Nitrogen Fertilizer Sources and Timing before Flooding Dry-Seeded, Delayed-Flood Rice

Urea is the primary N source used for the large preflood N application in delayed-flood rice (Oryza sativa L.) production in the southern United States. Urea is prone to substantial NH3 volatilization losses, however, if fields are not flooded quickly. Most delayed-flood rice fields require 5 to 10 d to flood. Consequently, a study was conducted to evaluate the use of less NH3-volatde N sources for the preflood N application. The objectives were to evaluate the NH3 volatilization loss and impact on N uptake and rice yield when urea, urea plus the urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT), (NH4)(2)SO4, or a urea-(NH4)(2)SO4 (UAS) blend were applied preflood and a flood established 1, 5, or 10 d after N application. When flooding was delayed for 5 or 10 d after N application, NH3 volatilization was the least for urea + NBPT (2-10%) and (NH4)(2)SO4 (4-5%) and they produced the highest rice N uptake and yield. The UAS blend that had NH3 volatilization losses (11-15%) at 5 and 10 d after application that were intermediate between urea (17-24%) and (NH4)(2)SO4 or urea + NBPT also had N uptake and grain yield intermediate between these N sources. Urea should only be used if similar to 2 d are required to flood a field. If 3 to 5 d are required to flood a field, then UAS has some merits but it is not as consistent as (NH4)(2)SO4 or urea + NBPT. When >5 d are required to flood, (NH4)(2)SO4 or urea + NBPT should be used.