Trait-Based Mechanical Dispersal of Weed Seeds by Combine Harvesters in Rice–Wheat Continuous Cropping Systems

Mechanized agriculture has created important pathways for weed seed dispersal; yet, the influence of seed morphological and phenological traits on harvester-mediated dispersal remains poorly understood. Field surveys and simulated dispersal experiments were conducted to investigate the factors affecting weed seed dispersal in rice–wheat cropping systems. Field investigations across 60 fields identified 54 weed species retained on combine harvesters after rice and wheat harvesting, with Poaceae species dominating the retained seed assemblage. Seed retention was strongly associated with plant height, seed size, appendage type, and phenological synchrony with crops. Species with a tall stature, large seeds, and synchronized maturity were disproportionately retained by harvesters. Cluster analysis classified weed species into four mechanical dispersal types according to their dispersal potential. Simulated dispersal experiments demonstrated that seed deposition followed an exponential decay pattern with increasing harvesting distance. Seeds possessing rough surfaces or appendages showed lower attenuation coefficients and longer dispersal distances than smooth seeds. Harvester-mediated dispersal distances exceeded 2 km during continuous field operation. These findings demonstrate that combine harvesters act not only as dispersal vectors but also as selective filters favoring weed species with specific trait combinations. Integrating harvest weed seed control technologies with machinery hygiene practices will therefore be essential for sustainable weed management.