AGRONOMIC AND ECONOMIC-EVALUATION OF SESBANIA-ROSTRATA GREEN MANURE ESTABLISHMENT IN IRRIGATED RICE

Research on crop establishment methods may improve green manure perfor mance, reduce costs, and increase the adaptability of pre-rice green m anure technology in lowland rice-based cropping systems. A two-season field experiment was conducted at the international Rice Research Inst itute (IRRI) in Los Banos, Philippines in 1991-1992 to compare four es tablishment practices of Sesbania rostrata green manure (zero tillage, with tillage, relay cropping in rice for 2 or 4 weeks) with four mine ral N fertilizer levels (0, 30, 60, and 90 kg urea N/ha) in an intensi ve irrigated lowland system with three rice crops per year. S. rostrat a was grown twice a year during the 43-day dry-wet and wet-dry transit ion periods between the wet and dry season rice crops. Grain yield pot ential and fertilizer responsiveness of rice was generally highest in the dry season. On the other hand, S. rostrata growth was more vigorou s in the wet season (long-day period) than in the dry season, regardle ss of establishment method. Green manure N accumulation was lowest wit h zero tillage (30 and 90 kg N/ha in dry and wet season, respectively) and highest when it was relay-cropped for two weeks (60 and 180 kg N/ ha in dry and wet season, respectively). Land preparation for Sesbania ensured best green manure stand (> 100 plants/m(2) vs 20-40 plants/m( 2) at no-till establishments) but increased costs of green manuring by US$I6/ha compared with other establishment methods. A quadratic respo nse function between mineral fertilizer equivalence and green manure N indicated that up to 75 kg N/ha, lowland rice uses green manure N mor e efficiently than urea. Depending on season and establishment method, S. rostrata substituted for 35 to 90 kg of split-applied urea N. Bene fit-cost ratios indicated that pre-rice green manure use in the wet se ason under the current fertilizer and labor prices in the Philippines was a less attractive economic option than mineral N fertilizer. This was true for all establishment methods. In the dry season, S. rostrata established by relay cropping gave the highest rate of return. The 2- week relay cropping of green manure with irrigated rice gave highest g reen manure N accumulation and rice grain yield, and may be economical ly viable where fertilizer prices are higher or labor costs are lower than in the Philippines.