The development of high-yielding rice varieties has led to the green revolution in a number of rice producing countries. Before 1950, farmers in tropical climate areas of Asia planted mostly traditional indica varieties and yields from farmers' fields rarely exceeded 2.5 tonnes/ha. Varietal improvement activities, especially since 1950, have increased the yield potential of rice varieties. The first high-yielding variety, IRS, was developed in 1966. During the 1966 dry season at the experimental farm of the International Rice Research Institute (IRRI), Los Banos, the Philippines, IRS yielded 10.3 tonnes/ha (De Datta, 1981). Tests carried out on IRS at rice experiment stations across tropical Asia indicated yields of 4-8 tonnes/ha (Chandler, 1979). H owever, evidence demonstrates that factors other than rice varieties have also contributed greatly to the success of rice production during the green revolution. Duwayri, Tran and Nguyen (1999) reported that rapid yield growth in Asia was obtained only d uring the 1980s, regardless of the release of IRS in 1968 and series of other high-yielding rice varieties by IRRI and national research systems in Asia during the 1970s.
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