Supplementing Fertilizer Nitrogen Application to Irrigated Wheat at Maximum Tillering Stage Using Chlorophyll Meter and Optical Sensor

In the Indo-Gangetic plain in South Asia, fertilizer nitrogen (N) application to irrigated wheat in two split doses at planting and at crown root initiation (CRI) stages of the crop is the general recommendation. Farmers have a tendency to apply another dose of N at maximum tillering (MT) stage to avoid the risk of N deficiency. However, appropriate criteria to decide MT stage application of N are lacking. Field experiments were conducted for three seasons with four wheat cultivars to develop criteria for using a chlorophyll (SPAD) meter and an optical sensor (GreenSeekerTM) for guiding MT stage N application. Application of 0, 60, 80, 100 and 120 kg N ha−1 in two equal split doses at planting and at CRI stage constituted the main plots. The two subplots consisted of applying 0 and 30 kg N ha−1 at MT stage immediately after taking SPAD meter and GreenSeeker readings. Greenness of wheat leaves at MT stage was found to be a function of N applied at planting and CRI stages. Wheat grain yield at maturity was determined by the level of greenness of leaves at MT stage—whether measured by SPAD meter or GreenSeeker (expressed as in-season estimate of yield (INSEY) computed by dividing normalized difference vegetation index by number of days between planting and sensing). Grain yield response to N application at MT regressed against SPAD readings at MT revealed that an application of 30 kg N ha−1 will increase wheat yield by 1.0 or 0.5 t ha−1 when color of the wheat leaves was equivalent to or less than SPAD value of 32.5 or 42.5, respectively. Robust regression between grain yield response and INSEY values revealed that application of 30 kg N ha−1 at MT will lead to a grain yield increase by 1.0 or 0.5 t ha−1 if INSEY values were found to be 0.005 or 0.011, respectively. It should serve as an easy and useful criterion for applying a supplementary dose of fertilizer N at MT stage of wheat.