Rice Aboveground Nitrogen Content, Development, and Yield as Affected by Nitrogen Fertilization and Flood Timing

Delaying fertilizer N and flooding delays maturity of drilled, delayed flood rice. Delaying the permanent flood may increase uptake of native soil N in some soils. The optimal preflood N and flood timing window is narrowest on the most N‐deficient soil. Urea fertilizer is typically applied near the five‐leaf stage to rice ( Oryza sativa L.) grown in a dry‐seeded, delayed‐flood production system. How long the preflood N can be delayed without adversely effecting yield is poorly understood. We examined the effects of delaying preflood N application and flooding on rice growth, aboveground N content, heading date, yield components, and grain yield. Trials were established on silt loam soils at the Pine Tree Research Station (PTRS) and Rice Research and Extension Center (RREC) during 2015 and 2016 and included up to five cultivars. Urea was applied at 0, 45, 90, 135, and 180 kg N ha −1 on five to seven different dates with applications beginning near the three‐leaf stage and ranging from 127 to 1035 growing degree units (GDU) among five site‐years (PTRS‐2015a, PTRS‐2015b, PTRS‐2016, RREC‐2015, RREC‐2016). Aboveground N content increased by 25 to 59 kg N ha −1 as N fertilization and flooding were delayed at PTRS‐2015a, PTRS‐2016, and RREC‐2016 due to increased uptake of soil N. Preflood N fertilization and flooding time had no effect on grain yield at the RREC. At the PTRS, relative grain yield of rice receiving no fertilizer N increased from 20 to 41% of the maximum yield as fertilization and flooding were delayed. Maximal relative yield (91%) was produced when fertilization and flooding were performed between 164 and 531 GDU and declined to 62% by 1035 GDU at PTRS. Delaying preflood N fertilization beyond 531 GDU may reduce grain yield on some silt loam soils.