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Article

The Effect of Nitrogen Fertilizer Placement and Timing on Winter Wheat Grain Yield and Protein Concentration

by
Brent Ballagh
1,
Anna Ballagh
2,
Jacob Bushong
3 and
Daryl Brian Arnall
2,*
1
Pro Agronomics, Ponca City, OK 74604, USA
2
Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 74078, USA
3
Apex Field Solutions, LLC, Stillwater, OK 74075, USA
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(8), 1890; https://doi.org/10.3390/agronomy15081890
Submission received: 5 July 2025 / Revised: 28 July 2025 / Accepted: 2 August 2025 / Published: 5 August 2025
(This article belongs to the Special Issue Fertility Management for Higher Crop Productivity)
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Abstract

Nitrogen (N) fertilizer management in winter wheat production faces challenges from volatilization losses and sub-optimal application strategies. This is particularly problematic in the Southern Great Plains, where environmental conditions during top-dressing periods favor N losses. This study evaluated the effects of a fertilizer placement method, enhanced-efficiency fertilizers, and application timing on grain yield and protein concentration (GPC) across six site-years in Oklahoma (2016–2018). Treatments included broadcast applications of untreated urea and SuperU® (urease/nitrification inhibitor-treated urea). These were compared with subsurface placement using single-disc and double-disc drilling systems, applied at 67 kg N ha−1 during January, February, or March. Subsurface placement increased the grain yield by 324–391 kg ha−1 compared to broadcast applications at sites with favorable soil conditions. However, responses varied significantly across environments. Enhanced-efficiency fertilizers showed limited advantages over untreated urea. Benefits were most pronounced during February applications under conditions favoring volatilization losses. Application timing effects were more consistent for GPC than for the yield. Later applications (February–March) increased GPC by 0.8–1.2% compared to January applications. Treatment efficacy was strongly influenced by soil pH, equipment performance, and post-application environmental conditions. This indicates that N management benefits are highly site-specific. These findings demonstrate that subsurface placement can improve nitrogen use efficiency (NUE) under appropriate conditions. However, success depends on matching application strategies to local soil and environmental factors rather than adopting universal recommendations.

1. Introduction

Winter wheat (Triticum aestivum L.) cultivation plays a vital role in sustaining the economic viability of numerous agricultural operations in the Southern Great Plains. Nitrogen (N) fertilizer management represents a critical determinant of both productivity and profitability, with NUE serving as a key performance indicator. Recent evidence reported an average NUE of cereal production in the United States as 41%, indicating that approximately 59% of the N is lost within the soil–plant system [1]. In a typical wheat system, this represents a 60–80 kg N ha−1 loss through various avenues, contributing to yield and economic losses, while potentially leading to declines in air and water quality.
The economic implications are substantial, with N fertilizer costs representing 40–50% of variable production expenses. Efficient and effective fertilization practices must be implemented to mitigate the effects of rising input costs and narrowing operating margins. Producers must still produce high yields to meet the increasing demand for food production [2]. Due to the responsiveness of wheat yield to N applications, some producers apply higher rates as a type of “yield insurance” [3]. However, this approach often results in diminishing returns. Previous research has shown that applying an excessive amount of N fertilizer to produce higher crop yields usually leads to lower NUE (<50%) [4]. This practice also potentially contributes to environmental issues from N losses [5,6].
The fundamental challenge lies in synchronizing the N supply with crop demand [7,8]. One practice commonly implemented in wheat production is applying fertilizer while the wheat is actively growing, generally referred to as top-dressing. Top-dressing winter wheat within the Southern Great Plains typically occurs during the late winter to early spring months (January to April). This timing coincides with the period of maximum N accumulation (60–80% of the total N uptake) [9,10]. Unfortunately, climatic conditions during these months are often highly conducive to N losses through NH3 volatilization. This is especially true for delayed applications and ammonium-producing fertilizers, such as urea [11]. Potential losses range from 10% to 50% of the applied N, depending on the environmental conditions [12,13].
The mechanisms governing volatilization losses are well-established. Soil moisture and pH have a direct impact on the amount of NH3 volatilization. Each unit increase in pH above 7.0 can potentially double volatilization rates [14] (Volk, 1959). The degree and rate of volatilization occur at varying rates among N sources due to differences in chemical composition. Ammonium-containing sources, such as ammonium nitrate, require no reaction to supply plant-available N and are less prone to N loss through volatilization. However, the mobility of NO3 could result in losses through leaching or denitrification. Conversely, NH4-producing sources such as urea undergo a chemical conversion known as hydrolysis. During the hydrolysis process, urea, H2O, and the urease enzyme react to rapidly convert urea into NH4 [15]. Complete conversion typically occurs within 2 to 5 days under favorable conditions [16]. These conditions include a soil temperature of >10 °C and adequate moisture [17]. It is during this process that the risk of NH3 volatilization is highest. This occurs because NH4 can readily be converted to NH3.
Due to the difficulty in readily obtaining ammonium nitrate, urea is often the best option for producers in the Southern Great Plains for top-dressing applications. This makes it vital to improve the management and application methods of the area to maximize its effect on grain production. To minimize the risks of potential N loss, enhanced efficiency fertilizers have been designed to reduce N loss when environmental conditions favor volatilization. SuperU® (Koch Agronomic Services LLC; Wichita, KS, USA), a urea fertilizer stabilized with urease and nitrification inhibitors, is one such fertilizer product that has been utilized for top-dress applications in winter wheat. When conditions favor N loss, it has been shown to maintain or increase the grain yield and/or GPC [18]. Reported yield improvements range from 5 to 15% under high-risk volatilization conditions.
Previous research evaluating N application methods found that incorporating urea into the soil can significantly improve NUE by reducing volatilization [19,20]. In a comprehensive study, Rochette et al. [20] found that applying 162 kg N ha−1 to slightly acidic soils more than 7.5 cm below the soil surface in a concentrated band resulted in negligible N losses (<5% of the applied N). When urea was applied to the surface, NH3 losses were 50%. However, those losses were reduced by 14% in comparison for each cm the fertilizer was incorporated into the soil [20]. Both broadcasting N on the soil surface and incorporating the fertilizer in bands under the soil surface can amplify the concentration of hydroxides, effectively increasing the soil pH. This rise in soil pH may increase the risk of NH3 volatilization. This is particularly true when an adequate precipitation event does not occur following application and there is not sufficient soil cover [20]. In a similar study, Rochette et al. [21] reported a comparison between broadcasted urea, broadcasted and then incorporated urea, and incorporated urea in bands. They found the highest N losses were from the banded application, with a 27% loss of applied N. These extensive losses were attributed to the placement of urea in areas with higher moisture content. This resulted in an increase in hydrolysis and volatilization, as well as a rise in pH from 6.0 to 8.7 in the concentrated band.
Given the mixed results regarding the effectiveness of banding and incorporating urea into soil to mitigate volatilization, further investigation into this method would be beneficial. Additionally, exploring the incorporation of urea into the soil in the context of top-dressing N into a growing wheat stand would potentially provide producers with additional options in terms of improving their NUE.
Despite extensive research on N fertilization in winter wheat, significant knowledge gaps persist in several critical areas. First, most previous studies have evaluated single factors (timing, method, or source) rather than their interactive effects under field conditions [22,23,24]. This limits the practical applicability of findings for producers who must make decisions about all three factors simultaneously. Second, the effectiveness of conventional grain-drilling equipment for subsurface N placement in established wheat stands has received limited attention. Most incorporation studies have focused on pre-plant applications or specialized equipment not readily available to producers. Third, the performance of enhanced-efficiency fertilizers, such as SuperU®, relative to placement methods, remains poorly understood. It is unclear whether the benefits of urease inhibitors are maintained when urea is placed subsurface, where volatilization risks are already reduced.
The economic implications of these knowledge gaps are substantial. With N fertilizer representing 30–40% of variable production costs and wheat prices exhibiting increasing volatility, producers require evidence-based recommendations that optimize both yield and profitability. Current extension recommendations often provide general guidelines that may not account for the practical constraints of existing farm equipment. Therefore, a study was established across multiple winter wheat systems in Oklahoma from 2016 to 2018. The objectives of this study were to (1) determine the impact of various top-dress N fertilizer methods and sources on winter wheat grain yield and GPC; (2) evaluate the timing of those top-dress N applications on both grain yield and GPC. We hypothesized that (1) subsurface placement of urea using conventional grain-drilling equipment would improve grain yield and GPC compared to broadcast surface applications, and (2) enhanced efficiency fertilizers (SuperU®) would demonstrate superior performance compared to untreated urea under broadcast conditions.

