A wheel mounted grain harvester includes a harvesting head for depositing grain in a grain handling assembly comprised of a plurality of grain moving parts. Control means are located on the harvester for selectively interrupting grain flow along the grain moving parts causing harvested grain from a new separate row segment to temporarily accumulate. Means are provided for transporting the harvested grain from separate row segments into separate collection bins permit the separate evaluation of the harvested grain in each row segment. Means are provided for moving the harvester along the row at a constant rate of speed to avoid the necessity of stopping the harvester between row segments to effect the separate evaluation of the harvested grain. The selective interruption by the control means is accomplished by either interrupting at least one of the grain moving parts or by selectively closing a movable blocking wall mounted on the harvester.