Abstract: The intensity and transformation of intra- and inter-specific competitions in mixed sowing grassland is affected by the relative density of mixed sowing grass species. From June to September 2010, a replacement series experiment was conducted to establish an Avena sativaVicia villosa mixed sowing grassland in the upper reaches of Shiyang River. Seven treatments with the relative density of A. sativa and V. villosa being 1∶0 (KY), 8∶2 (A), 6∶4 (B), 5∶5 (C), 4∶6 (D), 2∶8 (E), and 0∶1 (KM) were designed to study the changes of the inter-specific competition between annual grass species in mixed sowing grassland under effects of the grasses densities. Affected by the relative density of A. sativa and V. villosa, the intra- and inter-specific competitions of the grass species in different phonological periods were transformed in different degrees. In all mixed sowing treatments, the relative yield of A. sativa (RY_y) increased with its growth, while the relative yield of V. villosa (RY_m) decreased with its growth in treatments A, B, and C but increased in treatments D and E. At the seeding and tillering stages of A. sativa, the two grass species were in a hostile relationship (RYT<1) in all mixed sowing treatments except A, but at later growth stages of A. sativa, the relationship transformed to symbiotic (RYT>1) in all mixed sowing treatments, and the competition of A. sativa was stronger than that of V. villosa (RCC_y>1,RCC_m<1). The competition in all mixed sowing treatments was beneficial to A. sativa (AG<1) in its whole growth period. It was suggested that the intra-and inter-specific competitions in mixed sowing grassland in each phonological period showed an obvious density-dependent phenomenon, realizing the cooperative use of resources between the mixed sowing grass species.

Key words: maize plant type, soybean, dry matter accumulation, yield.