A mass flow sensor apparatus (30) for a billet harvester(10) for sugar cane of the type having an elevator (20) with a plurality of flights (24) that transfer batches of billets (5) through an elevator housing and having a floor section (23) proximate an upper end (22) of the elevator housing, the mass flow sensor apparatus including: a weigh pad (31) for receiving batches of billets (5); a first transducer means (34) operatively associated with the weigh pad for producing mass signals (39) indicative of a mass received by the weigh pad and error correction mechanism (37, 38) for correction of measurement errors in the mass signals; whereby, in use, the weigh pad (31) is substantially co-planar with the floor section (23) in the elevator housing such that batches of billets (5) traverse the weigh pad and the error correction mechanism (37) is operatively associated with the crop harvester (10). A yield mapping system employing DGPS positioning techniques with the mass flow rate sensor is also disclosed.