Recovery of Beef Cattle Manure Nitrogen in a Long-Term Winter Wheat Fertility Study

Abstract

Beef-cattle manure (BCM) can be an effective source of nitrogen (N) for crop production. However, N availability from manure can be difficult to quantify across varying environments. The Magruder Plots are a continuous winter wheat (Triticum aestivum L.) fertility study established in 1892 in Stillwater, OK, USA. In the study, wheat grain yield responses to BCM that is applied every four years are compared to those following annual applications of inorganic nitrogen (N), phosphorus (P), and potassium (K) fertilizers. This long-term, comprehensive dataset facilitates the evaluation of manure N availability and uptake across a wide range of growing environments; thus, the objective of this paper was to use 56 years of data from the Magruder Plots to benchmark current N-based manure application guidelines for Oklahoma. The results from this analysis revealed some discrepancies compared to regionally accepted guidelines for manure N availability. Existing guidelines for Oklahoma suggest that 50 to 70% of total N in BCM will become plant-available in the first year after application; however, the Magruder Plots have only averaged 23% total N availability in year one. The three seasons after manure application averaged total N availability of 20, 16, and 14%, respectively, which much more closely align with the existing estimates for Oklahoma of 10 to 20% N availability after year one. This study suggests that N availability of BCM in the first year after application in Oklahoma has a more accurate estimate of 10 to 30% of total N.

1. Introduction

Manure from livestock production can be a valuable source of plant nutrients in modern cropping systems. However, accounting for the inherent temporal variability of manure nutrient availability can be challenging. Nitrogen (N) contribution from manure can be especially difficult to quantify, as the mineralization of organic N to plant-available forms is highly dependent on weather conditions, as is N uptake and assimilation by plants.

Current manure management guidelines for Oklahoma estimate that 50 to 70% of total beef-cattle manure (BCM) N becomes plant-available in the first year after application and 10 to 20% in subsequent growing seasons without accounting for N losses [1]. This estimation varies considerably from those published for neighboring states. For example, Kansas State University estimates that solid BCM will contribute 25% of total N in the first year after application, 12% in year two, and 6% in the third year [2]. The University of Nebraska estimates that 15% of organic N will be available from composted beef feedlot manure in the first year followed by 20%, 10%, and 5% in subsequent years, while the estimates for solid manure are 40% in the first year and then 20%, 10%, and 5% [3]. Several research studies have quantified first-year N availability from manure applications and the results ranged from 5 to 40% and averaged approximately 21% [4,5,6,7]. As demonstrated by these studies, there are a wide range of manure N mineralization estimates in the literature. Research methodology, manure source, crop, and weather are factors that likely played a role in the varying conclusions on manure N contribution. However, as noted by Chang and Janzen, as well as Schröder et al., many manure-based experiments are not maintained long enough to accurately estimate the annual N contribution as part of a continually manured system [8,9]. The lack of long-term experiments observing nutrient cycling dynamics from manure somewhat limits the applicable literature from which to reference but provides substantial opportunity for this work as an addition to the current body of literature.

The Magruder Plots are a long-term, continuous fertility study located in Stillwater, OK, USA, that provides 125 growing seasons of data on winter wheat production under a manure-based fertility program. The long-term nature of the experiment has provided ample resources on the study of nutrient cycling dynamics as well as other facets of continuous winter wheat production on prairie soils of the Great Plains. These publications focus on wide array of subjects such as yield of the control treatment over time [10], effects on soil carbon, N, and soil pH both over time and in the short term [11,12], biological characteristics of the treatments [13,14], disease and weed dynamics over time [15,16], as well as economic studies on fertilization of winter wheat in the region [16].

Work on N removal from the Magruder Plots indicates that there is no significant differences between manured and NPKL treatments total soil N levels over time [17,18]. Work by Omara et al. [19] estimates a 20% increase in N uptake in the NPKL treatment versus the manured treatment, insinuating a significant departure of current manure N recovery guidelines in Oklahoma and observations in the Magruder Plots. This is explained by the assumption that if N availability of manure was 50% or greater in the first year after application as the current guideline suggests, the N uptake would be greater under the manured treatment than the NPKL as only 25% of the manure total N rate is applied annually as inorganic fertilizer N in the NPKL treatment.

