In-season estimation of yield and nitrogen management in irrigated wheat using a hand-held optical sensor in the Indo-Gangetic plains of South Asia

Bijay-Singh¹, R.K. Sharma², Jaspreet Kaur¹, M.L. Jat³, Yadvinder Singh¹, Varinderpal Singh¹, Parvesh Chandna⁴, O.P. Choudhary¹, R.K Gupta¹, HS Thind¹, Jagmohan Singh¹, H.S. Uppal¹, H.S. Khurana¹, Ajay Kumar¹, R.K. Uppal¹, Monika Vashistha¹ and Raj Gupta⁵

Department of Soils, Punjab Agricultural University, Ludhiana, India, ² Directorate of Wheat Research, Karnal, India, ³ PDCSR, Modipuram, India, and  ⁴Rice-Wheat Consortium-CIMMYT, NASC Complex, Pusa, New Delhi, India

Corresponding author Email: BijaySingh20@hotmail.com

Abstract

To account for large field-to-field variability of soil N supply and year-to-year variability in yield which restricts efficient use of N fertilizer when broad-based blanket recommendations are followed in irrigated wheat in the northwestern Indo-Gangetic plain, different combinations of prescriptive and field specific corrective N management strategies were tried.  Hand-held GreenSeeker^TM  optical sensor was used to work out corrective fertilizer N doses  based on expected yields as well as achievable greenness of the leaves. As per nitrogen fertilizer optimization algorithm for using the optical sensor, relationships of in-season estimate of yield (INSEY) defined as NDVI/day with potential yield (YP₀) of  wheat were developed using data from multi-location and multi-year field experiments. For relationships of the type YP₀=a*(INSEY)^b, R² values were 0.61 and 0.76 at Feekes 5-6 and Feekes 7-8 stages of wheat, respectively.  On the day of fertilizer N application using GreenSeeker, ratio of NDVI in a N-rich strip and the test plot (response index) was multiplied with YP₀ to calculate grain yield that can be achieved by applying fertilizer N.   Difference in N uptake between predicted yields with and without N fertilizer application allowed calculating the corrective dose of fertilizer N to be applied.  Application of at least 90 kg N ha^-1 at planting resulted in wheat yields equivalent to those recorded with blanket fertilizer N recommendation provided these were supplemented with application of corrective N dose at Feekes 5-6 or 7-8 stages.  Similarly, application of 40 or 50 kg N ha^-1 both at planting and at crown root initiation stage followed by optical sensor guided N application at Feekes 7-8 stage worked out to be the best strategy to obtain high yields as well as high N use efficiency. These studies suggest that GreenSeeker optical sensor can be an important tool for efficient management of fertilizer N in irrigated wheat in the Indo-Gangetic plains of South Asia.

Introduction

With decreasing profit margins and increasing awareness regarding non-point source pollution, it is imperative that N management in wheat be further improved.  Traditionally, farmers in the Indo-Gangetic plains of South Asia and elsewhere apply nitrogen uniformly as a blanket recommendation for large regions in wheat growing tracts. Many farmers often use uniform rates of N fertilizers based on expected yields (yield goal) that could be inconsistent from field-to-field and year-to-year depending on factors that are difficult to predict prior to fertilizer application.  Large temporal and field to field variability of soil N supply restricts efficient use of N fertilizer when broad-based blanket recommendations are used (Adhikari et al., 1999, Dobermann et al., 2003).  Under such situations, real-time N management can effectively replace the blanket fertilizer N recommendations for achieving high N use efficiency.  Also, many a times, ignorant farmers apply fertilizer N in doses much higher than the blanket recommendations to ensure high crop yields.  Over application of N in cereal crops is known to reduce fertilizer use efficiency.  It can be taken care of by using real-time N management.

