2 charts; 10 graphs; 4 tables; 8 ref. Summary (En) AIBA-SEARCA, College, Laguna 4031 - Philippines

The rainfed lowland ricelands prone to waterlogging in the prerice season are a major potential area for growing green manure (GM) crops tolerant of saturated soils. But year-to-year variability in GM growth and N accumulation would be large due to widely varying field hydrology in the dry-to-wet transition months. A simulation model of the GM-rainfed rice system was developed to estimate the yields and relative stability of prerice GMs and the subsequent rice crops at three representative sites in Southeast Asia. Average estimated N yield and yield stability over 26 yr were higher at Los Banos, Laguna, Philippines, than at Ubon, Thailand, or Tuguegarao, Philippines. The effects of planting date on sesbania N accumulation and following rice yields were compared: the late-planted (at 100 mm cumulative rainfall) crop accumulated more N than did the early- planted (at 50 mm rainfall) one because of higher soil moisture. Yields of following rice were similar for both sesbania planting dates, indicating that late planting of GM would not reduce rice yields. Optimum N rates for rice were also similar for both planting dates; consequently, more purchased N would be needed for rice following early-planted sesbania. The costs per unit of green manure N were higher than those of urea N when full wage rates and intensive tillage were assumed. However, with zero tillage and/or a modest cumulative soil fertility benefit, the cost of green manure N would be similar to or lower than that of urea N. The implications of these findings for future research are discussed

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