2. Methods and Materials

2.1. Experimental Sites and Environmental Characterization

Field trials were conducted at three Oklahoma State University research stations during the 2016–2017 and 2017–2018 growing seasons. Site selection was based on representative soil types and climatic gradients typical of winter wheat production in the Southern Great Plains. The sites included the Cimarron Valley Research Station near Perkins (CVRS; 35.9858° N, 97.0473° W), the Raymond Sidwell Research Station near Lahoma (RSRS; 36.3893° N, 97.1053° W), and the South-Central Research Station near Chickasha (SCRS; SCRS; 35.0410° N, 97.9077° W).
The experimental sites encompassed a range of soil taxonomic classes and physicochemical properties critical for N cycling processes (Table 1). Trials were established within a no-till continuous winter wheat cropping system for all site-years. Total precipitation for each growing season and location can be found in Table 2.

2.2. Treatments and Experimental Design

The experimental design employed a two-factor arrangement of treatments within a randomized complete block design. The factorial structure consisted of four application methods and three application timings, resulting in a total of 12 treatment combinations plus an unfertilized control. Replications varied from 3 to 4 per site-year, depending on the available field area. Plot dimensions were standardized at 3.1 m × 6.1 m (18.9 m2) to provide adequate area for combine harvesting while minimizing edge effects. Each plot contained 7–8 wheat rows (19 cm spacing) with a 1.5 m buffer zone between plots to prevent contamination from fertilizer drift.
Treatment factors were arranged to address specific hypotheses regarding N loss mechanisms and crop response patterns. Nitrogen source treatments included urea (46-0-0) as standard granular urea representing the predominant N source in regional production systems, and SuperU® (Koch Agronomic Services LLC; Wichita, KS, USA) as 46% urea stabilized with 0.06% N-(n-butyl) thiophosphoric triamide (NBPT) urease inhibitor and 0.85% dicyandiamide (DCD) nitrification inhibitor at manufacturer-recommended concentrations.
Application method treatments comprised broadcast (BR) surface application using a calibrated spreader with uniform distribution verified through collection pan analysis (CV < 10%), single-disc drill (SD) subsurface placement using single-disc opener grain drills targeting 2–5 cm depth with minimal soil disturbance, and double-disc drill (DD) subsurface placement using double-disc opener grain drills targeting 2–5 cm depth with wider furrow opening (Table 3).
Application timing treatments were scheduled for January, corresponding to Zadoks growth stage 25–29 (tillering), February, representing late-dormant to early green-up, corresponding to Zadoks growth stage 30–31 (stem elongation initiation), and March, during an active growth period, corresponding to Zadoks growth stage 31–32 (first node detectable) (Table 4). However, due to excessive rainfall prior to the third application period, the Perkins and Chickasha locations in 2017 only received the first and second in-season applications.
All fertilizer treatments received 67 kg N ha−1, representing approximately 50% of the regional recommended N rate for expected yield levels. This sub-optimal rate was deliberately chosen to maximize treatment discrimination by ensuring N-responsive conditions while avoiding yield plateau effects that could mask treatment differences. The rate selection was validated through preliminary trials, indicating linear yield responses up to 100–120 kg N ha−1 under similar conditions.