Application of  fertilizer N that corresponds to the spatial variability of the N needs of the crop should not only lead to increased nitrogen use efficiency but also to reduced possibility of fertilizer N related environmental pollution (Khosla and Alley, 1999). For example, according to Kranz and Kanwar (1995) as much as 70 % of the total N leached comes from as little as 30 % of the total field area. With more than 50% or more operational land holdings in South Asia having size less than 2 hectares (remaining 30-40% up to 10 ha) (Agricultural Research Data Book, 2007), it seems that high fertilizer N use efficiency can be improved through field specific fertilizer N management that takes care of both spatial and temporal variability in soil N supply.  Successful strategies will comprise of management options based on location specific fertilizer N requirements of crops according to year-to-year variations in climate (particularly solar radiation) and spatial as well temporal variations of indigenous soil N supplies (Giller et al., 2004).  Although generally good correlations with grain yield have been observed with methods based on soil tests and laboratory analyses of tissue samples to predict cereal N needs during vegetative growth stages (Fox et al., 1989; Hong et al., 1990; Magdoff et al., 1990; Justes et al., 1997; Lemaire and Gastal, 1997), these are time consuming, cumbersome, and expensive.  And prospects remain bleak for accurate N prescriptions developed using soil tests prior to the cropping season. Tissue tests are also of less value for the support of decisions on N supplementation than indicators that are directly related to measurement of leaf and canopy greenness (Schröder et al., 2000).  

Dynamic N management requires rapid assessment of leaf N content - a sensitive indicator of changes in crop N demand during the growing season.  The chlorophyll or SPAD meter (SPAD-502, Minolta, Ramsey, NJ, USA) , and its inexpensive and simple alternative, the leaf colour chart (LCC) can quickly and reliably monitor relative greenness of leaf as an indicator of leaf N status.  These tools have helped in developing real-time N management strategies for rice (Ladha et al., 2005) but these do not take into account photosynthetic rates or the biomass production and expected yields for working out fertilizer N requirements. Application of optical sensors in agriculture is increasingly rapidly through measurement of visible and near-infrared (NIR) spectral response from plant canopies to detect N stress (Peñuelas et al., 1994; Ma et al., 1996; Raun et al., 2001). Chlorophyll contained in the palisade layer of the leaf controls much of the visible light (400-720 nm) reflectance as it absorbs between 70 and 90 percent of all incident light in the red wavelength bands (Campbell, 2002).  Reflectance of the NIR electromagnetic spectrum (720-1300 nm) depends upon structure of the mesophyll tissues which reflects as much as 60 percent of all incident NIR radiation (Campbell, 2002).  Spectral vegetation indices such as the normalized difference vegetation index (NDVI) have been shown to be useful for indirectly obtaining information such as photosynthetic efficiency, productivity potential, and potential yield (Peñuelas et al., 1994; Thenkabail et al., 2000; Ma et al., 2001; Raun et al., 2001; Báez-González et al., 2002) and  have been found to be sensitive to  leaf area index, green biomass (Peñuelas et al., 1994), and photosynthetic efficiency (Aparicio et al., 2002). Raun et al. (2001) found expected yield as determined from NDVI to show a strong relationship with actual grain yield in winter wheat.

Using NDVI measurements of wheat at different times during crop growth period, Raun et al (2001, 2002) developed concepts of response index and potential yield and these were used to define a fertilizer nitrogen algorithm for working out the fertilizer N requirement in winter wheat based on expected yields as well as achievable greenness of the leaves. Raun et al. (2002) showed that prediction of wheat response to N applications guided by optical sensor was positively correlated to measured N response and increased nitrogen use efficiency.  In the present investigation we developed relationships between NDVI measurements and yield of irrigated wheat grown in the Indo-Gangetic plains of South Asia.  Using these relations and response indices, fertilizer N doses to be applied at Feekes 5-6 or Feekes 7-8 stages of irrigated spring wheat were worked out under different scenarios of fertilizer management at planting and at crown root initiation stage.  Different combinations of prescriptive and corrective N management scenarios were evaluated vis-à-vis blanket recommendations for N in the region.  In irrigated wheat, fertilizer N application in split doses along with 2nd or 3rd irrigation events should nearly coincide with Feekes 5-6 and Feekes 7-8 stages of growth.

