A rotary cutter unit for an agricultural mower, comprising a housing (14) that carries a rotary bearing (18) having an axis of rotation, and a rotor assembly that is supported by the bearing for rotation relative to the housing about the axis of rotation, the rotor assembly including a shaft (24) that extends through the bearing, wherein the housing includes a sleeve (14a) having a first end and a second end, a first flange (14b) at the first end of the sleeve and a second flange (48) at the second end of the sleeve, wherein the first and second flanges extend radially outwards from the sleeve to define a channel on an outer surface of the sleeve between the first and second flanges. A frangible element (40) is provided between the drive input element (28) and the drive output element (32), 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 (40) breaks but permits rotation of the drive output element relative to the housing. The retention mechanism includes a retention element that extends substantially radially between the drive output element and the housing to restrict axial movement between the drive output element and the housing