2.3. Equipment Specifications and Calibration Protocols

Six different grain drills were utilized across the study period due to equipment availability constraints at individual research stations (Table 3). While this introduced potential equipment variability, it also provided broader applicability of results to diverse farm equipment configurations. All drills were calibrated prior to each application using a standardized protocol consisting of static calibration, where fertilizer delivery rate was verified over 30.5 m test runs with collection and weighing of applied material. We used dynamic calibration through field application rate verification using real-time monitoring during initial plot applications, and depth verification where furrow depth was measured at 10 random locations per plot using a penetrometer with a target depth of 2–5 cm maintained across all drilled treatments. The single-disc openers were characterized by minimal soil disturbance and reliable furrow closure under no-till conditions. Double-disc openers created wider furrows but exhibited variable closure effectiveness depending on soil moisture and residue conditions.

2.4. Trial Management

Prior to trial establishment, composite soil samples consisting of 15 cores were collected from each trial location at a depth of 15 cm. These composite samples were submitted to the Oklahoma State Soil, Water, and Forage Analytical Laboratory for comprehensive chemical analysis. Soil pH was determined using a 1:1 soil-to-deionized water ratio following standard protocols. Nitrate (NO3-N) concentrations were determined using 2 M potassium chloride as the extractant and subsequently analyzed on a flow-injection analyzer using cadmium reduction methodology [25] (Mulvaney, 1996). Plant-available phosphorus and potassium levels were determined using Mehlich 3 extraction solution [26] (Mehlich, 1984) and analyzed using Spectro Blue inductively coupled plasma-optical emission spectrometry (SPECTRO Analytical instruments GmbH). Sikora buffer analysis was performed on all samples with a soil pH of less than 6.3 to assess lime requirements and buffering capacity [27] (Sikora, 2006).
Wheat varieties and starter fertilizer rates were selected to optimize performance for local conditions at each research station, varying between locations and years. All wheat was planted between October 10 and 15 across all site-years to ensure consistent establishment timing relative to regional planting recommendations. Standard agronomic practices were followed at each location, including weed control through herbicide applications as needed and disease management following integrated pest management recommendations. No additional N fertilizer was applied beyond the experimental treatments during the growing season. Phosphorus and potassium fertilization were provided based on soil test recommendations to ensure these nutrients were not limiting factors in crop response.
Weather data, including daily temperature, precipitation, and soil moisture measurements, were obtained from the Oklahoma Mesonet weather stations located at or near each research site. Specific weather metrics for the 10-day periods following each fertilizer application were compiled to assess environmental conditions that could influence N transformation and loss processes (Table 5). These data included average daily temperature, cumulative rainfall, maximum rainfall intensity, and average soil moisture as measured by calibrated Delta-T sensors at 5 cm depth.

2.5. Harvest Procedures and Sample Analysis

At physiological maturity, plots were harvested using a Kincaid 8XP plot combine equipped with an onboard Harvest Master Yield monitoring system (Juniper Systems; Logan, UT, USA). The yield monitoring system provided real-time collection of total plot grain weight, test weight, and moisture for each individual plot. Harvest timing was determined when grain moisture content reached approximately 13.5% to ensure consistent maturity across treatments while minimizing field losses. All grain samples were further analyzed using the Perten DA 7200 (PerkinElmer, Shelton, CT, USA) near infrared spectroscopy Diode Array Analysis System (NIR) to determine moisture content and GPC.

2.6. Data Analysis

All statistical analyses were performed using the JMP version 15.2.1 statistical software. Due to variability in grain yield and GPC responses, differences in equipment used, and the number of timings applied, all site-years were analyzed separately. Analysis of variance was conducted for each site-year using a randomized complete block design with treatments as fixed effects and replications as random effects.
Mean separation tests for treatments that showed significant differences were conducted using Fisher’s protected least significant difference (LSD). Predetermined contrasts were utilized to determine differences among broadcast and drilled urea treatments and broadcast urea versus broadcast SuperU® treatments. The unfertilized check was to ensure there was no damage from the drill traffic across a growing wheat crop; thus, it was not included in the data analysis. An alpha level of 0.05 was utilized for statistical significance, with treatment differences showing p-values < 0.10 reported as tendencies to account for economic significance.

3. Results

The primary objectives of this study were to evaluate the effectiveness of different N fertilizer application methods and sources on winter wheat grain yield GPC under diverse environmental conditions across Oklahoma. We sought to determine whether subsurface placement of urea using conventional grain-drilling equipment would improve NUE compared to broadcast applications. Additionally, we evaluated whether enhanced efficiency fertilizers would demonstrate superior performance under field conditions.