Substantial differences in manure N recovery estimates between Oklahoma and other regionally accepted guidelines exist, specifically for the first year after application. Therefore, our objective was to use 56 years of data from the Magruder Plots to quantify the N availability from BCM applications and to benchmark current manure application guidelines for Oklahoma in relation to other regionally accepted guidelines in the Great Plains region USA.

2. Materials and Methods

2.1. Site Characteristics

The Magruder Plots were established in 1892 to evaluate wheat production on unfertilized prairie soils, but over time, the design and objectives evolved. In 1899, BCM was added as a fertility treatment, and in 1930, eight additional fertility treatments were added. In 1947, a university construction project forced a reduction in the size of the study to the six treatments included today (

Table 1. Manure and inorganic fertilizer treatments included in the Magruder Plots long-term winter wheat experiment located in Stillwater, OK, USA, established in 1892.

Treatment	N Rate	P2O5 Rate	K2O Rate	Lime
	kg ha−1
Unfertilized Check *	0	0	0	0
Manure (every 4th year) *	268	0	0	0
NPKL (annually) *	67	34	34	as needed
NPK (annually)	67	34	34	0
NP (annually)	67	34	0	0
P (annually)	0	34	0	0

* Treatment utilized in the current analysis.

). The experiment is situated on a Kirkland silt loam (fine mixed, thermic Udertic Paleustoll) and has been continuously managed as a conventionally tilled system since initiation. Plot sizes are 9 m wide by 30.5 m long and arranged in an un-replicated strip trial design. Fertilizer sources have changed over time to match producer practices in the region, and N rates were increased in the fall of 1967 to account for increasing winter wheat yield potential. Thus, only data from 1968 through 2023 were used in this analysis for consistency in N rates. Specific treatments utilized for this analysis are indicated in Table 1. Beef feedlot manure was stockpiled prior to application and immediate incorporation at the Sparks Beef Feedlot Research facility in Stillwater, OK, USA. A calendar depicting management practices for the Magruder Plots over a four-year cycle is provided as Supplementary Table S1. Further details of the history and methodology of the Magruder Plots can be found in Girma et al. [18].

As expected, substantial P and K is received via the manure treatment and is undocumented within this work. However, previous work by Omara et al. [20] on the effect of long-term BCM application on soil-test P (STP) in the Magruder Plots demonstrates that vast separation in STP values between manure and NPKL treatments have not materialized. Across the four time periods analyzed, differences in STP averaged only 10.81 mg kg⁻¹. These differences are unlikely to affect this work, especially when considering STP values are above current Oklahoma P sufficiency levels. Additionally while K dynamics have not been studied in the Magruder Plots, previous work in Oklahoma on wheat response to additional K found only 2 of 59 sites responded to fertilizer K [21]. The absence of response corresponds with high levels of native soil K found in the region and provides confidence that additional K applied in the Magruder Plots is inconsequential in the context of this work.

2.2. Measured and Calculated Variables

Grain yield was determined in each year of the study by harvesting the center rows of each plot using a conventional combine. The harvest width was three meters until 1994, and then two meters for the subsequent years. Grain yield was adjusted to 12.5% moisture.

Grain N concentration in the Magruder Plots was determined using various methods over the years but was not measured in every year. Over the course of the study, the average grain N concentration was 2.16%. To calculate grain N uptake for this analysis, the following equation was used:

N uptake (kg ha⁻¹) = grain yield × 2.16%

(1)

Annual fertilizer N recovery was calculated by comparing grain N uptake from the manure or inorganic fertilizer treatment with that in the unfertilized check using the following equation:

Fertilizer N recovery (kg ha⁻¹) = (N uptake_fertilized − N uptake_check)

(2)

This approach is similar to those used by other authors, such as Motavalli et al. and Zhang et al. [7,22].

