Effects of cereal-legume intercrop system design on weed suppression

Cereal-legume intercrops offer benefits in weed suppression. Complementarity (enhanced resource-use efficiency) and facilitation (positive crop-crop interactions) have been proposed as mechanisms for enhanced weed suppression. However, recent research points to selection, the phenomenon where the more suppressive crop species disproportionately acquires resources making the intercrop resemble this crop species, as the key mechanism. Yet, the conditions under which enhanced weed suppression occurs, and the expression of selection across intercrop designs, require further study to inform the design of weed-suppressive intercrops. This study evaluated whether selection drives weed suppression in cereal-legume intercrops and examined if intercrop canopy cover aligned with a selection effect. We conducted four field experiments over three years (2022-2024), focusing on crop species composition, mixing ratio, and spatial design. Crop species with faster canopy development showed stronger weed suppression, and intercrop canopy cover resembled the stronger weed-suppressive cereal species more than the weaker weed-suppressive legume species. Triticale–faba bean intercrops suppressed weeds better than faba bean sole crops, while this benefit occurred with even relatively small fractions of triticale. Close proximity of component species was another prerequisite, as weed suppression benefits were larger in systems with narrower row distances or within-row mixing designs. Additionally, a prediction model accounting for the selection effect accurately predicted intercrop weed biomass based on the pure stand weed biomass of the component crop species. Our findings establish selection as the dominant mechanism of weed suppression in cereal–legume intercrops and demonstrate how both crop species composition and intercrop configuration influence this effect.