Mid-season adjustment of nitrogen fertilizer for rice with two plant spacings

Abstract

ContextFertilizer recommendations for rice (Oryza sativa L.) in Asia often split a predetermined amount of N into several applications to match N supply with anticipated crop demand for N. Within-season deviations from anticipated N demand could necessitate mid-season adjustments in N applications to ensure closure of exploitable yield gaps. ObjectiveWe hypothesized that application of additional fertilizer N to satisfy an underestimated crop demand for N could be delayed until two weeks after panicle initiation without a yield penalty. We additionally examined whether effects of mid-season N adjustments on rice differed between two plant spacings with different plant populations. MethodsField experiments with irrigated, transplanted rice were conducted at two sites in two seasons in North Sumatra, Indonesia. The experimental design was a factorial combination of two rice cultivars, two plant spacings, and seven N treatments. ResultsRice growth and yield components sometimes differed between equally spaced rows (square planting) and alternating narrow and wide rows (legowo), but grain yield with legowo consistently matched and sometimes exceeded yield with square planting. Effects of N treatments — with varying rates and split applications of N — on yield were comparable across the two plant spacings and two cultivars. A predetermined fertilizer recommendation with three split applications of N underestimated the N needed for high yield in all experiments. Application of 30 kg ha−1 more N than the recommended 155 kg ha−1 in dry season (DS) and 125 kg ha−1 in wet season (WS) increased yield by up to 0.5 Mg ha−1 in DS and 0.7 Mg ha−1 in WS and consistently increased net income in both seasons. Additional N at tillering in DS increased vegetative biomass but not yield. Additional N in both seasons could be delayed until 12–14 days after panicle initiation for highest yield. ConclusionsOur study represented circumstances where a fertilizer recommendation provided to farmers before a growing season merited an upward mid-season adjustment in N rate to achieve higher yield. The upward mid-season N adjustment of 30 kg ha−1 in our study could be delayed until two weeks after panicle initiation with no yield penalty in both seasons, for both cultivars, and for both equally spaced rows and alternating narrow and wide rows of transplanted rice. ImplicationsFarmers and advisory services using crop and/or weather monitoring to identify fields meriting additional mid-season N can delay the application of additional N until panicle initiation or two weeks after panicle initiation.