3.1. Grain Yield Nitrogen Application Methods

Analysis of variance revealed significant main effects of application method on grain yield at two of six site-years (p < 0.05; Table 6, Table 7 and Table 8). No significant method × timing interactions were detected across all locations and years (p > 0.10). At SCRS 2017, subsurface placement methods significantly outperformed broadcast urea applications (p < 0.05) (Figure 1A). Single-disc drilling achieved 3238 kg ha−1, double-disc drilling yielded 3171 kg ha−1, while broadcast urea produced 2847 kg ha−1 when averaged across application timings. Both drilling methods achieved statistically similar yields. Conversely, at RSRS 2018, single-disc drilling produced the lowest grain yields among all application methods. Single-disc yields averaged 2156 kg ha−1 compared to 2446–2537 kg ha−1 for other methods (p < 0.05) (Figure 1B). The remaining four site-years (SCRS 2018, RSRS 2017, CVRS 2017, and CVRS 2018) showed no significant differences among application methods (p > 0.05). Grain yields ranged from 1847 to 4021 kg ha−1 across methods with coefficients of variation below 12%.
Predetermined contrast analysis revealed significant method differences within specific timing periods. At SCRS 2017, drilling methods exceeded broadcast urea by 984 kg ha−1 during February applications (p < 0.001) (Table 6). At CVRS 2017, SuperU® outperformed broadcast urea by 404 kg ha−1 and exceeded drilled treatments by 472 kg ha−1 during February applications (p < 0.05) (Table 8).

3.2. Grain Yield Response to Application Timing

At SCRS 2017, application timing significantly affected grain yield (p = 0.08) (Figure 1B). January applications outperformed February applications by 324 kg ha−1 when averaged across all application methods. At CVRS 2018, timing effects were more pronounced (p < 0.05). Both January and February applications achieved statistically similar yields (2517 and 2456 kg ha−1, respectively). However, both early timings significantly exceeded March applications by approximately 400 kg ha−1 (March yield: 2117 kg ha−1). The remaining four site-years showed no significant timing effects on grain yield (p > 0.05).

3.3. Grain Protein Content Response to Nitrogen Treatments

3.3.1. Protein Response to Application Methods

The application method significantly influenced GPC at two site-years (SCRS 2018 and CVRS 2018) (Figure 2). At SCRS 2018, broadcast SuperU® achieved the highest GPC among all treatments when averaged across application timings (p < 0.1) (Figure 2A). SuperU® significantly exceeded both single-disc drilling and broadcast urea treatments. At CVRS 2018, method effects were more complex (p < 0.05). The double-disc treatment applied with the Great Plains 1006NT drill produced significantly higher GPC than other methods when averaged across all timings. The remaining four site-years showed no significant method effects on GPC (p > 0.05). The GPC ranged from 11.2% to 13.8% across methods with coefficients of variation below 8%, indicating adequate experimental precision despite the absence of treatment differences.

3.3.2. Protein Response to Application Timing

Application timing consistently influenced GPC across multiple environments, with later applications producing higher GPCs (Figure 2B). At SCRS 2017, timing effects were highly significant (p = 0.01). February applications increased GPC by 1.3% compared to January applications when averaged across all methods (13.6% vs. 12.3%) (Figure 2). At RSRS 2018, March applications achieved the highest GPC among all timings (p < 0.01) (Figure 2A). March applications exceeded both January and February timings by more than 0.5 percentage points (12.8% vs. 11.6–12.0%). Similarly, at CVRS 2018, GPC increased progressively with delayed application timing (p < 0.05). Applications for January, February, and March achieved GPC rates of 12.7%, 13.4%, and 13.7%, respectively.
Predetermined contrast analysis revealed significant treatment differences within specific timing periods that were not apparent in main effect analyses. At SCRS 2017, SuperU® significantly outperformed drilled treatments durinMulg January applications (+0.7%; p < 0.05) (Table 7). Furthermore, at RSRS 2017, drilled treatments significantly outperformed SuperU® during January applications (+0.9%; p < 0.01). However, this pattern reversed during February applications, where broadcast urea exceeded both drilled treatments and SuperU® (+0.8 and +0.9%, respectively; p < 0.05). At RSRS 2018, broadcast urea achieved significantly higher GPC than drilled treatments during February applications (+0.8%; p < 0.05). Additionally, broadcast urea outperformed SuperU® by 0.9% during the same period (p < 0.05). At CVRS 2018, several significant contrasts emerged. SuperU® exceeded broadcast urea during January applications (+1.0%; p < 0.01) (Table 8). However, drilled treatments outperformed SuperU® during March applications (+0.6%; p < 0.05). Equipment differences were also significant during March, with the Great Plains drill achieving 0.8% higher GPC than the John Deere drill (p < 0.01).

4. Discussion

4.1. Nitrogen Placement Effects on Grain Yield

The optimization of N management strategies in winter wheat production systems represents a critical nexus between agronomic efficiency and environmental stewardship. Contemporary NUE in cereal systems rarely exceeds 50% globally [1]. Volatilization losses alone account for 10–50% of applied N under field conditions [13]. This inefficiency necessitates innovative approaches that synchronize nutrient supply with crop demand. These approaches must simultaneously minimize environmental losses through ammonia volatilization, nitrate leaching, and nitrous oxide emissions [28,29].
Our experimental framework tested two fundamental hypotheses. Both received partial support under specific environmental conditions. The first hypothesis, that subsurface placement would enhance grain yield relative to broadcast applications, was conclusively supported at SCRS 2017 but not consistently across all environments. The second hypothesis, that SuperU® would outperform untreated urea under broadcast conditions, was supported at CVRS 2017 and SCRS during February applications.
Complex interactions fundamentally govern the efficacy of subsurface N placement. These involve soil buffering capacity, equipment performance characteristics, and environmental conditions during the critical post-application period. The significant yield enhancement observed at SCRS 2017 (324–391 kg ha−1 increase over broadcast applications) exemplifies the potential for placement optimization. This potential is realized under conditions where soil physicochemical properties favor N retention and equipment performance enables proper fertilizer–soil contact [30]. Subsurface placement can reduce ammonia losses by 60–80% compared to surface applications under favorable conditions [31,32]. Conversely, the absence of treatment effects at acidic sites (pH < 5.5) reflects the suppression of volatilization-driven losses. This demonstrates the pH-dependent nature of ammonia volatilization processes [20].

