Variable speed drive for chaff mill of combine harvester

A residue processing system for an agricultural harvester, said residue processing system comprising:

• a chaff mill for processing a stream of chaff containing weed seeds;
• and
• a drive system for powering the chaff mill, wherein the drive system comprises a belt-driven gearbox having (i) an input shaft that either directly or indirectly receives power from an engine or other power source of the agricultural harvester, and (ii) an output shaft that is either directly or indirectly connected to a rotor of the chaff mill for rotating the rotor,
• wherein the belt-driven gearbox is configured to selectively rotate the output shaft at at least two different constant speeds such that the rotor of the chaff mill can be rotated at at least two different constant speeds, wherein the gearbox has a first gear or gear set having a first drive ratio and a second gear or gear set having a second drive ratio that differs from the first drive ratio,
• wherein the chaff mill is a weed seed mill that is configured to devitalize the weed seeds in the stream of chaff,
• wherein the weed seed mill comprises the rotor and a stator surrounding the rotor, and the rotor is configured to rotate with respect to the stator for damaging and devitalizing the weed seeds passing between the rotor and stator.