Nitrogen availability in manure was calculated as the ratio of manure N recovery relative to recovery values obtained from the NPKL treatment. This approach provided an estimate of plant-available N derived from the manure in each growing season. Manure N availability was calculated using the following equation:

N availability (%) = (fertilizer N recovery_manure/fertilizer N recovery_NPKL) × 100

(3)

2.3. Statistical Analysis

Established in 1892, the Magruder Plots were initiated prior to the advent of modern statistics. Consequentially, the Magruder Plots are un-replicated. However, the long-term nature of the experiment offsets the absence of replication by the sheer number of years and data points, which provides a robust temporal dataset that facilitates a valid statistical analysis. The validity of using non-replicated data from historical experiments like the Magruder Plots is supported by the consistent methodology and maintenance of the experiment over time, as it captures a wide range of environmental conditions and natural variability. With over 100 years of data, the Magruder Plots provide a unique opportunity to observe long-term trends and treatment effects that might not be apparent in shorter-term, replicated trials [19], especially in the case of manure applications as noted by Chang and Janzen [8], and Schröder et al. [9].

In this study, data were analyzed using a mixed-effects model (PROC MIXED, SAS 9.4) to account for both fixed and random sources of variation. Specifically, 56 years of data were structured into 4-year cycles, coinciding with the manure treatment being applied every fourth year. These 4-year blocks were treated as a random effect to capture the inherent temporal variability, while within each 4-year block; individual years were treated as fixed effects to assess the specific impact of the fertilizer treatment in relation to each year within a manure cycle. This approach enabled a more accurate understanding of the effect of fertilizer treatment by accounting for both the temporal structure of the data and the underlying random variability among different 4-year periods. Single-degree-of-freedom orthogonal contrasts were used to compare treatments means for grain yield, grain N uptake, fertilizer N recovery, and fertilizer N availability at p < 0.1.

3. Results

3.1. Grain Yield

Over the course of the study, wheat grain yield in the unfertilized check ranged from 0.1 to 1.9 Mg ha⁻¹, from 0.2 to 4.1 Mg ha⁻¹ in the manure treatment, and from 0.4 to 4.6 Mg ha⁻¹ in the NPKL treatment (Figure 1). Significant differences in mean grain yield were observed between the two fertilized treatments (manure and NPKL) and the unfertilized check in each year of the manure cycle (

Table 2. Wheat grain yields as affected by fertilizer treatment in each year following manure applications. Reported yields are averaged over 14, four-year manure application cycles from 1967 to 2023.

Treatment †	Year 1 ‡	Year 2	Year 3	Year 4
	Mg ha−1
Check	1.3 a ¶	0.9 a	1.2 a	1.1 a
Manure	2.6 b	2.0 b	2.1 b	1.9 b
NPKL	2.7 b	2.5 c	2.4 c	2.5 c

^† Check: unfertilized; Manure: received 268 kg N ha⁻¹ as manure every fourth year; NPKL: received 67 kg N ha⁻¹ as inorganic N fertilizer annually. ^‡ Year after manure application. ^¶ Means in a column followed by the same letter are not significantly different at p < 0.1.

). No difference was found between the manure and NPKL treatments in the first year of the cycle; however, the mean grain yield was significantly different in years two through four of the cycle (Table 2), with grain yields following the manure application being reduced by 20, 13, and 24%, respectively.

3.2. Nitrogen Uptake

As would be expected, grain N uptake results mirrored the grain yield results, with significant differences in grain N uptake being observed between the manure and NPKL treatments and the unfertilized check in each year of the manure cycle (

Table 3. Grain N uptake as affected by fertilizer treatment in each year following manure applications. Reported yields are averaged over 14, four-year manure application cycles from 1967 to 2023.

Treatment †	Year 1 ‡	Year 2	Year 3	Year 4
	kg N ha−1
Check	28.2 a ¶	20.5 a	26.3 a	24.3 a
Manure	56.3 b	44.4 b	45.9 b	41.4 b
NPKL	58.4 b	54.7 c	52.8 c	53.8 c

^† Check: unfertilized; Manure: received 268 kg N ha⁻¹ as manure every fourth year; NPKL: received 67 kg N ha⁻¹ as inorganic N fertilizer annually. ^‡ Year after manure application0 ^¶ Means in a column followed by the same letter are not significantly different at p < 0.1.