4.2. Equipment Performance and Placement Efficacy

Equipment performance heterogeneity across research sites provides valuable insights into the practical constraints that govern N placement efficacy. Single-disc openers achieved consistent placement depth (2–5 cm target) with minimal soil disturbance and reliable furrow closure mechanisms. They proved effective for in-season N applications in established wheat stands [33,34]. The cutting action creates a narrow slot that minimizes root damage. It also provides adequate soil coverage for volatilization reduction [35]. Double-disc systems created wider furrows for improved fertilizer distribution. However, they exhibited variable closure effectiveness depending on soil moisture conditions and residue management practices [36].
The compromised performance observed with the TYE drill (RSRS 2018) illustrates how equipment limitations can increase N losses. This occurs through the creation of exposed, concentrated fertilizer bands that promote localized pH elevation. When furrow closure is inadequate, banded urea applications can experience greater volatilization losses than broadcast applications. This occurs due to rapid hydrolysis, which consumes soil hydrogen ions and elevates the pH in the immediate vicinity of the fertilizer placement zone [37,38,39].

4.3. Enhanced Efficiency Fertilizer Performance

The performance of SuperU® relative to untreated urea provides insights into dual inhibitor technology efficacy under diverse environmental conditions. The significant yield advantage observed at CVRS 2017 and SCRS during February applications demonstrates the potential for urease and nitrification inhibitors to enhance N utilization efficiency. This occurs when environmental conditions favor transformation losses [40,41]. The urease inhibitor component (NBPT) temporarily blocks the conversion of urea to ammonium. This reduces the initial ammonia pulse during the critical 2–5-day period following application [42,43]. Simultaneously, the nitrification inhibitor (DCD) maintains N in the ammonium form. This reduces susceptibility to leaching and denitrification losses while preserving availability for plant accumulation [44].
Variable SuperU® performance across environments suggests that several factors mediate inhibitor efficacy. These include soil temperature, moisture, and microbial activity patterns that influence enzyme kinetics and inhibitor persistence [45,46]. Under cool, dry conditions that naturally slow transformation processes, inhibitor benefits may be minimal [47]. Under warm, moist conditions that accelerate urea hydrolysis and subsequent volatilization, inhibitors can provide substantial protection against N losses [48]. Economic analysis of enhanced-efficiency fertilizers must consider both the premium cost (typically 15–25% above conventional urea) and the probability of achieving yield responses sufficient to justify investment.

4.4. Application Timing Effects

Application timing significantly influenced both grain yield and protein accumulation patterns. This reflects the dynamic nature of crop N demand and temporal variability in environmental loss processes. The yield advantage observed in earlier applications (January) at SCRS 2017 and CVRS 2018 likely reflects improved synchronization between nitrogen (N) availability and peak crop demand. This occurs during tillering and early stem elongation phases [49,50]. Winter wheat N accumulation follows a predictable temporal pattern. Between 60 and 80% of total accumulation occurs between Zadoks growth stages 30 and 65 [10]. January applications typically coincide with the initiation of rapid N accumulation. This ensures optimal availability during maximum demand periods. Delayed applications may experience reduced accumulation efficiency due to the advancement of crop development and declining root activity [51].
The contrasting results with previous Oklahoma research [52] highlight the site-specific nature of timing responses. They also emphasize the importance of considering environmental conditions during the post-application period. Years characterized by adequate precipitation and moderate temperatures may show minimal timing effects. Seasons with limited rainfall or extreme temperature fluctuations may enhance the benefits of early applications [53]. The progressive increase in GPC with delayed applications (RSRS 2018, CVRS 2018) demonstrates preferential allocation of late-season N accumulation to storage protein synthesis rather than vegetative growth. Post-anthesis N accumulation directly contributes to grain protein accumulation. This occurs without dilution effects from increased grain number or size [10,54].

4.5. Environmental Risk Assessment and Management Implications

Environmental conditions during post-application periods provide critical insights into factors governing N transformation and loss processes across experimental sites. Temperature and moisture interactions during these critical windows largely determine the magnitude of treatment differentiation. Losses increase exponentially as temperature rises above 10 °C and soil moisture approaches field capacity [55,56]. Minimal precipitation recorded at several sites during application created conditions conducive to volatilization losses. This was particularly true for surface-applied treatments. Under these circumstances, subsurface placement provided protection against atmospheric exposure. It also maintained fertilizer in the soil solution, where accumulation processes could occur. Wind speed, relative humidity, and solar radiation also influence volatilization rates. They do this through their effects on atmospheric ammonia concentration gradients and soil surface drying patterns [57].
The variable treatment responses observed across environments underscore the conditional nature of the benefits of nitrogen management. They also highlight the importance of matching application strategies to site-specific conditions. Equipment performance, environmental conditions during post-application periods, and crop developmental stage emerge as critical factors. These findings provide producers with evidence-based criteria for evaluating N management investments. They also contribute to a deeper understanding of sustainable nutrient management in winter wheat production systems.

