After germination, 50 uniform barnyard grass seedlings were transferred onto a holed plate of polystyrene foam and grown for 10 days as described above. The medium in the container was collected and filtered through filter paper (No. 2), and the filtrate was loaded onto a column of synthetic polystyrene adsorbent (Diaion HP20). The column was first eluted with water to remove inorganic compounds such as nutrient components, and then eluted with methanol. The methanol elution was evaporated to dryness, dissolved in 5 mL methanol and added to a sheet of filter paper (No. 2; 15 cm × 10 cm) in incubation container (15 cm × 10 cm × 10 (height) cm). Methanol was evaporated in a draft chamber. Hoagland nutrient solution (1 L) was added into the container, and uniform 50 rice seedlings were then transferred onto a polystyrene foam plate as described above. Final concentrations of barnyard grass root exudate in the medium were 0, 10, 30, 100, 200 and 300 mg/L. Root exudates of 50 barnyard grass seedlings produced approximately 200 mg. After 10 days incubation, rice seedlings were harvested for barnyard grass bioassay and momilactone B determination was preformed.