MATERIALS AND METHODS

Site Description

Field experiments were conducted at Ludhiana (30°56′ N, 75°52′ E), Karnal (29°42′ N, 77°02′ E) and Modipuram (29°40′ N, 77°46′ E) in the Indo-Gangetic Plain in northwestern India. The three sites have subtropical climates. Mean monthly temperature and rainfall for the sites are shown in Fig. 1.  Soils were mildly alkaline loamy sands (Typic Ustipsamment) at the Punjab Agricultural University farm, Ludhiana, mildly alkaline sandy loam (Typic Ustochrept) at the Directorate of Wheat Research farm, Karnal, and alkaline sandy loams (Typic Ustochrept) at the farms of Project Directorate for Cropping Systems Research, Modipuram.  Initial soil samples collected from each field experiment were mixed, combined by field replication, air dried, sieved, and analyzed for  some physical and chemical characteristics. The pH and electrical conductivity (H₂O, 1:2), bicarbonate-extractable (Olsen) P, exchangeable K, cation exchange capacity with ammonium acetate at pH 7, and particle size by the hydrometer method are shown in Table 1.

Experiments for developing relationships for predicting yield potential of wheat from in-season optical sensor measurements

Field experiments were conducted in three wheat seasons (2004-05 to 2006-07) at Ludhiana, Karnal and Modipuram.  During 2005-06 and 2006-07 wheat seasons, the treatments consisted of application of fertilizer nitrogen as urea at 60, 120, 180 and 240 kg N/ha applied at planting of wheat and 60, 120 and 180 kg N ha^-1 in 2 equal split doses at planting and at crown root initiation stage that occurs around 21 days after planting and coincides with first irrigation.  A no-N control plot was also maintained.  During 2004-05 wheat season, two on-going field experiments at Ludhiana and one on-going experiment at Karnal were used for generating data for developing relationships for in-season estimation of wheat yields.  In these experiments, doses of urea-N varying from 0 to 90 kg N ha^-1 were applied in two equal split doses at planting and crown root initiation stage of wheat.  During 2006-07 at Ludhiana, two experiments were conducted with zero-till sown wheat - one with rice straw mulch and the other without mulch.  Some details of the experiments such as planting date, sensing date and variety are given in Table 2.   All field experiments were laid out in a randomized complete block design with three or four replications.

During the month of January in 2005, 2006 and 2007, spectral reflectance readings were taken at the time of applying second and third irrigation to wheat crop (the first irrigation was applied at crown root initiation stage three weeks after planting of crop) coinciding with Feekes (Large, 1954) growth stages 5-6 and 7-8.  Sensing dates in different experiments are listed in Table 1. Sensor measurements were taken from treatments with varying levels of N nutrition within each replication.  Spectral reflectance expressed as NDVI was measured using a handheld GreenSeeker^TM optical sensor unit (NTech Industries Incorporation, Ukiah, CA, USA).  The unit senses a 0.6 × 0.01 m spot when held at a distance of approximately 0.6–1.0 m from the illuminated surface.  The sensed dimensions remain approximately constant over the height range of the sensor. The sensor unit has self-contained illumination in both the red [656 nm with ~25 nm full width half magnitude (FWHM)] and NIR (774 with ~25 nm FWHM) bands (http://www.ntechindustries.com/datasheets.php, confirmed on 01 September 2008). The device uses a patented technique to measure the fraction of the emitted light in the sensed area that is returned to the sensor (crop reflectance) and to calculate NDVI as:

where F_NIR and F_Red  are respectively the fractions of emitted NIR  and red radiation reflected back from the sensed area.  The sensor outputs NDVI at a rate of 10 readings per second.  The sensor was passed over the crop at a height of approximately 0.9 m above the crop canopy and oriented so that the 0.6 m sensed width was perpendicular to the row and centered over the row. With advancing stage of growth, sensor height above the ground increased proportionally. Travel velocities were at a slow walking speed of approximately 0.5 m s^-1 resulting in NDVI readings averaged over distances of <0.05 m.

            In-season estimated yield (INSEY) proposed by Raun et al. (2002) as the measure of the daily accumulated biomass from the time of planting to the day of sensing was calculated by dividing the NDVI data by the number of days from planting to sensing. The yield potential with no additional fertilization (YP₀) was calculated using an empirically derived function relating INSEY to yield potential as: YP₀=a*(INSEY)^b.

