Nitrogen and energy utilization and methane emissions of sheep grazing on annual pasture vs. native pasture

Abstract

The purpose of this study was to evaluate the differences in annual pasture and native pasture on dry matter (DM) intake, nutrient digestibility, nitrogen (N) and energy utilization, and methane (CH₄) emission of grazing sheep, and to provide the basis for rational livestock grazing in salinized regions. The study used 10 male Hu sheep ♀ × thin-tailed Han sheep ♂ rams (20 ± 5 kg) aged 5 mo. Sheep grazing was conducted in annual pasture and native pasture using a 2 × 2 Latin square design. After a 15-d adaptation period for grazing, the digestion and metabolism experiment of sheep were conducted, while CH₄ emissions were measured using sulfur hexafluoride tracer gas. DM intake did not differ between annual pasture and native pasture (P = 0.386). Meanwhile, the digestibility of DM (P < 0.001), neutral detergent fiber (P < 0.001), acid detergent fiber (P < 0.01), crude protein (P < 0.001), and ether extract (P < 0.001) of sheep grazing on native pasture was significantly higher than that of annual pasture. Sheep grazing on native pasture had increased N intake (P < 0.001) and N retained (P < 0.001) compared with those grazing on annual pasture. Digestion energy (P < 0.05) and metabolic energy (P < 0.01) of sheep grazing on annual pasture were significantly improved compared with those on native pasture, while fecal energy (P < 0.001), urine energy (P < 0.001) and CH₄ energy (CH₄-E) output (P < 0.001) and CH₄ emission (P < 0.001) of sheep grazing on annual pasture were significantly decreased. The CH₄-E/gross energy (GE) values of sheep grazing on annual pasture and native pasture were 0.09 and 0.10, respectively. In conclusion, grazing sheep have higher N utilization on native pasture, whereas grazing sheep have higher energy utilization and low CH₄ emissions in annual pasture. In conclusion, annual pasture has a lower CH₄-E/GE compared to native pasture, which helps in reducing environmental pollution.

Grazing sheep have higher nitrogen utilization in native pasture, but higher energy utilization and lower methane emission in annual pasture, providing a theoretical basis for rational grazing in salinized meadows.

Introduction

Livestock production accounts for about three-fifths of global greenhouse gases (GHGs) from agriculture (Mohammad et al., 2020). Methane (CH₄) from grazing ruminants accounts for 29% of total CH₄ from animal agriculture, 26.4% of which occurs in arid and semiarid regions (Xie et al., 2023). Livestock production contributes significantly to the gross domestic product of 1.3 billion people and accounts for more than one-third of agricultural gross domestic product in developing countries, and this proportion is rising (Field et al., 2014). Annual pasture and native pasture are dominated by extensive grazing, and more than half of the land area is dominated by grassland management and production with low input and high output/input ratio (Richmond et al., 2015; Simpson et al., 2015). The main objective of pasture management is to improve nutrient digestibility without worsening the environmental footprints, including from GHG emissions and urinary nitrogen (N) output, which depend on the relationship between forage quality and mass and grazing animal intake, nutrient digestibility, energy metabolism, as well as from CH₄ emissions, which depend on the quality and mass of the forage (Zhao et al., 2016a; Alvarado-Bolovich et al., 2021). Despite the significant contribution of grassland and livestock to ecological security and food security at local, national, and global scales (Hou et al, 2021), the lack of information on global grazing ruminant production and CH₄ emissions is a serious impediment to global productivity and GHG emission assessments based on the IPCC database (Yang et al., 2021). It is thus important to investigate the effects of annual pasture and native pasture on the N utilization, energy utilization, and CH₄ emission of grazing livestock, especially in grazing ecosystems.

