Efficient and flexible management of nitrogen for rainfed lowland rice

Abstract

Nitrogen (N) is the most limiting nutrient in the rainfed lowland rice soils of Laos. Indigenous N supply of these soils was low, ranging from 12 to 64 kg N/ha and was correlated with soil organic matter content. Resource-poor farmers and erratic rainfall are characteristic features of Lao rainfed lowland rice systems. Such climatic and economic factors influence farmers' ability to apply N at the ‘recommended’ time and therefore efficient and flexible recommendations are required. Research on N management focused on the timing of N applications. Splitting the N recommendation into three equal splits at transplanting, active tillering and panicle initiation increased yields by 12% compared to a single application at transplanting. Agronomic efficiency (AE = kg increase in grain yield/kg N applied) was further increased by 9 kg/kg N if a higher proportion of the N was applied during active tillering and panicle initiation when crop N demand is high. Under conditions of suboptimal N supply, the first N application can be applied from transplanting to 30 d after transplanting without lowering grain yield or AE (for medium duration varieties transplanted 1 month after sowing). The last N application can be made between two weeks before to one week after panicle initiation without lowering yield. These findings provide the basis for an efficient (AE of 20 to 25 kg/kg N) and flexible N management strategy for Lao rainfed lowland rice under conditions of suboptimal N supply.