Abstract
The effects of organic fertilization (cow and chicken manure) and pesticide application on the yield of rice (cultivar Nipponbare) were examined for ten years from 1990 to 1999 in the paddy field of Okayama University. On average, the yield on organic fertilized and pesticide-free plot (only barnyard grasses were eradicated .by hand) was 10% lower than that of chemical fertilized and pesticide applied plot because of the decease in the number of panicles. This reduction might be due to the competition with weeds especially monochoria for nutrients. Although infection of sheath blight was observed, this disease did not cause a serious damage to the rice yield. In late Sept. in 1990, 1991 and 1997 the hopper burn by the brown planthopper occurred in the pesticide-free plot. However, it brought no reduction in yield because hopper burn occurred just before the harvesting. Yield-reduction may be diminished by using the early-maturing cultivar. Close correlation was observed between population densities of leafhoppers and spiders, the major predators, and it was considered that the increase in the density of spiders did not affect the survival rates of leafhoppers. Yearly fertilization of manure increased the total carbon and nitrogen concentration of the soil, i.e., soil fertility. In this experiment, rice was grown by pesticide-free organic cultivation with only about 10% yield-reduction. A possibility of cultural and biological control of weeds and insects was discussed.
