This study was conducted to determine the effects of seeding rate and harvest stage of barley as a companion crop on the hay yield and number of red clover and weeds. Red clover was established with barley (150, 300, 450, 600 seed m(-2)) and cut at the flag leaf emerged or milk dough stage of barley. Also, red clover was sown alone and harvested at 50% flowering stage. Two separate field experiments were conducted. Experiment 1 was conducted in 2004 - 2007; Experiment 2 was conducted in 2005 - 2008. In both experiments, hay yield increased with addition of barley. The suppression of companion crop varied depending on the weeds. As barley efficiently suppressed Matricaria sp., Sinapis arvensis L. and Veronica sp., red clover seedling losses were not great. In this way, the effects of companion crop continued during the 3 years. When Alopecurus myosuroides Huds. and Vicia sp. presented densely, barley was not able to compete sufficiently; ultimately, much patchier area occurred. At the end of the 3 years, the highest total yield was obtained that red clover was sown with barley at the seeding rate of 450 seed m(-2) in Experiment 1, and 600 seed m(-2) in Experiment 2 (36.69 and 31.90 t ha(-1), respectively) and harvested at the milk dough stage of barley for hay in the establishment year.