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An optical sensor-based fertilizer nitrogen (N) management strategy that relies on visible and near-infrared spectral response from plant canopies was evaluated for irrigated wheat in the northwestern India. GreenSeeker™ optical sensor-guided fertilizer N dose, computed from an estimate of potential yield and response index, takes into account both the temporal and field-to-field variabilities and is applied only once after measuring in-season spectral response from the crop canopy. Seven field experiments were conducted in four wheat seasons to decide whether to apply the optical sensor-guided fertilizer N dose at 2nd or 3rd irrigation stage and to work out the appropriate N management before applying it. Robust relationships between in-season sensor-based estimates of yield and actual wheat yields were observed both at 2nd (R 2 = 0.64) and 3rd (R 2 = 0.86) irrigation stages of the crop. GreenSeeker-based fertilizer N management resulted in high yield levels and high N-use efficiency. Application of 30 kg N ha−1 at planting and 45 kg N ha−1 at 1st irrigation was found to be the appropriate N management before applying the GreenSeeker-guided dose at 2nd irrigation stage. Grain yields obtained by following sensor-guided N management were at par with those observed with the blanket recommendation of 120 kg N ha−1, but with greater recovery efficiency (by 6.7–16.2%) and agronomic efficiency [by 4.7–9.4 kg grain (kg N applied)−1]. The major outcome of this study was that applying a moderate amount of fertilizer N at planting and enough fertilizer N to meet the high N demand during the period between crown root initiation stage and maximum tillering stage before applying a sensor-guided fertilizer N dose at 2nd irrigation stage results not only in high yields but also in high fertilizer N-use efficiency in irrigated wheat.

Application of fertilizer N doses to rice (Oryza sativa L.) whenever leaf greenness as measured by chlorophyll meter (SPAD) and LCC falls below a fixed threshold is being extensively used for site-specific management of fertilizer N. As fixed threshold greenness varies with regions, rice cultivars and seasons, a dynamic threshold greenness concept in the form of SPAD-based sufficiency index has been introduced. For rice, the dynamic threshold greenness is defined as 90 % of the SPAD meter reading of leaves in an Nrich or over fertilized strip. In the present investigation, four field experiments with six cultivars were conducted at two locations to test fertilizer N management following two variants of the LCC-based dynamic threshold greenness concept—the leaf greenness one or 0.5 unit less than the LCC reading of the Nrich strip (LCC-Nrich), vis-a-vis SPAD meter-based sufficiency index approach and blanket recommendation of 120 kg N ha−1 applied in three equal split doses. The fertilizer N management following threshold greenness of one unit less than the LCC-Nrich resulted in total N application of only 60 kg ha−1 and thus sub-optimum rice grain yields. The grain yield of rice, N uptake and fertilizer N use efficiency measured with N management following threshold leaf greenness of 0.5 units less than LCC-Nrich were at par with the SPAD meter-based 90 % sufficiency index approach and were better than the blanket fertilizer recommendation in the region. The LCC-based dynamic threshold greenness strategy holds promise to efficiently manage fertilizer N rice as it can effectively take care of variations in the rice cultivars, locations and seasons.

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 R2 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−1 at transplanting and 45 kg N ha−1 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)−1]. 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.

In the quest of enhancing nitrogen (N) use efficiency in irrigated transplanted rice beyond that observed with blanket recommendation, leaf colour chart (LCC) is being used to apply N whenever colour of the first fully opened leaf from the top is less green than a critical colour shade. So as to avoid frequent (every 7–10 days) monitoring of leaf colour, criteria were developed to apply fertilizer N at critical growth stages of rice but by adjusting the dose of N as per colour of the leaf measured with LCC. A series of experiments were carried out at Ludhiana and Gurdaspur locations from 2007 to 2010 with treatments refined progressively to work out appropriate combination of fixed and adjustable rates of fertilizer N at critical stages of transplanted rice. A dose of 30 kg N ha −1 at transplanting as prescriptive N management proved to be adequate for achieving high yields of rice. Corrective N management consisting of adjustable N doses was worked out as application of 45, 30 or 0 kg N ha −1 depending upon leaf colour to be −1 only if leaf colour is less green than LCC shade 4 at initiation of flowering. A combination of these prescriptive and corrective N management strategies resulted in optimum rice grain yield and high N use efficiency with less fertilizer N application than the blanket recommendation. For some rice cultivars, particularly in years with favorable climate, fixed date adjustable dose N management produced yield levels higher than those achieved by applying blanket recommendation for fertilizer N and resulted in agronomic efficiency higher than 25 kg grain/kg N.

Blanket fertilizer nitrogen (N) recommendations for large irrigated wheat tracts lead to low N-use efficiency due to field-to-field variability in soil N supply and seasonal variability in yield. To achieve high N use efficiency, a site-specific N management strategy using GreenSeeker™ optical sensor was evaluated. We conducted seven field experiments during 2004–2006 at three locations to define relationships between in-season sensor measurements and yield of wheat and to know whether response of wheat to fertilizer N can be estimated by sensor measurements. During 2005–2007, four field experiments were conducted to assess the sensor-based N management strategy and to work out prescriptive N management to be followed prior to applying sensor-guided fertilizer dose. We observed robust relationships between in-season sensor-based estimates of yield at Feekes 5–6 and 7–8 stages and actual wheat yields. Response of wheat to fertilizer N defined by the sensor was highly correlated with harvest response index. Sensor-guided fertilizer N applications resulted in high yield levels and high N-use efficiency. Application of 90 kg N ha-1 at planting or in two equal doses at planting and crown root initiation stage was the appropriate prescriptive fertilizer N management. This study reveals that high N-use efficiency in irrigated wheat can be achieved by replacing blanket fertilizer recommendation by an optical sensor-based N management strategy consisting of applying moderate amount of fertilizer N at planting and crown root initiation stages and sensor-guided fertilizer N dose at Feekes 5–6 or 7–8 stages of wheat.

Large field to field variability restricts efficient fertilizer N management when broad based blanket recommendations are used in maize (Zea mays L.). To achieve higher yields and to avoid nitrogen (N) deficiency risks, many farmers apply fertilizer N in excess of crop requirement in maize. Field experiments were conducted for five years (2005–2009) to establish and evaluate threshold leaf colour to guide in-season need based fertilizer N topdressings in four maize genotypes. Colour (of the first top maize leaf with fully exposed collar) as measured by comparison with different shades of green colour on a leaf colour chart (LCC) and maize grain yield was significantly correlated. The Cate–Nelson plot of chlorophyll (SPAD) meter/leaf colour chart values against relative grain yield of 0.93 for the experiments conducted during first two years indicated that LCC shade 5 during vegetative growth stages and LCC shade 5.5 at silking stage (R1) can guide crop demand driven N applications in maize. Evaluation of the established threshold leaf greenness during the next three years revealed that fertilizer N management using LCC 5 starting from six-leaf (V6) stage to before R1 stage resulted in improved agronomic and N recovery efficiency in different maize genotypes. There was no response to fertilizer N application at R1 stage. The study revealed that in maize, fertilizer N can be more efficiently managed by applying fertilizer N dose based on leaf colour as measured by LCC than blanket recommendation.