). Also following the trend observed in the grain yield data, no difference in N uptake occurred between the manure and NPKL treatments in the first year of the cycle; however, grain N uptake was significantly different in years two through four of the cycle (Table 3). An average of approximately 10 kg ha⁻¹ more N was recovered in the grain following the NPKL treatment compared with that following the manure treatment in years two through four of the manure cycle (Table 3).

3.3. Fertilizer Nitrogen Recovery

Fertilizer N recovery is the portion of grain N uptake derived from fertilizer (Equation (2)). The fertilizer N recovery in the NPKL treatment averaged 30 kg N ha⁻¹ over the course of the study (

Table 4. Fertilizer N recovery as affected by fertilizer treatment in each year following manure application. Reported yields are averaged over 14, four-year manure application cycles from 1967 to 2023.

	Treatment †
Year After Application ‡	NPKL	Manure
	kg ha−1
1	30.1 a ¶	28.1 a
2	34.2 a	24.0 b
3	26.4 a	19.6 b
4	29.3 a	17.4 b

^† NPKL: received 67 kg N ha⁻¹ as inorganic N fertilizer annually; Manure: received 268 kg N ha⁻¹ as manure every fourth year. ^‡ Year after manure application. ^¶ Means in a row followed by the same letter are not significantly different at p < 0.1.

). Given the 67 kg N ha⁻¹ annual application for this treatment, the fertilizer use efficiency (FUE) was calculated to be approximately 45%, which would be considered typical for winter wheat production in the region. The mean fertilizer N recovery in the manure treatment was 28.1, 24.0, 19.6, and 17.4 kg ha⁻¹ for the four years of the manure application cycle, respectively (Table 4). Except for year one, fertilizer N recovery rates were significantly lower following the manure applications compared with the annual applications in the NPKL treatment (Table 4).

3.4. Manure Nitrogen Availability

Annual manure N availability was calculated using Equation (3). This calculation assumes that the FUE of the plant-available N derived from the total manure N is the same as that of the NPKL in each growing season.

The average manure N availability values were 23.3%, 20.3%, 16.3%, and 14.4% of the total manure N for each year after application, respectively.

4. Discussion

For optimal fertilization with BCM, it is essential that winter wheat producers implement a comprehensive nutrient management plan that accounts for nutrient contribution from manure accurately. Current Oklahoma guidelines for BCM management indicate that 50 to 70% of manure total N will become plant-available in the first year after application and 10 to 20% in the subsequent years [1]. Our findings suggest that this estimate of first year availability may be greater than what can be reliably obtained under the climatic conditions of Oklahoma (Figure 2). Furthermore, we found that in 14, four-year application cycles amounting to 56 growing seasons, manure N recovery reached 50% only once (Figure 3). The extreme temporal variability in manure N availability can also be seen in Figure 3, where approximately 2% to 50% of total manure N became plant-available in the first year after application across the fourteen application cycles analyzed.

This conclusion that N availability in manure is overestimated is supported by the grain yield data reported in Table 2. If N availability from the manure was 50% or greater in the first year after application, then it would be reasonable to assume that grain yield would be substantially greater compared to the NPKL treatment because the 67 kg ha⁻¹ annual N rate applied in the NPKL treatment is sub-optimal relative to the current wheat yield potential of the region. The manure treatment (according to the current guidelines) should contribute at least twice as much N as the NPKL treatment in the first year after application, resulting in adequate N availability for optimum grain yield. However, mean grain yield for the manure and NPKL treatments were not significantly different in the first year after manure application (Table 2). In fact, grain yield following the manure treatment was only nominally higher than that in the NPKL treatment in five of the 14 year-one after application growing seasons and was never significantly different (α = 0.10; data not reported). Additionally, this is further corroborated by the previously mentioned work of Omara et al. [19], who observed a nearly 20% increase in N uptake by the NPKL treatment compared to the manured, further suggesting that manure N recovery is substantially below the current guideline in the first year after application.

