A forage harvester comprises feeder means (20,21,26,27) for feeding harvested crop material to processing means along a crop feed path. It is equipped with a foreign object detector (58) for signalling the presence of foreign objects in said harvested crop. The detector means comprises a sensor (60) having a detection field extending into at least a portion of said path. The sensor (60) is substantially symmetric with respect to a direction transverse to said crop feed path and the detector (58) further comprises restriction means (86) distinct from said sensor (60), making said detection field asymmetric with respect to said transverse direction. As a result, the detector becomes less sensitive to noise caused by the other components of the harvester.
