Water and Nitrogen (N) Use Efficiency of Upland Rice (Oryza sativa L. × Oryza glaberrima Steud) under Varying N Application Rates
Upland rice (Oryza sativa L.) production systems in sub-Saharan Africa are faced with challenges of water stress and nitrogen (N) deficiency, which reduce grain yield, water use efficiency (WUE), and nitrogen use efficiency (NUE). The objective of the study was to determine the response of upland rice to N fertilizer rates under well-watered conditions and to clarify the relationships between WUE and NUE. Upland rice variety Nerica 10 was grown under well-watered conditions and varying N rates in 2014/2015 (Y1) and 2015/2016 (Y2) on the same field at the University of Pretoria’s Hatfield Experimental Farm, South Africa. Yields at harvest increased with increasing N rates, linearly in Y1, and following a quadratic trend in Y2. Mean grain yield was highest (4.5 t ha−1) at 120 kg N ha−1 and lowest (2.4 t ha−1) at 0 kg N ha−1. Agronomic NUE was lowest for 160 kg N ha−1 (11.7 kg kg−1 N), while WUE was highest for the 120 kg N ha−1 (7.58 ± 1.7 kg mm−1) and lowest for 0 kg N ha−1 (4.1 ± 0.9 kg mm−1). Findings revealed that at high N levels, compensative N uptake during tillering can reduce harvest index, WUE, and to a lesser extent, grain N concentration.