Effects of cereal-legume intercrop system design on weed suppression through selection and canopy dynamics

Cereal-legume intercrops offer notable benefits in weed suppression compared to the respective sole crops. Complementarity, where crop species collectively utilize more resources than when grown separately, and facilitation, where crops positively influence each other, have been proposed as mechanisms for this enhanced weed suppression. But research so far 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, is insufficiently understood, while this is essential to design intercrops for weed suppression. This study evaluates the extent to which selection drives weed suppression in various cereal-legume combinations and intercrop designs, and examines if intercrop behavior 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. We found that in general, a cereal is a better weed suppressor than a legume. Weed biomass was lower in most intercrops compared to the averages of the sole crops, demonstrating effective weed suppression. The harmonic mean, a predictive model that accounts for selection, corresponded much more to observed intercrop weed biomass than the arithmetic mean, which averages sole crop weed biomasses. Enhanced weed suppression was absent in systems where component crops had similar competitiveness or where wide row spacing limited interaction between species. Together these findings support the prominent role of selection. Canopy cover was primarily influenced by cereals, the more competitive crop. We showed that selection is the dominant mechanism for weed suppression across diverse cereal-legume intercrop systems, and demonstrated the impact on weed suppression of intercrop design, in particular species composition, mixing ratio, and spatial design of the intercrop.