5. Conclusions

Our study addressed the challenge of optimizing nitrogen management in winter wheat. We investigated whether subsurface placement methods, enhanced-efficiency fertilizers, and application timing improve grain yield and GPC compared to conventional broadcast applications under diverse environmental conditions across Oklahoma production systems. Our results provided partial support for subsurface placement benefits, with drilling systems achieving yield increases of 324–391 kg ha−1 over broadcast applications at sites with favorable soil conditions. However, treatment effects were inconsistent across environments, indicating that soil pH, equipment performance, and environmental conditions determine placement efficacy. The hypothesis that SuperU® would outperform untreated urea under broadcast conditions received limited support. Enhanced-efficiency fertilizer advantages were most pronounced during February applications at sites experiencing conditions favorable to volatilization losses. This suggests that inhibitor technology provides protection primarily when environmental conditions accelerate N transformation processes. Application timing effects demonstrated greater consistency for GPC than grain yield. Later applications (February and March) preferentially enhanced protein accumulation, likely reflecting the physiological prioritization of late-season N accumulation for storage protein synthesis rather than yield components. Future research should focus on developing predictive models that integrate soil properties, weather patterns, and equipment capabilities to provide site-specific N management recommendations. This knowledge could lead to more targeted application strategies that balance the competing demands of high yield, optimal GPC, and efficient N use in winter wheat production systems.

Author Contributions

B.B.: Data curation, Investigation, Project administration, Resources, Writing—original draft; Writing—review and editing. A.B.—Formal analysis, Investigation; Writing—review and editing. J.B.: Formal analysis, Writing—review and editing. D.B.A.: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Visualization, Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Oklahoma Fertilizer Checkoff Board.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

Author Brent Ballagh was employed by the company Pro Agronomics. Author Dr. Jacob Bushong was employed by the company Apex Field Solutions. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

CVRS, Oklahoma State University Cimarron Valley Agriculture Experiment Station near Perkins, Oklahoma; DD, double disc; GPC, grain protein content; NUE, nitrogen use efficiency; RSRS, Oklahoma State University Raymond Sidwell Agriculture Experiment Station near Lahoma, OK; SCRS, Oklahoma State University South-Central Agriculture Experiment Station near Chickasha, OK; SD, single disc.

