In the Muda irrigation area, about 24% of yield increase per unit area was attained during the 14 years since the beginning of the double cropping practice through the introduction of higher-yielding varieties and the application of a large amount of fertilizer. However, after double cropping spread to the whole area, the yield became extremely unstable mainly due to the insufficient supply of irrigation water and severe occurrence of rice tungro disease. Consequently the yields in 1982 decreased to the levels recorded in the initial year of double cropping.
It was considered that the severe occurrence of rice tungro disease was caused by yearround cultivation mainly resulting from the insufficient supply of irrigation water. To overcome the yield instability a new double cropping system incorporating a complete fallow period to be implemented over the whole Muda area during the dry season was proposed.