More than half of the world's population lives in Asia where only 30% of the world's 1,346 million ha arable land are found (Table 1, FAO 1994). To produce enough food for the rapidly growing population, agricultural land use in Asia has become very intense. Cereal production in Asia makes up half of the world's total. The most important crop is rice,followed by wheat and maize. Double cropping of rice makes up most of the 55% of the harvested irrigated rice area (IRRI 1993). Rice- wheat is a basic rotation practiced on 21 million HA (woodhead et al., 1994). The area cropped to a rice-maize rotation is smaller than that devoted to rice - wheat, but it is rapidly increasing. Moreover, the rate of conversion of prime rice land for industrial and residential use continues at an alarming rate. In China, Indonesia and the Philippines, this trend threatens self-sufficiency in cereal production.