Advances in Low-Damage Threshing Technologies for High-Moisture Maize: Damage Mechanisms and Threshing Boundary Regulation

Under high-moisture conditions, maize threshing faces a conflict between effective kernel detachment and mechanical damage due to changes in kernel properties, kernel–cob connections, and structural constraints within the ear. This review examines the mechanical mechanisms of kernel detachment and damage formation, evaluates existing damage assessment methods, and summarizes advances in low-damage threshing technologies. Based on the relationship between the load required for kernel detachment and the damage threshold of kernels, existing technologies are classified into three categories: mechanical action optimization, contact-load regulation, and coordinated regulation of material state and threshing boundaries. The review discusses the effects of threshing-device configurations, operating parameters, flexible contact structures, and pretreatment methods on material motion and load transfer. Existing studies demonstrate that structural and parameter optimization can modify kernel loading conditions, whereas kernel dispersion, ear loosening, and cob fragmentation may alter internal structural constraints. However, experimental validation of pretreatment-assisted threshing remains limited, particularly regarding total kernel damage, threshing completeness, and adaptability across cultivars and moisture conditions. This review identifies the need to coordinate material-state regulation with mechanical threshing and provides a framework for evaluating low-damage threshing technologies under high-moisture conditions.