The total GHGs emissions associated with animal production via various production systems have been estimated using the life cycle assessment approach, including pasture scale and geographical scale (Bhatt and Abbassi, 2021), most of which were based on the default values of the IPCC dataset (Minx et al., 2021). CH₄ emissions are affected by the types and productivity of ruminant agricultural systems, including the animals’ dietary compositions, feeding model, production performance, and grassland type (Zhao et al., 2016a; Vargas et al., 2022). Enteric methane emission is a significant energy loss during the metabolic process of livestock, which consequently leads to a reduction in their production levels (Keller et al., 2022). The N content primarily determines the energy loss in urine, while livestock’s N utilization determines the excretion of N in urine (Hristov et al., 2019). Forage crude protein (CP) content showed a negative correlation with N digestibility, but a positive correlation with CH₄ energy (CH₄-E) (Xie et al., 2023). CH₄ production was reported to be highly correlated with rumen fiber digestibility and rumen microorganisms, and forage fiber content was negatively correlated with digestibility, so improving forage digestibility may be an effective option for reducing the CH₄ emission associated with grazing livestock (Stergiadis et al., 2015a). The concentration, composition, and type of lipid could reduce enteric CH₄ emissions from ruminants by influencing rumen fermentation patterns (Boadi et al., 2004). Therefore, the ratio of dietary crude fiber to crude fat has been the main variable used for estimating enteric CH₄ emissions in livestock. The CH₄ emissions were calculated using the IPCC model to calculate the gross energy (GE) intake and standard CH₄ emission factors (CH₄ energy/GEI, 5.5%–7.5%) (IPCC, 2006), in which the CH₄ emission factor was mainly derived from experimental data. However, there is considerable uncertainty in our current understanding of the IPCC CH₄ emission factors because of the substantial variation among breeds and forage types and the lack of experimental data on grazing pasture. Therefore, knowledge of digestibility and metabolism is critical for the optimal utilization of both annual and native pastures as a whole and for properly evaluating the consequences of the comprehensive utilization of saline meadows to improve the utilization of ecological resources.

Salinized areas are home to approximately one-fifth of the global population, which accounts for nearly one-fifth of the world’s gross domestic product (Gao et al., 2015). Annual pasture and native pasture are the two most widely distributed grass types in saline areas and can provide more than half of the forage for livestock in these areas (Xie et al., 2023). Annual pasture has high quality, fewer species, and weak selective feeding, whereas native pasture has high species diversity and average forage quality (Thomas et al., 2021), and livestock selectively ingests high-quality forages or organs in native pasture grazing (Miller et al., 2005). Hence, we hypothesize (Figure 1) that there is a large difference in nutrient digestibility, N and energy utilization, or CH₄ emissions between annual pasture and native pasture, due to the large differences in their forage nutrient compositions. This study tested this hypothesis by conducting sheep grazing trials on annual pasture and native pasture in the areas with the most typical salinity distribution in China. The purposes of the present study were to investigate the differences in dry matter (DM) intake, nutrient digestibility, N and energy utilization, and CH₄ emissions of grazing sheep between annual and native pasture on grazing sheep, so as to provide basic for rational livestock grazing in arid areas.

Figure 1.

Sketch diagram of the correlation hip between forage quality, forage diversity, and methane (CH₄) emissions and annual pasture and native pasture.

Materials and Methods

The study and all animal procedures therein were approved by the ethics committee of Lanzhou University (Nos. 2010-1 and 2010-2).

Research area

The study was conducted at the Grassland Agriculture Experiment Station of Inland Arid Zone of Lanzhou University, Zhangye City, Gansu Province, China (39° and 99°51 E–100°30 E, 1,390 m a.s.l.). The region has a continental desert steppe (Hou et al., 2021), in which the annual average temperature and annual precipitation are 8 °C and 119 mm, respectively. Although native pasture is extensively exploited for food production in this ecoregion, it remains one of the most important grazing systems in the region. The dominant grassland agriculture system is a mixed crop-livestock system (Hou et al., 2008), with ruminants used for animal production and Triticum aestivum L., Sorghum bicolor L., and Medicago sativa L. as main crops.

