SOYBEAN YIELD VARIABILITY OF DRAINAGE AND SUBIRRIGATION SYSTEMS IN A CLAYPAN SOIL

Claypan soils with less than 1% slope are poorly drained because of an argillic claypan layer 45 to 60 cm below the soil surface. Field research was conducted near Bethel, Missouri, to evaluate soybean (Glycine max [L.] Merr.) grain yields and plant populations above subsurface drain tile lines and 3.1 m distances from the tile lines of laterals installed at 6.1 and 12.2 m wide spacings for drainage (DO) or drainage plus subirrigation (DSI). The site was arranged as a split-plot design with four replications. In some years, sub-sub-plots included multiple cultivars or fungicide/insecticide management systems. This resulted in 30 year-cultivar-management (YCM) treatments from 2002 to 2015. Averaged over all of the 30 YCM systems, the highest yields (4,050 kg/ha) were observed above the 6.1 m DSI drainage tile line. Subsurface drainage tile spacings (6.1 and 12.2 m) and distances from the tile lines for DO or DSI yielded 11% to 21% greater than the ND control. Due to extreme weather events among YCM systems, data were separated into low (LYE, < 3,360 kg/ha) and high (HYE, > 3,360 kg/ha) yield environments. In LYEs, yields were more variable above the tile line and generally decreased as the distance from the subsurface tile lines increased for DSI, but yields were greater and more variable between the tile lines for DO. In HYEs, yields were greatest and more variable between the 6.1 or 12.2 m spaced DO treatments, while yields were greatest above the drain tiles with lower variability compared to between the tile lines with DSI. A narrower drain tile spacing may be needed to reduce yield variability in LYEs, but this was less evident in HYEs.