Loss mechanisms and performance optimization of alfalfa seed harvesting with negative-pressure airflow adsorption

Alfalfa seed harvesting suffers from severe seed shattering, splashing and carry-back losses due to the weak pod–stem attachment and the poor adaptability of conventional full-feed headers. To address these problems, a mechanical–pneumatic hybrid pre-harvest threshing header was developed, featuring a transversely arranged stripping drum and a negative-pressure throat airflow for enhanced seed adsorption. A full-plant flexible DEM model of alfalfa was established and calibrated, and a bidirectional CFD-DEM coupling approach was applied to reveal the seed–flow–mechanism interactions. The loss mechanisms of shattering, splashing, unstripped loss and carry-back were quantitatively clarified. Single-factor simulations showed that both drum height and forward speed significantly influenced the loss characteristics, and that a drum center height of 430 mm and a forward speed of 0.8 m·s −1 effectively suppressed losses, yielding a simulated collection rate of 77.94%. A two-factor CCD field experiment further optimized the operating parameters, and the optimal combination (430 mm drum height, 0.67 m·s −1 forward speed) achieved a seed collection rate of 83.4%. The proposed mechanical–pneumatic header and the revealed loss mechanisms provide theoretical and technical support for the design of specialized alfalfa seed harvesting equipment.