2.1. Study Area

A field study was conducted during the rabi season of 2023–24 at the P.G. Research Farm, Centurion University of Technology and Management (CUTM), Odisha. The meteorological data was collected from an automatic weather station situated at CUTM, Parlakhemundi campus. During the experimental period, the maximum and minimum temperatures varied from 28.20°C to 32.69°C and 14.37°C to 21.39°C, respectively, with the peak temperature observed during the 45th Standard Meteorological Week (SMW). A total rainfall recorded during entire crop period was 38.20 mm, primarily distributed across the 45th, 47th and 49th SMWs. These were light and intermittent showers, insufficient to cause measurable N leaching. Sunshine hours ranged from 5.29 to 8.80 hours per day, while pan evaporation and wind velocity varied between 1.47–3.09 mm and 3.29–5.86 km hr⁻¹, respectively. Relative humidity was consistently higher in the morning compared to the afternoon throughout the crop growing season.

2.2. Physico-Chemical Details of Experimental Site Soil

The soil of the study field was characterized as sandy loam, with 72% sand, 10% silt, and 18% clay. It had a slightly acidic pH of 6.1 and an electrical conductivity of 0.62 dS/m. The organic carbon (0.39%) and the available N was also low (233.7 kg ha⁻¹) in the experimental soil. The soil had moderate levels of available phosphorus (14.1 kg ha⁻¹) as extracted by Bray’s method and potassium (227.2 kg ha⁻¹). In our study, the conversion of %N to kg ha⁻¹ was performed by multiplying the %N with soil mass factor (2.24 × 10₆ kg ha⁻¹), derived from the measured bulk density (1.49 g cm⁻³) and the sampling depth (0–15 cm).

2.3. Treatment Details

The study was carried out using the pearl millet hybrid 'Kaveri Boss 65', following the recommended fertilizer dose of 80:40:30 kg N:P₂O₅:K₂O ha⁻1. The experiment was laid out in a Randomized Block Design comprising eight treatments replicated three times. N was applied according to the treatments (Table 1). In treatments receiving 75% and 50% of recommended N, the soil-applied N was split as 50% basal, 25% at 20 days after sowing (DAS), and 25% at 45 DAS. The entire dose of phosphorus was applied basally through single super phosphate by side-dressing in a band 5 cm away from the sowing rows, while potassium was applied as muriate of potash by broadcasting uniformly over the plots at the time of sowing. The nano urea used in the study was a commercially available liquid formulation developed by the IFFCO and was applied as a foliar spray at a concentration of 4 ml l⁻¹ of water using a knapsack sprayer. The nano urea used in the study was a commercially available liquid formulation developed by IFFCO and was applied as a foliar spray at a concentration of 4 ml L-1 water using a knapsack sprayer. Spray solution was applied at 500 L ha-1 to ensure uniform canopy coverage. As per the manufacturer’s specifications, the product contains 4.0% total N (w/v) and is formulated using N nanoparticles with an average particle size range of 20-50 nm. Thus, each foliar spray supplied approximately 0.08 kg N ha-1, and treatments receiving two and three sprays supplied a cumulative nano urea-N of 0.16 and 0.24 kg N ha-1, respectively. The treatments receiving two sprays were applied at 40 and 50 DAS, while treatments receiving three sprays were applied at 40, 50 and 60 DAS during early morning hours under calm weather conditions to ensure uniform leaf coverage and minimize spray drift. The foliar nano urea spray timings (40, 50, and 60 DAS) were selected to coincide with critical physiological stages of pearl millet viz., panicle initiation, flowering and early grain filling to maximize synchronization between N supply and crop demand.

Table 1. Treatment details. [Click here to view]

2.4. Data Collection

At harvest, the plant height was measured in centimetres using a measuring tape on five randomly selected pearl millet plants from each treatment, and the mean value was recorded. Measurements were taken from the base of the stem to the base of the earhead. Tiller number at harvest was determined by randomly placing a 0.25 m² quadrat within the net plot area of each treatment, and the number of tillers within the quadrat was counted and later mathematically converted to a per square metre basis. Dry matter accumulation at harvest was estimated by uprooting pearl millet plants from 1 m row length of the second or third row from the bund to avoid border effects. The collected plant samples were first sun-dried to remove surface moisture and then oven-dried in a hot air oven at 65°C until a constant weight was achieved. The dry weight was recorded and dry matter accumulation was finally expressed on a per square metre basis by multiplying the per metre row dry weight with the respective row spacing.