Experiments for evaluating optical sensor based N management

            In all, four field experiments were conducted to evaluate optical sensor based nitrogen management in wheat vis-à-vis blanket recommendation for the region.  During 2005-06 wheat season, an experiment was conducted at Ludhiana whereas during 2006-07 experiments were located at Modipuram, Karnal and Ludhiana. Blanket recommendations for N management in wheat in northwestern India consisting of applying half of the total dose of 120 or 150 kg N ha^-1 at planting and remaining half at crown root initiation stage coinciding with first irrigation event 3-4 wks after planting, constituted the reference treatments for evaluating the GreenSeeker based N management.   Since fertilizer N application to wheat has to coincide with an irrigation event, GreenSeeker based N management treatments were planned to work out fertilizer N applications to wheat at Feekes 5-6 or Feekes 7-8 stages with different doses of N applied as prescriptive N management at planting and at crown root initiation stages.  Also, Feekes 5-6 and Feekes 7-8 stages almost coincide with 2^nd and 3^rd irrigation events and relationships between INSEY and potential yield of wheat at these stages have been worked out.  Treatments tested in the four experiments are listed in Table 3.  Dates on which fertilizer N was applied corresponding to different growth stages of wheat are listed in Table 4. 

            In all the four experiments, a N-rich strip was established by applying 200 kg N ha^-1 in split doses to ensure that nitrogen was not limiting.  The NDVI measurements form the N rich strip (NDVI_NRICH) and the test plots (NDVI_TEST) were used to calculate response index (RI) to fertilizer N (Johnson and Raun, 2003) as:

As advocated by Raun et al. (2002) the yield of the test plot achievable by applying additional fertilizer N (YPn) was estimated as the product of YP₀ and RI. The N fertilizer algorithm to compute fertilizer N to be applied using GreenSeeker optical sensor (Raun et al., 2002) is based on determining the difference in estimated N uptake between YPn and YP₀.  It was done by estimating the mean N content of the grain at harvest (1.85% N for spring wheat grown in Indo-Gangetic plains of South Asia; in Exp. 1 a value of 1.6% was used) and multiplying this number by YPn and YP₀, respectively.  The difference in N uptake between YP₀ and YPn was then divided by efficiency factor (taken as 0.5 to be reasonably achievable under South Asian conditions) to work out the fertilizer N dose using the equation:

In this equation, YPn and YP₀ are expressed in kg ha^-1 so as to calculate fertilizer dose in kg N ha^-1. 

            The values of YP₀ used in fertilizer algorithm for computing fertilizer N doses to be applied in experiments conducted in 2005-06 and 2006-07 were based on INSEY- YP₀ relationships developed from data collected from experiments conducted up to 2004-05 and 2005-06, respectively.

Crop management

Wheat was planted in rows 20 cm apart in 16.8 to 24 m² plots on dates as indicated in Tables 2 and 4.  Prior to seeding, the land was plowed twice to about 20-cm depth and leveled. After seeding with a seed-cum-fertilizer drill, a plank was dragged over the plots to cover the seed. All P [26 kg P ha^-1 as Ca(H₂PO₄)₂] and K (25 kg K ha^-1 as KCl) were drilled below the seed at sowing.  The basal dose of N per treatment was mixed in the soil just before sowing.  In 2006-07 season at Ludhiana, two experiments were conducted for developing relationship between INSEY and YP₀ when wheat was sown after the harvest of rice crop under zero-till conditions.  In these experiments, soil was not tilled after harvesting and wheat was planted using a zero-till seed-cum-fertilizer drill.  In one of the experiments rice straw was removed while in the other 6 Mg ha^-1 rice straw was allowed to remain in the field as mulch. 

Four to five  irrigations were applied at crown root initiation stage, Feekes 5-6, Feekes 7-8, flowering/booting and grain filling stage (depending upon rainfall events and climate) using both well and canal water.  The dates of irrigation-cum-fertilizer application in four experiments conducted to evaluate GreenSeeker guided N management vis-à-vis blanket recommendation are given in Table 4.   Weeds, pests, and diseases were controlled as required.

Crops were harvested by hand at ground level at maturity on dates listed in Tables 2 and 4. Grain and straw yields were determined from an area (8-13.2 m²) located at the center of each plot.  Grains were separated from straw using a plot thresher, dried in a batch grain dryer, and weighed.  Grain moisture was determined immediately after weighing, and subsamples were dried in an oven at 65°C for 48 h.  Grain weight for wheat was expressed at 120 g kg^-1 water content.  Straw weights were expressed on oven-dry basis. 