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Figure 1. Mean grain yield of nitrogen application method (A) and application timing (B) on winter wheat grain yield across multiple site-years. (A) Grain yield response to application methods: single-disc drilling (SD), double-disc drilling (DD), and broadcast application (BR) at South-Central Research Station 2017 (SCRS 2017) and Raymond Sidwell Research Station 2018 (RSRS 2018). (B) Grain yield response to application timing: January, February, and March applications at South-Central Research Station 2017 (SCRS 2017) and Cimarron Valley Research Station 2018 (CVRS 2018). Bars represent mean values ± standard error. Different letters above bars indicate significant differences between treatments within each site-year (p < 0.01, Fisher’s protected LSD test).
Figure 1. Mean grain yield of nitrogen application method (A) and application timing (B) on winter wheat grain yield across multiple site-years. (A) Grain yield response to application methods: single-disc drilling (SD), double-disc drilling (DD), and broadcast application (BR) at South-Central Research Station 2017 (SCRS 2017) and Raymond Sidwell Research Station 2018 (RSRS 2018). (B) Grain yield response to application timing: January, February, and March applications at South-Central Research Station 2017 (SCRS 2017) and Cimarron Valley Research Station 2018 (CVRS 2018). Bars represent mean values ± standard error. Different letters above bars indicate significant differences between treatments within each site-year (p < 0.01, Fisher’s protected LSD test).
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Figure 2. Mean grain protein concentration of fertilizer application method (A) and timing of winter wheat across multiple site-years. (A) Grain protein concentration response to different fertilizer application methods during CVRS 2018 and SCRS 2018 seasons. (B) Grain protein concentration response to fertilizer application timing during SCRS 2017, RSRS 2018, and CVRS 2018 seasons. Bars represent mean values ± standard error. Different letters above bars indicate significant differences between treatments within each site-year (p < 0.05). SD = single-disc drilling; DD = double-disc drilling; BR = broadcast; DDJD = double-disc John Deere; DDGP = double-disc Great Plains; SCRS = South-Central Research Station near Chickasha, Oklahoma; RSRS = Raymond Sidwell Research Station near Lahoma, Oklahoma; CVRS = Cimarron Valley Research Station near Perkins, Oklahoma.
Figure 2. Mean grain protein concentration of fertilizer application method (A) and timing of winter wheat across multiple site-years. (A) Grain protein concentration response to different fertilizer application methods during CVRS 2018 and SCRS 2018 seasons. (B) Grain protein concentration response to fertilizer application timing during SCRS 2017, RSRS 2018, and CVRS 2018 seasons. Bars represent mean values ± standard error. Different letters above bars indicate significant differences between treatments within each site-year (p < 0.05). SD = single-disc drilling; DD = double-disc drilling; BR = broadcast; DDJD = double-disc John Deere; DDGP = double-disc Great Plains; SCRS = South-Central Research Station near Chickasha, Oklahoma; RSRS = Raymond Sidwell Research Station near Lahoma, Oklahoma; CVRS = Cimarron Valley Research Station near Perkins, Oklahoma.
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Table 1. Pre-plant surface (0–15 cm) chemical characteristics and soil classification of sites utilized in this study.
Table 1. Pre-plant surface (0–15 cm) chemical characteristics and soil classification of sites utilized in this study.
Location aYearSoil Mapping UnitTaxonomic ClassificationpH bBI cNO3-N dP eK eLatitudeLongitude
-mg kg−1dec. degrees
SCRS2017Dale silt loam, 0 to 1 percent slopes, rarely floodedDale: Fine-silty, mixed, superactive, thermic Pachic Haplustolls6.6N/A133225635.0410 N97.9077 W
SCRS2018McLain silty clay loam, 0 to 1 percent slopes, rarely floodedMcLain: Fine, mixed, superactive, thermic Pachic Argiustolls5.87.0142218235.0410 N97.9077 W
RSRS2017Grant silt loam, 1 to 3 percent slopesGrant: Fine-silty, mixed, superactive, thermic Udic Argiustolls5.06.8233424536.3893 N97.1053 W
RSRS2018Grant silt loam, 1 to 3 percent slopesGrant: Fine-silty, mixed, superactive, thermic Udic Argiustolls5.67.0102513936.3893 N97.1053 W
CVRS2017Teller loam, 0 to 1 percent slopesTeller: Fine-loamy, mixed, active, thermic Udic Argiustolls5.66.6161814835.9858 N97.0473 W
CVRS2018Teller loam, 0 to 1 percent slopesTeller: Fine-loamy, mixed, active, thermic Udic Argiustolls4.76.793413535.9858 N97.0473 W
a SCRS, Oklahoma State University South-Central Agriculture Experiment Station near Chickasha, OK; RSRS, Oklahoma State University Raymond Sidwell Agriculture Experiment Station near Lahoma, OK; CVRS, Oklahoma State University Cimarron Valley Agriculture Experiment Station near Perkins, Oklahoma. b 1:1 water. c SMP Buffer Index. d 2 M KCl extract. e Mehlich III extract.
Table 2. Total precipitation for each month of the growing season for all site-years. All data obtained from the Oklahoma Mesonet.
Table 2. Total precipitation for each month of the growing season for all site-years. All data obtained from the Oklahoma Mesonet.
Precipitation (mm)
Location aYearSept.Oct.Nov.Dec.Jan.Feb.Mar.Apr.MayTotal
SCRS20178977023457010813157600
SCRS2018152883284751443164571
RSRS20171326591060538014882639
RSRS201854584203424080253
CVRS201760545512675060230101690
CVRS2018691447164832066100507
a SCRS, Oklahoma State University South-Central Agriculture Experiment Station near Chickasha, OK; RSRS, Oklahoma State University Raymond Sidwell Agriculture Experiment Station near Lahoma, OK; CVRS, Oklahoma State University Cimarron Valley Agriculture Experiment Station near Perkins, Oklahoma.
Table 3. Types of grain drills used for drilled urea applications for each site-year.
Table 3. Types of grain drills used for drilled urea applications for each site-year.
Location aYearType of Disc OpenerBrand of Drill b
SCRS2017Single-DiscJohn Deere 1590
Double-DiscKinkaid 2010
2018Single-DiscJohn Deere 1590
Double-DiscKinkaid 2010
RSRS2017Single-DiscTYE
2018Single-DiscTYE
Double-DiscJohn Deere 1560
CVRS2017Double-DiscJohn Deere 450
2018Double-DiscJohn Deere 450
Double-DiscGreat Plains 1006NT
a SCRS, Oklahoma State University South-Central Agriculture Experiment Station near Chickasha, OK; RSRS, Oklahoma State University Raymond Sidwell Agriculture Experiment Station near Lahoma, OK; CVRS, Oklahoma State University Cimarron Valley Agriculture Experiment Station near Perkins, Oklahoma. b The drills used were John Deere 450 (Deere and Company; Moline, IL, USA), Kincaid 2010 (Kincaid Equipment Manufacturing Corporation; Haven, KS, USA), TYE Grain Drill (AGCO Corporation; Duluth, GA, USA), John Deere 1560, John Deere 450, and Great Plains 1006NT (Great Plains Manufacturing, Inc.; Salina, KS, USA).