Earheads from the net plot area of each treatment were harvested separately, sun-dried and threshed. Grain yield was recorded from each plot and converted to kilograms per hectare (kg ha⁻¹), adjusting the grain moisture content to 14%. After removing the earheads, plants were cut at ground level and the stover was field-dried, weighed and expressed as stover yield in kg ha⁻¹. N uptake by grain was calculated by multiplying the N content of grain (%) with the oven-dry grain yield, and expressed in kg ha⁻¹. Similarly, N uptake by stover was calculated by multiplying the N content of stover (%) with the oven-dry stover yield and expressed in kg ha⁻¹. Total N uptake was computed as the sum of N uptake by grain and stover and expressed in kg ha⁻¹.

Agronomic nitrogen use efficiency (ANUE) was calculated to express yield increase per unit of applied N and was expressed in kg kg⁻¹.

$$\mathrm{ANUE}\mathrm{(}{\mathrm{kgkg}}^{-1})=\frac{\mathrm{Grain}\mathrm{yield}\mathrm{of}\mathrm{fertilized}\mathrm{plot}\mathrm{–}\mathrm{Grain}\mathrm{yield}\mathrm{in}\mathrm{control}}{\mathrm{Fertilizer}\mathrm{Napplied}}$$

Apparent recovery nitrogen use efficiency (RNUE) of applied N was calculated to estimate the percentage recovery of N in above-ground biomass and expressed in percentage.

$$\mathrm{RNUE}\mathrm{(\%)}=\frac{\mathrm{Nitrogen}\mathrm{uptake}\mathrm{in}\mathrm{f}\mathrm{ertilized}\mathrm{plot}\mathrm{–}\mathrm{Nitrogen}\mathrm{uptake}\mathrm{in}\mathrm{control}}{\mathrm{Fertilizer}\mathrm{N}\mathrm{applied}}\times 100$$

Partial factor productivity of nitrogen (PFPN) was calculated as an index of total crop productivity per unit of applied N.

$$\mathrm{PFPN}\mathrm{(}{\mathrm{kgkg}}^{-1}\mathrm{)}=\frac{\mathrm{Grain}\mathrm{yield}\mathrm{in}\mathrm{t}\mathrm{reatment}}{\mathrm{Fertilizer}\mathrm{N}\mathrm{dose}\mathrm{applied}}$$

2.5. Data Analysis

The collected data on growth, yield attributes, yield, and N removal were analysed with the Analysis of Variance to determine treatment effects. The statistical analysis was performed using [12,13] the “agricolae” package in R version 4.4.2 (R Core Team, 2024). To interpret the differences among treatments, Standard Error of Means (SEm±) and Critical Difference (CD) at 5% significance level (p = 0.05) were computed as per the standard procedure [14]. Results were presented with superscript letters to denote treatment comparisons, where treatments sharing the same letter were not significantly different at the 5% level of significance.

3.1. Growth Characters

The experiment results indicated that the treatment N75-3F was on par with N75-2F, full nitrogen standard (FNS) and full nitrogen modified (FNM) in terms of plant height, number of tillers (m-2) and dry matter production (Table 2). The comparable growth response observed under N75-3F and N75-2F may be associated with a more synchronized N supply through combined soil and foliar application. Such synchronization has been reported to support sustained N availability [15]. Adequate N availability during the active vegetative phase may be associated with relatively greater stem height and tiller number among treatments, reflecting the role of N in supporting vegetative growth. In addition, foliar-applied nano urea may have contributed to improved plant growth and dry matter accumulation, as beneficial growth and yield responses to nano-urea application have also been reported in rice [16]. Interestingly, foliar application of nano urea either twice at 40 and 50 DAS in treatment N75-2F or thrice at 40, 50, and 60 DAS in treatment N75-3F did not register any significant difference between these treatments in terms of plant height, number of tillers (m-2) and dry matter production. In the present study, pearl millet attained 50% flowering at 47 DAS (23rd December, 2023) indicating that the third foliar nano urea spray applied at 60 DAS was imposed during the post-flowering stage when crop N uptake had already begun to decline, thereby explaining the limited additional benefit of the third spray [17,18].

Table 2. Effect of N application strategies on growth parameters of pearl millet. [Click here to view]

3.2. Grain and Stover Yield

Grain and stover yields of pearl millet varied among N application strategies (Figs. 1 and 2). However, the differences in grain and stover yields were statistically non-significant among N75-3F, N75-2F, FNS, and FNM, indicating comparable yield performance across these treatments. Moreover, the former treatment showed significant superiority over full nitrogen early (FNE) in grain yield by expressing 21.28% higher values. This might be attributed due to a better synchrony between N supply and crop demand in N75-3F N75-2F, FNS, and FNM treatments, while poor synchrony in FNE led to N losses declining NUE. Moreover, delaying N application until 30 DAS in FNE treatment might have missed the early critical stages of crop growth, tillering, and canopy development [19].