Plant Sampling and Analysis

Grain and straw subsamples were dried at 70°C and finely ground to pass through a 0.5-mm sieve.  Nitrogen content in grain and straw was determined by digesting the samples in sulfuric acid followed by analysis for total N by a micro-Kjeldahl method (Yoshida et al., 1976).  The N in grain plus that in straw was taken as the measure of total N uptake.

Data Analysis

Analysis of variance was performed on yield parameters to determine effects of N management treatments using IRRISTAT version 5.0 (International Rice Research Institute, Philippines).  Power functions of the type YP₀=a*(INSEY)^b were fitted using MS EXCEL.

The N-use efficiency measures - recovery efficiency (RE), agronomic efficiency (AE), and physiological efficiency (PE) as described by Baligar et al. (2001) were computed as follows:

where N uptake is the total N uptake in grain and straw

RESULTS AND DISCUSSION

Predicting yield potential of wheat from in-season optical sensor measurements

Data from Karnal, Ludhiana and Modipuram generated in different years using different cultivars of wheat grown in tilled or zero-tilled soil and by applying fertilizer N levels either as whole basal or in two split doses were plotted as X-Y graph between INSEY and grain yield.  Figure 2 shows the INSEY-YP₀ relation for wheat for Feekes 5-6 stage.  With wheat planting dates ranging from 02 November to 23 November and sensing dates ranging from 02 January to 23 January, a value of R² as high as 0.61 suggest that wheat yields can be predicted fairly accurately as early Feekes 5-6 stage when first node appears on the wheat plant and second irrigation becomes due.  The relationship turned out to be even more robust (R² = 0.76) at Feekes 7-8 stage when data were available from more number of experiments than for Feekes 5-6 stage (Fig. 3).  At Feekes 7-8 stage, wheat crop demands irrigation again when a dose of fertilizer can also be applied along with it.

The concept of in-season estimated yield (INSEY) as developed by Stone et al. (1996) and Raun et al. (2002) is unique as it provides an estimate of the yield potential (YP₀) of the particular area without additional N fertilizer (i.e. what the field would yield, all factors being equal, without any additional fertilizer applied).  In fact, a robust relationship between INSEY (computed from NDVI data collected by GreenSeeker optical sensor) and yield potential constitutes the first step in determining fertilizer doses to be applied for correcting in-season N deficiencies in wheat. The INSEY-YP₀ functions ought to be unique for different geographic regions and irrigation practices. The results clearly indicate that for irrigated wheat as it is grown in the western Indo-Gangetic plains in South Asia, biomass produced per day was a reliable predictor of grain yield.  It was in spite of the fact that there exist so many uncontrollable variables (rainfall, planting date, temperature etc.) from planting to sensing having potential to adversely affect the relationships between INSEY and YP₀.  Lukina et al. (2001) showed that a single equation could be used to predict grain yield over a wide production range, diverse sites, and with differing planting and harvest dates. Although Raun et al. (2002) has shown that INSEY based on optical sensor readings collected once anytime between Feekes growth stages 4 to 6 of winter wheat was an excellent predictor of yield, yet one must consider that many post-sensing stresses such as rusts, weed infestation and abnormally high temperatures near grain filling stage in February/March (in South Asia) can result in reduced yields.  Thus relatively good fit of the INSEY-YP₀ relations as shown in Figs. 2 and 3 strongly support the argument that yield potential can indeed be predicted but the potential yield may not be realized because post-sensing condition could adversely impact the final grain yield. It seems very important that while developing INSEY-YP₀ relationships data from only those situations should be used where yields were unaffected by adverse conditions from sensing to maturity.

It is of further importance to note that differences from yield prediction equations formulated using the data collected up to 2004-05, 2005-06 and 2006-07 (Figs. 2 and 3) from different locations did not differ substantially when compared to each other.  Only exception seems to be the relationship for Feekes 5-6 stage in 2004-05 (Fig. 2) because it was based on data collected from only one experiment conducted in Karnal.  It suggest that it is possible to establish reliable yield potential prediction from at least 2 years of field data provided enough sites were evaluated during this period.  Decrease in regression significance (R²) was expected for relationships based on data collected up to 2004-05, 2005-06 and 2006-07 because these were based on increasing number of data sets.

