Effects of cereal-legume intercrop system design on weed suppression

Cereal-legume intercrops offer notable benefits in weed suppression. Complementarity, where crop species collectively utilize more resources than separately, and facilitation, where crops positively influence each other, have been proposed as mechanisms for enhanced weed suppression. Recent research points to selection, the phenomenon where the more suppressive crop species disproportionally acquires resources making the intercrop resemble this crop species, as the key mechanism. However, the requirements under which enhanced weed suppression occurs, and the level of expression of selection in different intercrop systems, needs to be further investigated to enable the design of weed suppressive intercrops. This study evaluates whether selection drives weed suppression in various cereal-legume combinations and intercrop designs and examines if intercrop behaviour concerning canopy cover aligns with such a selection effect. We conducted four field experiments over three years (2022–2024), focusing on crop species composition, mixing ratio, and spatial design. Cereals were generally stronger weed suppressors than legumes. In most intercrops weed biomass was lower than the average weed biomass of the sole crops. Just 25% cereal in the mixture achieved weed suppression comparable to that of sole crop cereals. A prediction model which accounted for the selection effect more accurately predicted intercrop weed biomass than the average of sole crop weed biomasses. Species choice and spatial design influenced weed suppression and the role of selection, as a difference in competitiveness between component crops and narrow row spacing promoting species interaction were required for the expression of a selection effect, resulting in enhanced weed suppression. These findings underscore the dominant role of selection in enhancing weed suppression of intercrops. Canopy cover was primarily influenced by cereals. Our findings confirm that selection is the dominant mechanism for weed suppression across diverse cereal-legume intercrop systems and demonstrated the impact of intercrop design on weed suppression.