Abstract: A foreign object detector comprises an inertial sensor mounted for displacement with one of the displaceable rolls in the feed mechanism of a forage harvester. When a foreign object is forced through the rolls, the acceleration of the rolls' displacement exceeds a predetermined threshold and the detector stops the feed mechanism. Preferably, the detector's sensor is a microswitch having an inertially-actuable lever and which is mounted for displacement with the upper feed roll. More preferably, the detector's circuitry ties into existing metal detector circuitry for reducing cost and complexity, while being easily retrofitted into existing forage harvesters.