Estimating fertilizer N dose using optical sensor for correcting in-season N deficiency

Estimating the amount of fertilizer N to be applied as per need of the crop in a given year not only depended upon identification of a yield potential (YP₀), but also on the extent to which the crop will respond to additional fertilizer N.  Pioneering work of Johnson and Raun (2003) provided the concept of a response index (RI) to quantify the later.  They found that RI measured as ratio of NDVI of the N rich strip and that of test plot was positively correlated with ratio of yield in the N rich strip and that in the test plot.  The RI allowed estimation of the yield level that can be expected by applying additional N. The inclusion of the N-rich strip reduces variability in the N fertilization optimization algorithm cause by localized weather and soil conditions by normalizing the output for the specific site.  Predicting the yield of the test plot  with additional fertilizer (YPn) allows quantifying the amount of fertilizer N to be applied and it is accomplished by using the product of YP₀ and RI.  Using YPn and YP₀, the amount of additional N fertilizer required was determined by taking the difference in estimated N uptake between YPn and YP₀ and an efficiency factor (Raun et al., 2002).  Amount of fertilizer N needed to be applied in the test plot varied from one year to another and is independent of whether or not previous year yield was high or low.  Since response to fertilizer N application depends not only upon supply of non-fertilizer N (mineralized from soil organic matter, deposited through rainfall or through irrigation etc.), the amount of fertilizer N applied at planting and crown root initiation stage (along with first irrigation event) also determined RI.  

As shown in Tables 5 to 8, the prescriptive N management in the form of applying different dose of fertilizer N at planting of wheat and crown root initiation stage and whether optical sensor based N management was practiced at Feekes 5-6 or Feekes 7-8 stage greatly influenced the dose of fertilizer N to be applied following N fertilizer optimization algorithm.  In general, amount of N to be applied at Feekes 5-6 stage as guided by optical sensor turned out be less than that worked out at Feekes 7-8 stage.  Data pertaining to YP₀ and RI as listed in Tables 5 to 8 reveal that for similar application of fertilizer N at planting and crown root initiation stage, higher optical sensor guided fertilizer N doses at Feekes 7-8 stage were due to higher RI values recorded at this stage rather than at Feekes 5-6 stage.  Obviously due to passage of more time after applying the prescriptive doses of N at planting and crown root initiation stage, RI values turned out to be higher at Feekes 7-8 stage than at Feekes 5-6 stage.  It can also be interpreted as optical sensor underestimates the fertilizer N needs of wheat when it is used too close to a fertilizer N application event.  For example, when total prescriptive dose of N was applied at planting, the amount of fertilizer N to be applied as guided by GreenSeeker optical sensor turned out to be higher than when similar amount of N was applied in two equal split doses at planting and crown root initiation stage.  Of course, the amount of N recommended by optical sensor was very sensitive to the total amount of N already applied to wheat.  More was the total amount of N applied at planting and crown root initiation stage, less was the recommendation of fertilizer N given by the optical sensor. In Tables 5 to 8, when only 60 or 80 kg N ha^-1 was applied at planting and no N was applied at crown root initiation stage, optical sensor guided recommendations were not as high that total fertilizer N applications turned out to be equal to or more than when 100 or more kg N ha^-1 were applied either all at planting or in two split doses.  It was due to that fact that at low prescriptive N levels, YP₀ turned out be less so that total fertilizer recommendation (prescriptive + optical sensor guided) remained low than when adequate amount of fertilizer N was applied as prescriptive dose.   

