The green revolution increased considerably the yield and production volume of wheat and rice from 1966 to 1982 as shown in Table 1. Each of these individual grain's yield has been increasing annually by 2.7% for wheat and 2.2% for rice, over the last thirty years. However, limitations can be seen recently on the increase of yield. The world average increase ratios of yield and grain production volume in the eighties fell sharply compared with the late seventies and also with the 1966-74 period as shown in Table 1. Although, the (theoretical) potential yield of new varieties of rice and wheat is clearly greater compared to the ordinary varieties, it is sometimes lower in experimental fields and farmers' fields. 13) Yield of new high-yield rice varieties at trial research at the International Rice Research Institute (IRRI), and national rice research centers and at farmers' fields in Asian countries, have recently been static or reducing. 14) There is a problem of excessive use of paddy fields as symbolized by triple rice harvests per year in Asia. The stagnation in yield is seen in the double cropping of rice and wheat, and also a fall in factor productivity of wheat is seen. 15)
Let's see the movement of crops' yield and production in China which is a very significant agricultural country on the globe. According to the FAO data, grain yield has increased substantially after the war from 1.9 tons per hectare in 1961 to 4.5 tons in 1994. Although grain yield has undoubtedly increased, I thought the yield in 1994 was too high. Surprisingly, it became clear from a recent investigation of the Chinese Science Academy that statistics of cultivated area in China are 40% less than the actual amount. If we re-calculate the yield in 1994 with this actual area, it is about 3.2 tons. Even with the revised yield, it is at the same level as the average grain yield in Japan, America and Europe in the same year, which is very high. Water shortage is a serious short-term and long-term problem in China. The prices of such agricultural materials, as chemical fertilizers have started to increase rapidly from the early nineties, and will remain at high level in the future. Agricultural research investment has stagnated. 16) Consequently a continuous increase in the crops yield will be difficult in the long term. Superior farm land has been rapidly converted to nonagricultural uses because of the extremely rapid
