There is an urgent need for more integrated weed management in commercial cereal production. However, there is gap in the general understanding of crop–weed interactions and the traits contributing to weed suppression by winter cereals. In this research, we measured the above-ground competitive traits crop height, leaf area index and early ground cover and below-ground allelopathic traits by quantification of benzoxazinoid (BX) content in the root zone by HPLC–MS/MS. A partial least squares regression was applied to identify the relative contribution of each trait to explaining the observed variance in weed biomass. The use of three winter cereals, wheat, triticale, and rye enabled us to cover a broad range of weed suppression strategies. For the first time, we were able to demonstrate a positive correlation between BX in plant material and phenoxazinoids in the soil. The highest concentration of BX in the rhizosphere was found at 3–10 cm. Furthermore, differences in crop weed interactions between wheat, rye, and dicotyledonous and monocotyledonous weeds were identified. Interestingly, there was no significant difference between the contribution of any single competitive or allelopathic trait to explain weed biomass. This is valuable information for the establishment of breeding programs for enhanced weed suppression by the cereal crops examined and suggests that an improvement of any of the competitive or allelopathic traits will lead to increased weed suppression. Furthermore, the transfer of traits from rye to wheat via triticale is promising as it may broaden the weed suppressive abilities of wheat.