Grain yields. Four field runs (4 by 3=12 m; planter width was 3 m) were considered during corn planting within each plot. Grain corn yield was geo-referenced and determined in all plots with an 8 rows combine harvester equipped with a DGPS (CR 960 New Holland) and an impulse type based yield monitor. The 8 rows (6-m wide) of each plot were harvested in one pass and yield records were obtained at about 1.35 m intervals. The yield in the 6-m centre of plots 1 and 2 was recorded, whereas the entire width of each plot in Zone 2 could be harvested in one combine pass (Fig. 2).