A drive assembly for an agricultural machine. A power take off (PTO) output shaft is connected to a driving motor for driving an attachment. A drive transmission is positioned between and interconnects the driving motor and the support wheels of the agricultural machine. An input device is connected to an electronic control unit. A sensor senses the torque transferred between an attachment and the PTO output shaft, and an actuator is configured to change the transmission ratio of the drive transmission, thereby setting the propulsion speed of the agricultural machine such that the torque transferred between the PTO output shaft and an attachment does not exceed a pre-determined value. The electronic control unit further receives data from the input device and calculates a pre-determined value based on the data received, and thereafter controls the actuator accordingly.