Abstract: Crop processing apparatus comprising crop processing means (18, 23, 24) driven by drive means (29) and an automatic control system comprising sensor means (50, 51) for sensing transmission loss in the drive means (29) and producing a corresponding transmission loss signal which is proportional to the load exerted by crop on the crop processing means, setting means (Sr) for selecting a desired level of crop load and producing a reference crop load signal (Sr) corresponding to a selected level of transmission loss, signal processing means (13) which compares the transmission loss signal with the reference crop load signal to produce a control signal (Se), and actuator means (14) that is controlled in accordance with said control signal and is effective to vary the rate of feed crop to the processing means to maintain a substantially uniform load of crop in the crop processing means. The transmission loss takes the form of relative rotation or slip between input drive and output driven shafts (31, 33).