The shredder is the main and most energy-consuming unit of any forage harvester. Its design determines the technological scheme of the machine and the location of its main joints. Pull-type and self-propelled forage harvesters are equipped with disc and drum shredders, which have become widespread. Beater-knife cutters are installed on balers. In general, the work which is done on grinding the grass mass includes grinding (compression, cutting, friction), transportation of leaf mass, as well as the cost of idle motion of the working parts. A characteristic feature of the bither-knife apparatus under the study is the use of active disc knives, which cut a portion of the leaf mass. Therefore, the determination of the moment of friction forces on the side surfaces of the disk knife of the shredder is important for an objective assessment of the energy density of the process of grinding the grass mass. The object of the study was the process of transporting the cut portion of the grass mass with the fingers of the feed rotor through the disc knives in the forming channel of the biter-knife shredder. The purpose of the work was to establish the analytical dependences of the moment and the work on the friction forces of the grass mass on the side surface of the disk knife of the biter-knife shredder. The task of the work was: to analyze researches and publications on cutting the material (soil, plant debris, plant stems) with a disk knife and their interaction during the process; to determine the area of the part of the disc knife that is in direct contact with the grass mass; to establish the total force of action of the cut layer of the stem mass and the analytical dependences of the torque and work on the force of friction of the grass mass on the side surface of the disk knife of the shredder. The research methodology was based on the analysis of the interaction of the cut portion of the grass mass with the side surface of the disc knives in the formation of channels of the grinding apparatus using the methods of theoretical mechanics. In the work were established the direction of the absolute vector of speed and friction force of the grass mass on the side surface of the disk knife; was determined the area of the surface of the blade disk bounded by the bottom of the shredder and the blade of the knife and the area (6) of the part of the disk knife that is in direct contact with the grass; was established the total force (7) of action of the cut layer of the stem mass. The analytical dependences of the torque (9) and work (10) on the friction force of the grass mass on the side surface of the disc knife of the shredder are given. It is established that the coefficient of friction of the movement while harvesting lucerne for soliage with a humidity of 45-55% depending on the speed og rotation and pressure on the side surface of the knife takes the magnitude within the limits of fд = 0,25 – 0,55.