Experimental design, animals, and measurements

Ten 5-mo-old Hu sheep ♂ × thin-tailed Han sheep ♀ rams were selected. Before the experiment started, all sheep were regularly sterilized, quarantined, and vaccinated to ensure similar body condition and body weight (BW) of ~20 ± 5 kg. A 2 × 2 Latin square design was used to evaluate two treatments (n = 10). Treatments consisted of two grazing systems with different compositions: annual pasture consisting of annual spring wheat (T. aestivum L.) and native pasture, which is one of the dominant rangelands in shallow groundwater-level areas. The main species of native pasture include Agrostis clavate, Phragmites communits, Triglochin palustre, etc. (Figure 2). Animals were assigned to two groups of five sheep each, and each batch was randomly assigned to the treatment in the first stage and then to the other treatments in the second stage. The initial 15 d of each stage was designated as the grazing adaptation period, whereas the subsequent 10 d were allocated for collecting feces and urine and measuring CH₄ emissions.

Figure 2.

Forage of composition of native pasture graze by sheep

The sheep were driven to the two grazing areas at 0700 hours every day, and then returned to the pens at 1900 hours, throughout the experiment. The sheep had free access to clean drinking water while grazing. After the grazing acclimation, CH₄ emissions were collected with sulfur hexafluoride (SF₆) tracer gas under grazing conditions (Deighton et al., 2013). In this technique, a single permeation tube containing 2.5 g of SF₆ was fed to each sheep 9 d before the start of the gas test. The permeability of the permeation tube was 3.135 ± 0.364 mg/d (mean ± SEM). In the entire gas collection experiment, each sheep was equipped with a 2.5 L gas collection tank and equipment for measuring the gas flow rate. Meanwhile, the same type of device was placed next to each grazing area to measure the flow of CH₄ and SF₆ in the atmosphere. The collected samples were analyzed immediately after the end of the experiment. The collected gas was analyzed using a gas chromatograph (GC-2014; Shimadzu Instrument (Suzhou) Co., Ltd., China). Capillary column is used for general-purpose gas capillary column (Model SH-1; Shimadzu Instrument (Suzhou) Co., Ltd., China). The injection method of the gas chromatograph analyzer involved automatic injection via a six-port value, and N₂ with purity of 99.999% was used as the carrier gas, along with a gas flow rate of 30 mL/min. The detector of the gas chromatograph analyzer mainly comprised an FID-2014 hydrogen flame ionization detector and a TCD-2014 thermal conductivity detector. The main detection component of the FID-2014 hydrogen flame ionization detector was CH₄ (detection limit: ≤5 × 10⁻¹⁰ g/s; calibration range: 0.9999–1). The main detection component of the TCD-2014 thermal conductivity detector was SF₆ (sensitivity: ≥800 mV·mL/mg, calibration range: 0.9999–1). Daily CH₄ emissions were calculated from the SF₆ release rate in the breath samples and the CH₄/SF₆ concentration ratio after correction for gas concentrations (Richmond et al., 2015).

$${\displaystyle \begin{array}{l}{\displaystyle \mathrm{C}{\mathrm{H}}_4\left(\mathrm{g}/\mathrm{k}\mathrm{g}\right)=\frac{\mathrm{C}\mathrm{S}{\mathrm{F}}_6\times ((\mathrm{C}{\mathrm{H}}_4)x-(\mathrm{C}{\mathrm{H}}_4)a)}{(\mathrm{S}{\mathrm{F}}_6)x-(\mathrm{S}{\mathrm{F}}_6)a}}\end{array}}$$

Here, CH₄ is the daily CH₄ production of the sheep, $\mathrm{C}\mathrm{S}{\mathrm{F}}_6$ is the SF₆ emission from the permeation tube, (CH₄)x and (SF₆)x are the gas concentrations in the collection cylinder, and (CH₄)a and (SF₆)a are the gas concentrations in the blank.

