Rice seedlings were incubated with (mix-incubation) or without (mono-incubation) barnyard grass seedlings for 10 days, and allelopathic activity of rice extracts was determined by barnyard grass bioassay (Fig. 1A). The extracts of mono-incubated rice inhibited the growth of barnyard grass roots and shoots by 15 and 12%, respectively, and the extracts of mixed-incubated rice inhibited the growth of barnyard grass roots and shoots by 79 and 75%, respectively. Thus, the mixed-incubation induced 5.3–6.3-fold increases in allelopathic activity in the rice extracts against barnyard grass roots and shoots, respectively.