An agricultural machine includes a plant processing apparatus for processing plant material to improve the biodegradability or the digestibility of the plant material. The plant processing apparatus comprises a cylindrical drum (10) that is rotatable about a drum axis (X), a rotary drive (12) for rotating the drum about the drum axis, and a curved shell (20) that extends around at least part of the circumference of the drum. A processing channel (22) is provided between an outer surface of the cylindrical drum and an inner surface of the shell, said processing channel having an inlet end (22a) and an outlet end (22b). In use plant material is transported through the processing channel from the inlet end to the outlet end by rotation of the cylindrical drum. The drum (10) has on its outer surface a first set of working elements (16) and the shell (20) has on its inner surface a second set of working elements (30), said first and second sets of working elements being configured to process the plant material as it is fed through the processing channel.