Animals were then placed in a separate pen for a 5-d digestibility trial with free access to water. Sheep were allowed to freely eat through a simulated mowing experiment with the residual forage, feces, and urine collected and combined. Feces were divided into two parts: one part was subjected to basic nutrition and energy analysis after being dried at 65 °C for 72 h, and the other part was directly measured for fecal N after fresh feces were combined with 10% H₂SO₄ (v/v).

Grassland measurement

The selected annual pasture and native pasture were divided into three grazing areas, each of 100 m × 100 m in size. To ensure consistent nutrients in the two grazing stages, there was an 8-d growth difference between the three grazing areas in the same treatment group prior to grazing. At each grazing stage, grazing areas at the same forage growth stage in the two treatment groups were grazed at the same time. Ten grazing cages were used to measure pre–post grazing differences, which were placed on the pasture at the beginning of each grazing period. The cages consisted of dense wire mesh with dimensions of 1 m $\times$ 1 m × 1 m and were arranged in a “W” shape on each pasture. Forage inside and outside the cages was collected from 0.25 m² quadrats at 0, 15, and 25 d in each experiment. The forage consumed by grazing animals was represented by the forage that disappeared between the forage inside and outside the cage. The feed intake of the sheep was as calculated (Undi et al., 2008):

$${\displaystyle \begin{array}{l}{\displaystyle \mathrm{D}\mathrm{M}\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{k}\mathrm{e}(\mathrm{k}\mathrm{g}{\mathrm{d}}^{-1})=\frac{[\mathrm{D}\mathrm{M}\mathrm{i}\mathrm{n}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{g}\mathrm{e}(\mathrm{k}\mathrm{g}\mathrm{h}{\mathrm{a}}^{-1})-\mathrm{D}\mathrm{M}\mathrm{o}\mathrm{u}\mathrm{t}\mathrm{s}\mathrm{i}\mathrm{d}\mathrm{e}\mathrm{c}\mathrm{a}\mathrm{g}\mathrm{e}(\mathrm{k}\mathrm{g}\mathrm{h}{\mathrm{a}}^{-1})]\times \mathrm{a}\mathrm{r}\mathrm{e}\mathrm{a}(\mathrm{h}\mathrm{a})}{\mathrm{N}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{g}\mathrm{r}\mathrm{a}\mathrm{z}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{d}\mathrm{a}\mathrm{y}\mathrm{s}\times \mathrm{S}\mathrm{h}\mathrm{e}\mathrm{e}\mathrm{p}\mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{s}}}\end{array}}$$

Chemical analysis

Fresh forage and feces and residual forage were dried continuously at 85 °C for 24 h to determine the DM content (method AOAC 930.15, AOAC, 1990). Forage and feces were crushed through a 1-mm mesh to analyze the chemical composition. Neutral detergent fiber (NDF), acid detergent fiber (ADF), and ether extract (EE) were determined in accordance with the procedure of Robertson and Van Soest (1981), method 973.18 (AOAC 2005), and method 991.36 (AOAC 1990). Organic matter (OM) was determined using a fully automatic moisture ash meter burning at 550 °C for 5 h (PrepASH340; Precise Weighing Equipment Systems Ltd., Zurich, Switzerland). GE in forage, feces, and urine were measured in an automatic oxygen bomb calorimeter (PARR 6400, PARR, Moline, USA). N concentrations in fresh feces and urine and forage were determined using a Hanon Kjeldahl Automatic K9840 analyzer (method AOAC 988.05, AOAC, 2005).

