Weed communities and their negative impact on rice yield in no-input paddy fields in the northern part of Japan

Although weeds cause serious yield losses in agriculture, relatively few studies have been conducted on the relationship between weeds and yield loss in no-input rice production systems, where no herbicides are used. This study investigated weed communities in rice paddy fields and their influence on rice yield, by comparing 16 no-input paddy fields in northern Japan. The weed seed community in the paddy field soils, before transplanting, were analyzed using a spontaneous germination test, and the established weed communities in the fields were examined at the full heading growth stage of the rice. The weed community structures showed a marked difference between the seeds in the soil and the established weed communities. Although eight weed species germinated from the soil, only four weed species were observed in the fields at the full heading stage of the rice. The two dominant weed species, Monochoria vaginalis and Cyperaceae, accounted for 84% of the total weed biomass in the field. The multiple linear regression analysis revealed that during the period of no-input cultivation, soil NO3 concentrations, air temperature, and sunlight hours had significant effects on the biomass of M. vaginalis, but not on Cyperaceae. Furthermore, the two dominant weed species had different impact on rice yield; Cyperaceae had a large negative effect on rice yield, whereas M. vaginalis did not. These results indicated that weed management in no-input paddies should be directed towards the control of specific weed species such as Cyperaceae sp., rather than the entire weed community.