Rice Grain Yield and Nitrogen Uptake as Affected by Urea Amendment and Rainfall Timing

Nitrogen loss in the delayed-flood method of rice (Oryza sativa L.) production is minimized by applying urea to a dry soil, use of a urease inhibitor, and immediate flooding. The time required to flood fields is oft en a factor limiting fertilizer-N recovery. Our objectives were to compare the effects of simulated rainfall timing, N-(n-butyl) thiophosphoric triamide (NBPT) rate, and nitrapyrin (NP) rate on rice aboveground-N content and grain yield. Field experiments evaluated rice growth as affected by NBPT rate (0 and 0.89 g NBPT kg(-1) urea), NP rate (0 and 572 g NP ha(-1)), and three simulated rainfall timings (no simulated rainfall [NOSR], simulated rainfall before nitrogen [SRBN], and simulated rainfall aft er nitrogen [SRAN]). Yield was unaffected by simulated rainfall timing when rice was fertilized with NBPT-treated urea (7904-8264 kg ha(-1)). When rice was fertilized with untreated urea, grain yields were greater with NOSR than with SRAN or SRBN. Within each simulated rainfall timing, rice yields were 6.9 to 21.3% greater when NBPT-treated urea was applied. Nitrapyrin rate had no effect on yield in 2013, but, compared to untreated urea, NP-treated urea decreased yield by 5.6% in 2014. Application of urea to moist soil followed by dry conditions or dry soil followed by rainfall are field environments that result in lower yields presumably to fertilizer N loss than when urea is applied to a dry soil that remains dry until rice is flooded. Use of urea amended with NBPT maximized grain yield in each simulated rainfall scenario.