Site-specific fertilizer nitrogen management in irrigated transplanted rice ( Oryza sativa) using an optical sensor.

Blanket fertilizer nitrogen (N) recommendations for large irrigated transplanted rice tracts lead to low N use-efficiency (NUE) due to field-to-field variability in soil N supply and seasonal variability in yield. To achieve high NUE, a fertilizer N management strategy based on visible and near-infrared spectral response from plant canopies using a GreenSeeker™ optical sensor was evaluated. Seven field experiments were conducted during 2005-2007 at two locations in the Indo-Gangetic plains of South Asia to define relationships between in-season sensor measurements at panicle initiation (PI) stage and up to 2 weeks later, and yield of rice. During 2006-2010, seven field experiments were conducted to assess the sensor-based N management strategy and to work out the prescriptive N management to be followed prior to applying sensor-guided fertilizer dose. During 2010 and 2011, the sensor- based N management strategy was evaluated versus farmers' fertilizer practice at 19 on-farm locations. Relationships with R values 0.51 (n = 131), 0.45 (n = 74) and 0.49 (n = 131), respectively, were observed between in-season sensor-based estimates of yield at 42 (PI stage), 49 and 56 days after transplanting of rice and actual grain yield of rice. Applications of 30 kg N ha at transplanting and 45 kg N ha at active tillering stage were found to be the appropriate prescriptive strategy before applying the GreenSeeker-guided dose at PI stage. Sensor-guided N management resulted in similar grain yields as the blanket rate farmer practice, but with reduced N rates, i.e. greater recovery efficiency (by 5.5-21.7 %) and agronomic efficiency [by 4.7-11.7 kg grain (kg N applied)]. This study revealed that high yields coupled with high NUE in transplanted rice can be achieved by replacing blanket fertilizer recommendation by an optical sensor-based N management strategy consisting of applying a moderate amount of fertilizer N at transplanting and enough fertilizer N to meet the high N demand during the period between active tillering and PI before applying a sensor-guided fertilizer N dose at PI stage of rice. [ABSTRACT FROM AUTHOR]