Weed‐suppressive ability and species dominance in intercropping: A meta‐regression

Abstract

In intercropping, weed biomass is usually lower than the average of the respective monocultures. Recent studies suggest that enhanced weed suppression in intercrops results from a selection effect, whereby the stronger weed‐suppressive species contributes disproportionately to overall suppression. Next to improving weed suppression, its dominance may also result in the weaker suppressive species being outcompeted. However, this hypothesis remains untested. Here, we analysed data from 47 experiments reporting weed and crop data in annual bi‐specific intercrops and pure stands to test whether species with stronger weed‐suppressive ability outperform their companions in intercrops. Weed biomass in pure stands was used to identify the stronger (s) and weaker (w) weed suppressor. For both species, the ratio of realized to expected yield in the intercrop (i.e. the net effect ratio, NER) was calculated. The competitive ratio CRsw (=NERs/NERw) was then calculated to assess whether the stronger weed suppressor outperformed its companion in the intercrop. We assessed species performance and explored how their weed suppressiveness related to dominance in intercrops. We also examined the influence of plant density, spatial arrangement and species combination. The stronger weed suppressors showed a competitive advantage over the weaker ones in intercrops (CRsw = 1.33), with their dominance becoming more pronounced as the difference in weed‐suppressive abilities between the pure stands increased. This dominance was evident in mixed configurations but not in alternate row or strip configurations. In both replacement and additive intercrops, the stronger weed suppressor consistently dominated the weaker species. Furthermore, the dominance of the stronger weed suppressor was found in small‐grain cereal/legume systems, but not in maize/legume or other intercrop systems. Synthesis. Our findings provide further evidence for the selection effect being the key mechanism explaining why intercrops suppress weeds more than the average of their pure stands. While the dominance of a stronger weed suppressor enhances weed suppression, it may also suppress the weaker species, potentially compromising overall production. These results highlight the need to design intercrops that balance weed control and individual crop yield. Combining species with complementary traits and managing spatial arrangement and plant density can help optimize this balance.