The impact of intercrop design on weed suppression of species mixtures: A model-based exploration

Intercropping has frequently been reported to enhance weed suppression. A recent study combining a plant competition model and empirical data demonstrated that improved weed suppression results from a so-called selection effect, whereby the more weed suppressive crop species contributes disproportionate to the weed suppressive ability of intercrops. Here, we build on this finding and used the plant competition model to explore how species composition, mixing ratio, planting density and spatial arrangement influence the weed suppressive ability of annual intercropping systems. Analysis identified species composition as the principal design factor, since a difference in weed suppressive ability between crop species appeared the prime driver responsible for the above-average weed suppression of intercrops: the larger this difference the stronger the effect. With greatly differing levels of weed suppressive ability between crop species, even a small proportion of the stronger suppressive species greatly enhanced the intercrop’s ability to suppress weeds. In such a situation, mixing ratio can thus be used to regulate the trade-off between weed suppressiveness and the risk of the less competitive crop species being overgrown. Plant density was found to be a useful modulator if crop species displayed similar levels of weed suppression. In this case, intercrops in additive design were the only option to enhance weed suppression. Proximity of component species proved a prerequisite for superior weed suppressiveness. Consequently, in strip cropping systems, the improved weed suppressive ability rapidly declined with wider strips. The acquired quantitative insights form a theoretical foundation for considering weed suppression when designing multifunctional annual intercropping systems.