Allelopathic activity and momilactone B concentration in rice seedlings increased in the presence of barnyard grass seedlings (Fig. 1). These increases were probably caused not only by nutrient competition between two species, but also by chemical constituents in barnyard grass root exudates (Figs. 2–5). As momilactone B possesses strong phytotoxic and allelopathic activities (Kato-Noguchi et al., 2002, 2008, 2010), the elevated production and secretion of momilactone B of rice may provide a competitive advantage for root establishment through local suppression of pathogens and inhibition of the growth of competing plant species including barnyard grass.