Early sowing increases nitrogen use efficiency of sunflower in wet clay soils

Abstract

Early sowing increases yield potential of sunflower on wet, saline clay soils but the effects on nitrogen (N) requirements have not been determined. In a 2-year study on a drainable field with wet, saline clay soils in southwestern coastal Bangladesh, we investigated nitrogen use efficiency (NUE) and yield responses to N rates (0, 60, 90, 120, 150, 180 and 210 kg ha−1) for zero-till sunflower cv. Hysun-33 sown from November 30 (early) to December 30 (late). Early sowing produced highest seed yield (3.59 ton ha−1) and oil yield (1400 kg ha−1) at 150 kg N ha−1. Delayed sowing (December 15–December 30) reduced seed and oil yields by 4%–15% and 7%–18%, respectively. In late sowing, crops suffered from multiple stresses (soil–water deficit, salinity, and heat), which decreased yields. The N fertilizer requirement in late sowing was higher (180 kg ha−1) to achieve maximum yield, yet the yield was lower than from earlier sowing. Compared with nil N supply, the rates of 60–150 kg N ha−1 increased seed and oil yields of the early-sown sunflower by 1.3–3.3 times and 1.2–2.9 times, respectively, whereas the rates of 180–210 kg N ha−1 decreased seed and oil yields by 2%–5% and 7%–14%, respectively. Higher net profit (US$764) was also achieved from earlier sowing with 150 kg N ha−1. We conclude that under wet and saline clay soils, higher sunflower yield in early sowing is attributable to lower stress that increased NUE and decreased N requirement.