It is important to note that the manure management practiced in the Magruder Plots is not currently recommended for Oklahoma wheat production. Applying the entire N requirement for four growing seasons at one time does not consider any N loss mechanisms that could affect plant-available N in the first or subsequent years of the cycle. Even with the uncertainty surrounding potential N losses in the Magruder Plots, the grain yield (Table 2), N uptake (Table 3), and fertilizer N recovery (Table 4) data in this study clearly demonstrate that manure N availability to a wheat crop in the first year after application in Oklahoma is considerably lower than 50% (Figure 2).

While these results deviate from the current guidelines for Oklahoma, it is important to note that the difference only applies to the first year after manure application. This is the time in a manure application cycle when other studies report the highest variation in research findings and recommendations as well [6]. Discrepancies between research findings and local guidelines could be attributed to crop type, manure application timing, geographic characteristics, and weather factors such as temperature and precipitation, which are important variables in the N mineralization process [23]. The results may be influenced by the manure production and storage methods, including livestock ration and manure stockpiling facility design [24], which affect manure decomposition and subsequently the N availability of the manure.

Oklahoma’s current guidelines for manure N availability differ substantially from other states in the Great Plains region, especially in the first year following application (

Table 5. State guidelines for N availability from beef-cattle manure in the Great Plains region, USA.

	Year After Manure Application
	1	2	3	4
Oklahoma	50 to 70%	10 to 20%	10 to 20%	10 to 20%
Kansas	25%	12%	6%	N/A
Nebraska	25%	15%	7%	4%

Sources: [1,2,3].

). Many factors may contribute to these dissimilarities, one being the crop utilized to indicate N recovery in field studies used to develop recommendations. Manure for winter wheat, the primary crop grown in Oklahoma currently and when the existing guidelines were established, would be applied in the fall prior to planting while the weather is still warm, allowing for some early mineralization prior to wheat vernalization. This is followed by a second period of mineralization activity during the spring, which is the predominant time for N uptake in OK winter wheat, as well as the surrounding region [25]. The guidelines for Kansas and Nebraska (Table 5) may have been established using data from corn (Zea mays L.) production trials, which would likely have utilized a spring-based application. Whatever the original sources for the N availability guidelines were, the results from this study suggest that the N availability from BCM in OK is much closer to that reported for KS and NE, compared with what is currently reported for OK.

While the Magruder Plots data agree with guidelines for manure N availability in OK after the first year after application (Figure 2; Table 5), the overestimation of N availability in the first season may lead to under-fertilized crops, reduced yields, and harmful economic outcomes for producers. Further exploration of the Magruder Plots data provides some indication of what a more accurate recommendation for first-year N availability may be. The histogram presented in Figure 4 displays the frequency of estimated N availability from manure in each year after application. Over the 14 application cycles spanning from 1967 to 2023, the first-year manure plant N availability was 0 to 10% of total manure N twice, 10 to 20% five times, 20% to 30% five times, 30% to 40% zero times, and >40% twice. The adoption of manure application guidelines for OK that are closer to what has been most frequently observed under these experimental conditions (10 to 30%) would lead to more accurate N-based manure applications, subsequently reducing the need for supplemental fertilization and helping to mitigate potential yield loss due to sub-optimal N fertilization.

5. Conclusions

Beef-cattle manure can provide valuable nutrients to crop production systems when utilized properly. For producers to optimize their use of manure, accurate estimates of nutrient availability are necessary. Utilizing 56 years of data from the Magruder Plots allows current manure management guidelines to be evaluated in a way that many shorter-term experiments cannot. The data from this analysis suggest that current Oklahoma guidelines grossly overestimate the N contribution from BCM in the first year after application. The nitrogen availability from manure in the first year after application reached the current guideline of at least 50% only once during the 56 years of manure applications analyzed. Reducing the estimates for N availability from manure in OK in the first year after application to a range more closely related to that observed in this study (10 to 30%) would likely lead to improved manure utilization as producers may more accurately account for the amount of N contributed from BCM in their nutrient management plans. However, this analysis of the Magruder Plots experiment alone provides only an indication of the research needed in the area of the N availability of BCM. Therefore, the development of experiments to evaluate manure nutrient availability that recognize the inherent variability of soils, environment, and cropping system across Oklahoma are needed to justify any changes to current Oklahoma manure management guidelines.