to crop rotation cultivation. Introducing inter-variety mixtures of spring barley into relation decreased by 21% the number of weeds in relation to the individual variety sowing. In the canopy of spring barley the following weeds dominated in monoculture: Matricaria maritima ssp. inodora, Echinochloa crus-galli, Stellaria media, Viola arvensis, Chamomilla recutita, Polygonum lapathifolium, Agropyron repens, Chenopodium album, Veronica arvensis, Galinsoga parviflora, Galeopsis tetrahit, Sonchus arvensis and Cirsium arvense.