Abstract
A field experiment was conducted to determine the optimum over-seeding rate of Italian ryegrass （IR） among four levels 0.7, 1.0, 1.5 and 2.5 g/m2） on cool-season production of giant stargrass （GS） stand in south-western Okinawa island, Japan. The effect of over-seeding IR was positive only at the second cutting among three defoliation, due to the lowest air temperature at 18.3℃ during the regrowth period. Cumulative dry matter yield among three cuttings tended to be higher at IR seeding rate of 0.7g/m2 than GS pure stand. The yield of IR increased with the increasing over-seeding rates, while that of GS decreased. Relative yield （RY） of GS and IR, determined by yields of mixed grown GS and IR relative to those of pure GS and IR stands, respectively, was over 0.5 only in one species, however relative yield total （RYT）, determined by sum of RY of both species, was over 1.0 only at IR over-seeding of 0.7 g/m2 in all three cutting times. The interspecific competition was causes of limiting growth of the species in mix culture. From these results, it is concluded that IR over-seeding at 0.7 g/m2 to GS stand might be effective for getting higher yield in cool-season in south-western Okinawa island, Japan.