Figure 1. Effect of N application strategies on grain yield of pearl millet. [Click here to view]

Figure 2. Effect of N application strategies on stover yield of pearl millet. [Click here to view]

Seasonal weather conditions, characterized by light and intermittent rainfall (38.2 mm) and moderate to high temperatures (28.2°C–32.7°C), may have influenced crop responses to N application strategies by affecting N availability, movement, and loss processes in the soil. Limited rainfall (38.2 mm) might have reduced N leaching but also restricted its proper dissolution and movement into the root zone, while higher temperatures (28.2°C–32.7°C) could have accelerated N losses through volatilization, especially from surface-applied fertilizers. These conditions might have led to inefficient N use in treatments with large basal applications, whereas strategies that supplied N gradually (N75-2F and N75-3F) better synchronized nutrient availability with crop demand, resulting in improved crop response.

The results confirm that the findings of Samanta et al. [20] in finger millet and Meena et al. [21] in pearl millet who similarly reported that foliar application of nano-urea combined with reduced soil N doses performed comparably to 100% conventional N application, confirming that temporally distributed N supply better synchronizes nutrient availability with crop demand. For stover yield, N75-3F showed its significant superiority over FNE by 24.77%. Such results corroborate the findings of Arya et al. [22] and Shree et al. [23]. The lowest grain and stover yield were noted by control highlighting the importance on N application strategy in pearl millet.

3.3. N Content and Uptake

The data on N content and uptake in grain and stover of pearl millet were significantly affected by the N application strategies (Table 3). N concentration in grain ranged from 0.926% to 1.201%, while stover N concentration varied from 0.323% to 0.451%. The highest N concentration in both grain and stover was recorded under N75–3F but it was statistically at par with N75–2F and FNS. N uptake primarily governs crop yield, as the amount of N absorbed and assimilated by the crop during critical growth stages directly determines the accumulation of dry matter and consequently the final grain yield. However, N75–3F recorded the highest N uptake in grain, stover, and total uptake by the crop but mean values in this treatment showed statistically at par with N uptake in grain, stover, and total N uptake by the crop in the treatments N75–2F and FNS. However, N75–3F significantly outperformed FNM and FNE, which were statistically at par with each other, with increase of 24.24% and 41.38% in grain N uptake, 19.04% and 38.88% in stover N uptake, and 20.37% and 35.41% in total N uptake, respectively. Applying 75% RDN with three foliar sprays of nano urea at 40, 50, and 60 DAS ensured a more continuous and readily available N supply throughout the critical growth stages of pearl millet. This likely enhanced N absorption efficiency and promoted better growth, leading to increased grain and stover yields. In contrast, FNM and FNE lacked such targeted supplementation either due to less synchronized splits or delayed application resulting in lower N uptake. Similar findings were observed by Patel et al. [24] and Manjunath et al. [25].

Table 3. Effect of N application strategies on N content and N uptake (kg ha-1) of pearl millet. [Click here to view]

3.4. Nitrogen Use Efficiency

Among NUE indices viz., agronomic nitrogen use efficiency (ANUE), recovery nitrogen use efficiency (RNUE), and partial factor productivity of nitrogen (PFPN), significantly higher values were observed under N75-3F (ANUE: 33.01 kg kg^-1, RNUE: 72.91%, PFPN: 55.99 kg kg-1) which was on par with N75-2F. These numerical trends indicate that combining reduced soil-applied N (75% RDN) with foliar nano-urea sprays tended to improve synchrony between N supply and crop demand, potentially reducing N losses and improving N uptake per unit of fertilizer applied. Moreover, the former treatments significantly outperformed FNS treatment in terms of AEN, REN and PFPN. The N50-2F treatments exhibited the lowest ANUE and RNUE and remained on par with N50-3F and FNE, respectively. However, FNE noted the lowest PFPN which was on par with FNM and FNS. This affirms that although foliar nano-urea sprays provided readily available N, the absolute quantity of N supplied through foliar application is relatively small and short-acting and therefore could not fully compensate for the reduced basal soil N supply. Based on the NUE data, N75-2F was statistically comparable to N75-3F, making it the most economical recommendation as it achieves near-optimal agronomic performance with only two foliar nano-urea sprays, thereby reducing labour costs and input use by 25% compared to full conventional N application Table 4.

Table 4. Effect of N application strategies on NUE of pearl millet. [Click here to view]