A rotary cutter unit (6) for an agricultural mower (2) comprises a housing (14) that carries a rotary bearing (18) having an axis of rotation (X), and a rotor assembly (20) that is supported by the bearing for rotation relative to the housing about the axis of rotation. The rotor assembly includes a shaft (24) that extends through the bearing, a drive input element (28) at an input end of the shaft for transmitting rotary drive to the shaft and a drive output element (32) at an output end (22) of the shaft to receive rotary drive from the shaft. A frangible element (40) comprising a weakened portion of the shaft (24) is provided between the drive input element and the drive output element, which is configured to break when subjected to an excessive load so as to interrupt the transmission of rotary drive between the drive input element and the drive output element. A retention mechanism (44) is provided, which prevents separation of the drive output element from the housing when the frangible element breaks but permits rotation of the drive output element relative to the housing. The retention mechanism includes a retention element (46, 48) that extends substantially radially between the drive output element (32) and the housing (14) to restrict axial movement between the drive output element and the housing.
