Apparatus for controlling the speed of a combine harvester (10) comprises one or more sensors (48) each generating one or more signals (28, 36) that correspond during use of the combine harvester to variables of a harvesting process; a processor; and one or more control devices operatively connected in a feedback arrangement comprising two loops in a first of which a first said signal (28) corresponding to the ground speed of the combine harvester (10) is fed back as an input to a first said control device (42) that is capable of adjusting the said ground speed relative to a set value; and in a second of which the said first signal (28) is fed together with a second said signal (34) indicative of the flow rate of crop through the combine harvester to a second said control device (32) that generates as a third signal (31) the said set value, the third signal being input to the first said control device (42). The combine harvester has a sieve equipped with a sieve sensor generating a signal, which is input to the processor.
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