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Digital design of modern agricultural equipment, which is one of the pillars of digital agriculture, is important for achieving a fully mechanized crop production system. Understanding how agricultural equipment interacts with agricultural materials is a prerequisite for designing superior agricultural equipment. Discrete element method (DEM) is a discontinuum-based numerical analysis and simulation method. Recent studies have shown that DEM is a promising method of addressing engineering problems in granular and discontinuous agricultural materials. The concept, development stages, and common software packages of the DEM were introduced. The determination of DEM contact models used in modeling agricultural soil and agricultural materials, including seed, biomass, feed, fertilizer, fruit and vegetables, were summarized and their parameter calibration processes were highlighted. It provided a state-of-the-art review of various DEM applications to improve crop farming systems such as soil tillage, seeding and planting, fertilizing and harvesting. The challenges faced by modeling and design engineers using DEM in modern agricultural equipment research and development were discussed and possibilities for improvements were proposed. The limiting factors of using DEM for wider and broader applications were computational intensity and the lack of an effective parameter determination method. Further fundamental studies on contact model and parameter calibration as well as the development of standardized simulation packages and methods needed to be prioritized. With advances in computing power and numerical algorithms for efficient computation, the DEM would emerge from the development stage and became a robust and cost-effective tool of equipment and process design and optimization in the agri-food industry.