of dwarfing genes in tall cultivars led to great increases in rice yield. The release and widespread adoption of short-du­ration, high-yielding, semidwarf cultivars triggered invest­ment in irrigation infrastructure and allowed more farmers to grow two or three rice crops each year. Tillage and in­tense management increased and soil remained submerged longer. Inorganic fertilizer and pesticide use increased, but the diversity of rice cultivars in the irrigated systems shrank. Higher yields resulted from the combination of increased yield of modern cultivars compared with the landraces they replaced. Improved crop nutrition was made possible by fertilizer application and improved plant resistance and pest management to minimize losses from weeds, insects and diseases (Cassman and Pingali, 1995). In the irrigated lowlands of Asia, which accounted for 75% of the world's rice production, average yield increased from 2 to 3 tonnes ha-1 in the 1950s to 5.3 tonnes ha-1 at the turn of the last century.
     Although the quest to further increase the potential yield of inbred rice after the release of IR8 was not as suc-

cessful as hoped, many new cultivars were better adapted to different environments and contributed to better nutri­tional quality and human health (Peng et al., 1999; Peng and Khush, 2003). Considerable progress was made, for example, in managing major rice diseases, such as bacte­rial blight, blast and tungro, through genetic improvement. The reason few disease outbreaks occurred in the past 15 years was the result of collaborative research between in­ternational research institutions and the national extension organizations in many developing countries.  Combining resistance to insect pests with ecological crop management principles (Heong et al., 1995) greatly reduced the incidence and effect of pest outbreaks (IRRI, 2006).
     Modern rice cultivars with origins in breeding research in the 1960s covered about 75% of Asian riceland. In ir­rigated areas, that proportion was often 80 to 100%. In Bangladesh, for example, 46 new cultivars developed from 1970 to 2001 had spread to 80% of the dry season irri­gated rice area by the early 2000s. These modern cultivars have contributed to a 2.3% growth in rice yield each year