Many factors decided about a competitive ability of crop plant towards weeds but the possibility of crop defence against the most aggressive ones is very important. The phenomenon was studied on the results collected from a field experiment under nutrient-rich and brown loess soil by split-plot method in the 1995/1996–1997/1998 seasons. Winter wheat Kobra cultivar was sown in half a field area (designed to crop) and the other half was left as fallow. The aboveground air dry weight (adw) produced by weeds (by species) was assessed in weed communities growing in a winter wheat canopy and in a canopy developed under fallow. Both cropped and uncropped fields were treated in the same way by means of experimental factors and agronomical measures. Experimental treatments concerned weed management: A – no treated; B – harrow after wheat emergence (autumn); C – harrow in tillering stage (spring); D – herbicide (thifensulfuronmetyl 40.92+chlorsulfuron 4.09 g ha-1) in tllering stage, and combination the treatments: BC; BD; CD; BCD. Biomass produced by weed species in the cropped field and the uncropped one was collected from 1 m2 (2 m × 0.5 m) of each plot at winter wheat harvest time. Winter wheat Kobra cultivar showed high potency to decrease the weed species number (up to 16) and PSM many of species. Seven species (Chenopodium album, Apera spica -venti, Matricaria maritima ssubsp. inodora, Echinochloa crus -galli, Galium aparine, Cirsium arvense and Convolvulus arvensis) demonstrated the greatest PSM reduction (jointly from 92% to almost 99%, dependence on weed management) in a wheat canopy in comparison to fallow. Kobra is a highly competitive cultivar to those and other species, even under no weed control. Hence, assuring Kobra wheat good vegetation conditions allows to decrease or even eliminate weed control treatments.