Evaluation of GreenSeeker guided N management vis-à-vis blanket recommendation

Application of 120 kg N ha^-1 in two equal split doses at planting and crown root initiation stage of irrigated wheat constitutes the blanket recommendation in the Indian state of Punjab where Ludhiana site is located.  In the neighboring states of Haryana and Uttar Pradesh where the other two sites - Karnal and Modipuram are located, the recommendation  is to apply 150 kg N ha^-1 (Yadvinder-Singh et al. 2007).  A survey in the northwestern belt of Indo-Gangetic plains conducted by Yadav et al. (2000) indicated that use of fertilizer N for wheat ranged from 95 to 200 kg N ha^-1.  Thus different GreenSeeker based fertilizer N management scenarios in wheat as listed in Tables 5 to 8 were evaluated vis-à-vis blanket recommendations of 120 and 150 kg N ha^-1.  Application of fertilizer N in two equal split doses - half at sowing and half at crown root initiation stage (along with first irrigation) has been found beneficial in increasing grain yield and N uptake of wheat and  it is a general recommendation for wheat over a vast area in the IGP (Meelu et al., 1987). Nitrogen uptake of irrigated wheat proceeds very slowly until tillering begins, and N flux (kg N ha^-1 day^-1) increases to a maximum around Feekes 6 stage (Doerge et al., 1991). Also, N management in irrigated wheat should not only consider crop demand but also the specific irrigation schedule that is followed. Fertilizer N applied at a time when crop needs are high, reduces the chances of losses of N from the soil-plant system and results in better N use efficiency.  Therefore, a number of prescriptive N management scenarios consisting of applying 60 to 120 kg N ha^-1 either at planting or in two split doses at planting and crown root initiation stage combined with corrective N management scenarios as guided by GreenSeeker optical sensor at Feekes 5-6 (second irrigation) or Feekes 7-8 (third irrigation) stage were tested at the three locations in the western Indo-Gangetic plain.  Since at planting of wheat there are no plants and at crown root initiation stage biomass is too little, optical sensor cannot be used for guiding fertilizer N application at these stages.

            Data from the four experiments conducted during 2005-06 and 2006-07 seasons at Ludhiana, Karnal and Modipuram as listed in Tables 5 to 8 reveal that GreenSeeker optical sensor can be successfully used to guide fertilizer N applications to irrigated wheat in the western Indo-Gangetic plain at Feekes 5-6 and Feekes 7-8 stages coinciding with 2^nd and 3^rd irrigation events.  When 60 kg N ha^-1 was applied at planting and no N was applied at crown root initiation stage of wheat, optical sensor guided fertilizer N applications at Feekes 5-6 or Feekes 7-8 stage were never adequate to produce optimum wheat yields.  This scenario is developed because low YP₀ are recorded when low levels of fertilizer N are applied at planting of wheat. Even with application of 80 kg N ha^-1 at planting and no N at crown root initiation stage at Karnal during 2006-07 season, similar trends were observed.  Application of at least 90 kg N ha^-1 at planting of wheat resulted in YP₀ values high enough to obtain wheat yields equivalent to those recorded with blanket fertilizer N recommendation provided these are supplemented with application of corrective N doses as guided by GreenSeeker optical sensor at 2^nd or 3^rd irrigation stages. 

            Prescriptive N management scenarios consisting of applying fertilizer both at planting and crown root initiation stages seem to work better with optical sensor guided corrective N management at Feekes 5-6 or Feekes 7-8 stages.  In some years (for example, Ludhiana, 2005-06 season, Table 5) due to small time gap between fertilizer N application at crown root initiation stage and Feekes 5-6 stage, response index turned out to be very less resulting in very low fertilizer N recommendation by optical sensor thereby resulting in low wheat yields.  Application of 40 or 50 kg N ha^-1 both at planting and at crown root initiation stage followed by GreenSeeker optical sensor guided N application at Feekes 7-8 stage seems to be the best strategy  to obtain high yields of wheat as well as high N use efficiency (Tables 5 to 8).  In a field study in Mexico, Ortiz-Monasterio et al. (1994) observed that a three way-split application of fertilizer N to wheat with one-third at planting, one-third at Feekes 6 stage (Z 31) and one-third at Feekes 8 (flag leaf just visible, Z 37) resulted in optimum grain yield of wheat.  These experiments were conducted in heavy clay soils. It is expected that in lighter textured soils, with potentially higher leaching problems, the three- or four-way split could be more efficient than the two-way split (Chaudhary and Katoch, 1981). In a study carried out by IAEA on irrigated wheat in 10 countries, it was found that N recovery in wheat was higher with fertilizer N application at Feekes 6 stage rather than at planting (IAEA, 2000).  It was concluded that most of the N should be applied by Feekes 6 stage to maximize grain yield and should not be delayed beyond Feekes 8 stage. 

            Removal of N by wheat was generally determined by the total quantity of fertilizer N applied in different treatment plots (Table 5 to 8).  High fertilizer N use efficiency parameters such as recovery efficiency and agronomic efficiency were observed in all those treatments where high yields were recorded by applying moderate amount of fertilizer N at planting and at crown root initiation stages as prescriptive doses and application of need based fertilizer N doses as guided by GreenSeeker optical sensor followed. 

