The Ag Leader Technology cotton yield monitor system utilizes multiple optical sensors to measure cotton volumetric flow rate in the chutes which convey cotton from the picking rotors to the basket of a cotton picker. Volumetric flow rates from at least two rows of the cotton picker are measured and are communicated to a PF3000 yield monitor unit in the cab of the picker over a serial communication bus. The PF3000 stores accumulated cotton volume for both fields and individual loads within fields, and combines these volumetric values with accumulated harvest areas, weight calibration values, and estimated percent lint turnout, to calculate total weights and average yields for fields and loads. When coupled to a GPS receiver, the PF3000 can also record data on a PCMCIA memory card for producing yield maps with desktop PC software.