Fallow weeds can be abundant in rice paddies without any inputs and provide ecosystem services like those of cover crops, such as reducing nitrogen (N) leaching and capturing carbon. Therefore, allowing fallow weeds to grow is a potential alternative to cover crops in rice cropping systems. To evaluate the feasibility of this strategy, the effect of fallow weeds on grain yield in rice needs to be clarified. In this study, 2-year field experiments were conducted to compare N uptake, biomass production, yield components, and grain yield in rice with and without application of fallow weeds (500 g...