Conclusions

Innovative fertilizer management practices aimed at managing N efficiently must integrate both prescriptive and corrective strategies to sustain the soil resource base and increase the profitability of irrigated wheat grown in the Indo-Gangetic plains in South Asia. A combination of prescriptive N doses at planting and crown root initiation stage and corrective N dose guided by GreenSeeker optical sensor at Feekes 5-6 and Feekes 7-8 stages of irrigated wheat holds promise in achieving high yields and N use efficiency. For irrigated wheat as it is grown in the Indo-Gangetic plain, biomass produced per day as estimated through NDVI measurements using optical sensor is a reliable predictor of yield potential.  Using yields potential and response index worked out from NDVI measurements of the test plot and N-rich strip, corrective doses of fertilizer N to be applied at Feekes 5-6 or 7-8 stages of wheat were as per supply of soil N as well expected yields in a particular year.  In general a combination of moderate prescriptive dose of fertilizer N and GreenSeeker based fertilizer N management always resulted in substantial savings in as compared to prevalent blanket recommendations but with no reduction in yield.

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Table 1. Soil (0-15 cm) properties of experimental sites, Ludhiana, Karnal and Modipuram, India

† 1:2 soil/water

‡ EC = Electrical conductivity

§ Walkley (1947)

¶ CEC = Cation exchange capacity

Table 2.  Some details of the optical sensor calibration experiments in which sensor and wheat grain yield data were collected

Table 3. Treatment details for the five field experiments conducted to evaluate optical sensor based N management in wheat in India

^†Crown root initiation

^§Fertilizer N applied as guided by GreenSeeker optical sensor

Table 4. Crop management details for the five field experiments conducted to evaluate optical sensor based N management in wheat in India

Table 5.  Evaluation of GreenSeeker based N management in wheat (cultivar PBW 343) at Ludhiana, India during 2005-06

*GreenSeeker guided N application     

^¥Crown root initiation stage

^†AE : Agronomic efficiency of applied N (kg grain kg^-1 N applied)

^¶RE : Recovery efficiency of applied N (%)      

 ^#PE :  Physiological efficiency (kg grain kg^-1 N uptake)

^‡YP₀ : Yield potential with no additional fertilizer N applied

^§RI : Response index

Table 6.  Evaluation of GreenSeeker based N management in wheat (cultivar PBW 343) at Karnal, India during 2006-07

*GreenSeeker guided N application     

^¥Crown root initiation stage

^†AE : Agronomic efficiency of applied N (kg grain kg^-1 N applied)

^¶RE : Recovery efficiency of applied N (%)      

 ^#PE :  Physiological efficiency (kg grain kg^-1 N uptake)

^‡YP₀ : Yield potential with no additional fertilizer N applied

^§RI : Response index

Table 7.  Evaluation of GreenSeeker based N management in wheat (cultivar PBW 343) at Modipuram, India during 2006-07

*GreenSeeker guided N application     

^¥Crown root initiation stage

^†AE : Agronomic efficiency of applied N (kg grain kg^-1 N applied)

^¶RE : Recovery efficiency of applied N (%)      

 ^#PE :  Physiological efficiency (kg grain kg^-1 N uptake)

^‡YP₀ : Yield potential with no additional fertilizer N applied

^§RI : Response index

Table 8.  Evaluation of GreenSeeker based N management in wheat (cultivar PBW 343) at Ludhiana, India during 2006-07

*GreenSeeker guided N application     

^¥Crown root initiation stage

^†AE : Agronomic efficiency of applied N (kg grain kg^-1 N applied)

^¶RE : Recovery efficiency of applied N (%)      

 ^#PE :  Physiological efficiency (kg grain kg^-1 N uptake)

^‡YP₀ : Yield potential with no additional fertilizer N applied

^§RI : Response index

Fig. 1.    Average monthly mean temperature and monthly rainfall at Ludhiana, Karnal and Modipuram, India

Fig. 2.  Relationships between in-season estimate of yield (INSEY) and potential grain yield of irrigated wheat at Feekes 5-6 stage

Fig. 3.  Relationships between in-season estimate of yield (INSEY) and potential grain yield of irrigated wheat at Feekes 7-8 stage