Cultivars and lines used in the cultivar experiments were mostly from upland environments, and are adapted to aerobic soil conditions. The doubled haploid lines (DHLs) were developed from a cross between the upland cultivar ‘Azucena’ and the irrigated lowland cultivar ‘IR64’ (Huang et al., 1997). The introgression lines of IR64 were developed by marker-assisted backcrossing to insert QTLs for improved root traits from Azucena (Shen et al., 2001). Dr. B. Courtois, IRRI, provided all seed.