Statistical analyses

Data analysis was based on a 2 × 2 Latin square design. Chemical composition of forage, digestibility data, energy, and CH₄ were statistically analyzed using independent sample T tests in SPSS statistical software (v.20.0; Inst., Chicago, IL, USA). Linear and quadratic regression of correlations was performed between nutrient digestibility, N utilization parameters, energy parameters and forage nutrients using the REG procedure of SAS (v.9.4; SAS Institute Inc., Cary, NC, USA). Residual diagnostics were assessed using normality plots. Significant differences in the effect of the mean, linear, and squared term, were accepted at P < 0.05.

Result

Nutritional composition of annual pasture vs. native pasture

The content of DM (41.6% vs. 40.37%) and CP (8.61% vs. 7.76%) in native pasture were 3.05% and 10.95% higher than that of annual pasture (P < 0.001; Table 1), respectively. However, the contents of OM (88.37% vs. 86.3%), NDF (52.94% vs. 36.11%), ADF (36.16% vs. 29.19%), and EE (2.46% vs. 1.7%) in annual pasture were 2.40%, 46.61%, 23.88%, and 44.71% higher those in native pasture (P < 0.001), respectively.

Table 1.

Nutrient composition of annual pasture and native pasture grazed by sheep (n = 20)

Item1	Annual pasture	Native pasture	SEM2	P
DM, %	40.37	41.60	0.165	<0.001
OM%, of DM	88.37	86.30	0.234	<0.001
CP%, of DM	7.76	8.61	0.092	<0.001
NDF%, of DM	52.94	36.11	1.137	<0.001
ADF%, of DM	36.16	29.19	0.591	<0.001
EE%, of DM	2.46	1.70	0.088	<0.001

¹DM, dry matter; OM, organic matter; CP, crude protein; NDF, neutral detergent fiber; ADF, acid detergent fiber; EE: ether extract.

²SEM = treatment standard error of the mean.

DM intake, nutrient intake, and nutrient digestibility

There was no significant difference in the DM intake of grazing sheep between annual pasture and native pasture (P > 0.05; Table 2). Based on the differences in the chemical composition between annual and native pasture, digestible OM intake and nutrient digestibility also varied greatly (P < 0.001). Compared with in annual pasture, sheep grazed in native pasture increased the digestibility of DM, OM, NDF, ADF, CP, and EE by 6.85%, 6.18%, 10.51%, 1.28%, 25.33%, and 15.16% (P < 0.001).

Table 2.

DM intake, digestible OM, and Nutrient digestibility of annual pasture and native pasture grazed by sheep (n = 20)

Item1	Annual pasture	Native pasture	SEM2	P
DM intake, g/d	0.91	0.93	0.017	0.386
DM intake/BW0.75, kg/kg	0.088	0.090	0.0016	0.199
Digestible OM intake, g/d	0.60	0.62	0.001	<0.001
Apparent digestibility, %
DM	62.46	66.74	0.092	<0.001
OM	67.85	72.04	0.234	<0.001
NDF	53.73	59.38	1.137	<0.001
ADF	38.92	39.42	0.165	0.007
CP	53.80	67.43	1.363	<0.001
EE	55.48	63.89	1.136	<0.001

¹DM: dry matter; BW, body weight; OM, organic matter; NDF, neutral detergent fiber; ADF, acid detergent fiber; CP, crude protein; EE, ether extract.

²SEM = treatment standard error of the mean.

CH₄ emission

Compared with annual pasture, sheep grazing on native pasture increased CH₄ emissions by 23.3% (P < 0.001; Table 3). Based on the metabolic weight, CH₄ emission in annual pasture was lower by 20% than in native pasture (P < 0.001). The two types of pasture had an effect on CH₄ emission per kilogram DM intake, digestible DM, and digestible OM; the CH₄/DM intake, CH₄/digestible DM, CH₄/digestible OM of sheep grazed on annual pasture was lower by 17.4%, 13.26%, and 12.8%, respectively (P < 0.001).

Table 3.

