Abstract
Since the year 1973, each of several herbicides commonly used in rice have been applied annually on paddy field in order to find out the effect on weed succession in early-season rice culture. The results of successive application for five years are reported here.
Weed population became specific to the applied herbicides depending upon their selectivities. In some plots where herbicides effective only for annual weeds were applied successively, some tolerant weed species were increased. In other plots where herbicides, effective for both annual and perennial weeds, were applied successively, little change in the weed population was observed.
These results suggest that the herbicides which are less effective for some weed species should be restricted in successive application for two or three years. In the case of further successive application, we should pay attention to give other weeding methods for remaining weeeds.