Table 4. Dates for all fertilizer applications made at each location over the two-year trial.
Table 4. Dates for all fertilizer applications made at each location over the two-year trial.
Location aYearTiming 1Timing 2Timing 3
SCRS201727 January 201728 February 2017N/A
20189 January 20188 February 20186 March 2018
RSRS201724 January 201710 February 20176 March 2017
201810 January 20188 February 201813 March 2018
CVRS201723 January 201823 February 2018N/A
20188 January 20185 February 20183 March 2018
a SCRS, Oklahoma State University South-Central Agriculture Experiment Station near Chickasha, OK; RSRS, Oklahoma State University Raymond Sidwell Agriculture Experiment Station near Lahoma, OK; CVRS, Oklahoma State University Cimarron Valley Agriculture Experiment Station near Perkins, Oklahoma.
Table 5. Average daily temperature, cumulative rainfall, max rainfall intensity, and average soil moisture calibrated Delta-T at 5 cm for the 10 days following fertilizer application. Data obtained from the Oklahoma Mesonet.
Table 5. Average daily temperature, cumulative rainfall, max rainfall intensity, and average soil moisture calibrated Delta-T at 5 cm for the 10 days following fertilizer application. Data obtained from the Oklahoma Mesonet.
Application
Timing		Weather IndicatorSCRS aRSRS aCVRS a
201720182017201820172018
JanuaryTemperature (°C)5.38−1.174.70−2.036.72−1.02
Rainfall (mm) 0.250.510.000.000.000.25
Max Rain Intensity
(mm h−1)		3.053.050.000.000.003.05
Soil Moisture Calibrated Delta-T at 5 cm (°C)1.461.501.552.461.601.86
FebruaryTemperature (°C)12.844.7710.323.7610.400.76
Rainfall (mm) 0.515.5951.565.590.000.00
Max Rain Intensity
(mm h−1)		3.053.0567.066.100.000.00
Soil Moisture Calibrated Delta-T at 5 cm (°C)1.702.401.612.651.671.46
MarchTemperature (°C)N/A9.277.9710.29N/A9.71
Rainfall (mm) N/A0.005.5914.73N/A0.25
Max Rain Intensity
(mm h−1)		N/A0.006.1054.86N/A3.05
Soil Moisture Calibrated Delta-T at 5 cm (°C)N/A1.401.661.76N/A1.46
a SCRS, Oklahoma State University South-Central Agriculture Experiment Station near Chickasha, OK; RSRS, Oklahoma State University Raymond Sidwell Agriculture Experiment Station near Lahoma, OK; CVRS, Oklahoma State University Cimarron Valley Agriculture Experiment Station near Perkins, Oklahoma.
Table 6. Analysis of variance and differences between means of predetermined contrasts for each application timing for the effects of fertilization method and timing on winter wheat at the South-Central Research Station near Chickasha, Oklahoma.
Table 6. Analysis of variance and differences between means of predetermined contrasts for each application timing for the effects of fertilization method and timing on winter wheat at the South-Central Research Station near Chickasha, Oklahoma.
Grain YieldGrain Protein
2017201820172018
p-Valuep-Value
Method0.04790.49780.66900.0975
Timing0.08850.56230.00610.4738
Method X Timing0.37750.53940.19920.2707
kg ha−1%
January
Urea vs. Drilled−324200.4−0.3
SuperU vs. Drilled −541550.7 *0.5
SD a vs. DD a−67−169−0.4−0.6
Urea vs. SuperU−270−135−0.2−0.8
February
Urea vs. Drilled−984 ***229−0.4−0.7
SuperU vs. Drilled −364−70.00.0
SD vs. DD−67−189−0.4−0.5
Urea vs. SuperU−627 *236−0.4−0.7
March
Urea vs. DrilledN/A−243N/A0.7
SuperU vs. Drilled N/A−7N/A1.2 **
SD vs. DDN/A−148N/A0.4
Urea vs. SuperUN/A−236N/A−0.5
a SD, single-disc opener; DD, double-disc opener. (* p < 0.05, ** p < 0.01; *** p < 0.001).
Table 7. Analysis of variance and differences between means of predetermined contrasts for each application timing for the effects of fertilization method and timing on winter wheat at the Raymond Sidwell Research Station near Lahoma, Oklahoma.
Table 7. Analysis of variance and differences between means of predetermined contrasts for each application timing for the effects of fertilization method and timing on winter wheat at the Raymond Sidwell Research Station near Lahoma, Oklahoma.
Grain YieldGrain Protein
2017201820172018
p-Valuep-Value
Method0.39070.02750.43100.7706
Timing0.38800.99200.12700.0097
Method X Timing0.28890.29890.29580.3113
kg ha−1%
January
Urea vs. Drilled−30394−0.50.4
SuperU vs. Drilled −88−61−0.9 **0.2
SD a vs. DD aN/A−290N/A0.5
Urea vs. SuperU−2221550.40.2
February
Urea vs. Drilled−1891890.00.8 *
SuperU vs. Drilled −438 *−2220.4−0.1
SD vs. DDN/A−303N/A−0.4
Urea vs. SuperU256404 *−0.40.9 *
March
Urea vs. Drilled−3461−0.4−0.6
SuperU vs. Drilled 189330 *−0.2−0.3
SD vs. DDN/A−458N/A−0.5
Urea vs. SuperU−222−270−0.2−0.3
a SD, single-disc opener; DD, double-disc opener. (* p < 0.05, ** p < 0.01).
Table 8. Analysis of variance and differences between means of predetermined contrasts for each application timing for the effects of fertilization method and timing on winter wheat at the Cimmaron Valley Research Station near Perkins, Oklahoma.
Table 8. Analysis of variance and differences between means of predetermined contrasts for each application timing for the effects of fertilization method and timing on winter wheat at the Cimmaron Valley Research Station near Perkins, Oklahoma.
Grain YieldGrain Protein
2017201820172018
p-Valuep-Value
Method0.23490.54600.72210.0508
Timing0.30250.03790.16590.0400
Method X Timing0.27220.51020.28040.2265
kg ha−1%
January
Urea vs. Drilled155−13−0.3−0.6
SuperU vs. Drilled 115−20−0.30.5
DDJD a vs. DDGP aN/A88N/A−0.2
Urea vs. SuperU47−1150.0−1.0 **
February
Urea vs. Drilled−67−2830.4−0.4
SuperU vs. Drilled 404 *3710.8−0.3
DDJD vs. DDGPN/A607N/A−0.5
Urea vs. SuperU−472 *−654 *−0.5−0.1
March
Urea vs. DrilledN/A391N/A−0.2
SuperU vs. Drilled N/A101N/A−0.6 *
DDJD vs. DDGPN/A67N/A−0.8 **
Urea vs. SuperUN/A290N/A0.4
a DDJD, double-disc opener for John Deere 450 grain drill; DDGP, double-disc opener for Great Plains 1006NT grain drill. (* p < 0.05, ** p < 0.01).
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Ballagh, B.; Ballagh, A.; Bushong, J.; Arnall, D.B. The Effect of Nitrogen Fertilizer Placement and Timing on Winter Wheat Grain Yield and Protein Concentration. Agronomy 2025, 15, 1890. https://doi.org/10.3390/agronomy15081890

AMA Style

Ballagh B, Ballagh A, Bushong J, Arnall DB. The Effect of Nitrogen Fertilizer Placement and Timing on Winter Wheat Grain Yield and Protein Concentration. Agronomy. 2025; 15(8):1890. https://doi.org/10.3390/agronomy15081890

Chicago/Turabian Style

Ballagh, Brent, Anna Ballagh, Jacob Bushong, and Daryl Brian Arnall. 2025. "The Effect of Nitrogen Fertilizer Placement and Timing on Winter Wheat Grain Yield and Protein Concentration" Agronomy 15, no. 8: 1890. https://doi.org/10.3390/agronomy15081890

APA Style

Ballagh, B., Ballagh, A., Bushong, J., & Arnall, D. B. (2025). The Effect of Nitrogen Fertilizer Placement and Timing on Winter Wheat Grain Yield and Protein Concentration. Agronomy, 15(8), 1890. https://doi.org/10.3390/agronomy15081890

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