Abstract
We investigated the mechanism of weed control by carp and the effect in growth and yield of paddy rice. Carp ate the soil with food materials and as a results carp mixed and agitated the surface of the soil. By the agitation, roots of the weeds were cut so that the weeds died. And more carp seemed to eat directly a part of weeds. We suggested that weeds production was inhibited by reduction of light penetration into soil surface. Because the water of paddy field was muddied by the agitation as the relative illuminance reduced by about 50% of non-carp plot. The deep flood irrigation for carp suppressed weed growth. Weed control was effective when carp were introduced in the field at early season (10-20 days after transplanting), long carp (10-15cm) were used and much more carp (>0.5 carp m^<-2>) were input. Compared to plot of non-carp, soil temperature was lower (about-1℃), and the number of stem decreased during the early season in the carp plot. However, there was not a difference in fruitful culms between both plots. Although the number of grain per head and of spiklets per ground increased in the carp plot, the grain yield decreased by reduction of percentage of ripened grain. We concluded that a discharge of carp could be used as effective weed control in paddy field, if the several conditions were prepared.
