Managing tillage, sowing rate and nitrogen top-dressing level to sustain rice yield in a low-input, direct-sown, rice - vetch cropping system

Direct sowing of rice into drained soil is a relatively new farming practice, replacing transplanting, being developed for use in External Low Input Sustainable Agriculture in Korea. Field experiments were conducted to investigate the importance of tillage, sowing rate, and top-dressing of fertiliser nitrogen (N) to rice growth and grain yield in a direct-sown, rice–vetch cropping system. From 1993 to 1997, Chinese milk vetch (Astragalus sinicus L. Vetch) was grown during the winter season (October–May) at Uiryong, Korea. Until 1995 transplanted rice (Oryza sativa L.) was grown during the summer season, but in 1996 and 1997 the direct-sowing method was used. In 1996, the treatments applied to rice were either tillage or no-tillage of the paddy before the rice crop was sown, combined with 2 rates (0 or 2 g/m2) of fertiliser N top-dressed onto the paddy just after the panicle initiation stage. In 1997, the treatments were either tillage or no-tillage of the paddy before the rice crop, combined with 2 rice sowing rates (400 or 800 seeds/m2), and 3 rates (0, 2 or 4 g/m2) of fertiliser N top-dressed just after the panicle initiation stage. Rice yielded 243–435 g/m2 in 1996 and 493–678 g/m2 in 1997, the lower yield in 1996 was attributed to fewer established seedlings and productive panicles. There was no effect of tillage treatment. Across years, yield was correlated with shoot N content, and grain N-use efficiency exceeded 50 g grain/g shoot N content. Rice residues contained about 5 g N/m2 and vetch residues released 6–8 g N/m2 during the 4 weeks after submergence, therefore the release of N from residues was potentially sufficient to supply the rice crop demand. In both years, however, the yield of unfertilised rice was N limited, so increasing the shoot N content appeared to be the most important issue to be resolved in sustaining yield in External Low Input Sustainable Agriculture. A low rate of top-dressed fertiliser N (2–4 g N/m2) increased yield by increasing the number of panicles and spikelets, and to a lesser extent, 1000-grain weight. Growing rice using no-tillage practices did not limit the yield of rice, except in 1996 when no fertiliser N was applied and the N uptake by rice was low. A high sowing rate combined with top-dressed fertiliser N in 1997 reduced yield through crop lodging and an increase in the number of unproductive tillers. This problem may be overcome by the use of a variety less susceptible to lodging.