Effect of Delaying the Flood and Preflood Nitrogen Application on Rice Nitrogen Uptake and Yield.

Urea is typically applied at the 5-leaf stage of rice (Oryza sativa L.) grown in a delayed-flood cultural system in Arkansas. The preflood urea application is sometimes delayed due to moist soil conditions caused by untimely rain and growers must decide whether to wait for the soil to dry or apply urea to moist soil. Our objective was to determine the effects of delaying preflood fertilizer-N application and flooding past the 3-leaf stage on rice grain yield. Trials were established at two locations (PTRS and RREC) during 2015 using five different varieties. This presentation will focus on the responses of ‘Roy J’ rice. Urea-N (0, 45, 90, 135, and 180 kg N/ha) was applied weekly for 6 weeks after the 3-5 leaf stage. Tiller number, aboveground-N content, and grain yield data for each site and cultivar were regressed across thermal time (heat unit accumulation after emergence, DD10s) and allowed to depend on fertilizer-N rate.

Aboveground-N content was a quadratic function of time with only the intercept depending on urea-N rate.  The predicted aboveground-N content peaked when urea was applied 896 and 377 DD10s at PTRS and RREC, respectively. At PTRS, grain yield was a quadratic response to fertilization time and was characterized by a uniform quadratic coefficient with each urea-N rate having unique intercept and linear slope coefficients.  At RREC, grain yield was a quadratic function of preflood-N application time and described by common linear and quadratic coefficients but different intercept terms. Maximal yield was produced when 180 kg N/ha was applied at 555 DD10’s for PTRS and 434 DD10’s at RREC.  At PTRS, the optimal cumulative DD10s for preflood-N application increased as the urea-N rate decreased.  At both locations, maximal yield was produced when fertilizer-N and flooding occurred later than the recommended time (200-300 DD10).