CH₄ emissions of sheep grazed sown pasture and native pasture (n = 20)

Item1	Annual pasture	Native pasture	SEM2	P
CH4, g/d	10.00	12.33	0.164	<0.001
CH4/BW0.75, g/kg	0.44	0.55	0.009	<0.001
CH4/DM intake, g/kg	11.01	13.33	0.202	<0.001
CH4/digestible DM, g/kg	16.02	18.47	0.047	<0.001
CH4/digestible OM intake, g/kg	17.57	20.15	0.086	<0.001

¹CH₄: methane; DM: dry matter; BW, body weight; OM, organic matter.

²SEM = treatment standard error of the mean.

Nitrogen and energy utilization

Based on the metabolic weight, N intake, urinary N, N retained, and N retained/N intake in native pasture were higher by 27.5%, 5%, and 60.82% than those in annual pasture, respectively (P < 0.001; Table 4). Meanwhile, compared with those on native pasture, fecal N, fecal N/N intake, and urinary N/N intake of grazing annual pasture were increased by 10%, 39.39%, and 22.22% (P < 0.001), respectively.

Table 4.

Nitrogen intake and utilization by sheep grazed annual pasture or native pasture (n = 20)

Item1	Annual pasture	Native pasture	SEM2	P value
N intake, g/kg BW0.75	1.20	1.53	0.027	<0.001
Fecal N, g/kg BW0.75	0.55	0.50	0.008	<0.001
Urinary N, g/kg BW0.75	0.40	0.42	0.006	0.027
N retained, g/kg BW0.75	1.71	2.75	0.027	<0.001
Fecal N/N intake, kg/kg	0.46	0.33	0.126	<0.001
Urinary N/N intake, kg/kg	0.33	0.27	0.005	<0.001
N Retained/N intake, kg/kg	0.20	0.40	0.017	<0.001

¹N, Nitrogen.

²SEM = treatment standard error of the mean.

Based on the metabolic weight, GE intake in annual pasture tended to be higher than that in native pasture (1.29 vs. 1.23 MJ/kg BW^0.75; P = 0.052; Table 5); urine energy (UE) output (P < 0.001), fecal energy (FE) (P < 0.001) output and CH₄ emission in native pasture were higher than those in annual pasture, while digestion energy (DE) (P = 0.01) and metabolic energy (ME) (P < 0.001) were lower in native pasture than in annual pasture. Sheep grazing on annual pasture showed higher DE to GE ratio (P = 0.001), ME to GE ratio (P < 0.001), and ME to DE ratio (P < 0.001) than those on native pasture. Compared with the values in annual pasture, sheep grazing on native pasture exhibited a significant increase in the ratio of CH₄-E to GE (P < 0.001), the ratio of CH₄-E to DE (P < 0.001), and the ratio of CH₄-E to ME (P < 0.001).

Table 5.

Energy intake and utilization by sheep grazed annual pasture or native pasture (n = 20)

Item1	Annual pasture	Native pasture	SEM2	P
GE, MJ/kg BW0.75	1.29	1.23	0.026	0.052
FE, MJ/kg BW0.75	0.31	0.33	0.003	<0.001
UE, MJ/kg BW0.75	0.17	0.19	0.002	<0.001
DE, MJ/kg BW0.75	0.98	0.90	0.027	0.010
ME, MJ/kg BW0.75	0.81	0.71	0.026	0.003
DE/GE, MJ/MJ	0.77	0.75	0.006	0.001
ME/GE, MJ/MJ	0.63	0.58	0.009	<0.001
ME/DE, MJ/MJ	0.82	0.79	0.005	<0.001
CH4-E, MJ/MJ	0.12	0.13	0.009	<0.001
CH4-E/ GE, MJ/MJ	0.09	0.10	0.002	<0.001
CH4-E/ DE, MJ/MJ	0.13	0.15	0.005	<0.001
CH4-E/ ME, MJ/MJ	0.15	0.18	0.005	<0.001

¹GE, gross energy; BW, body weight; FE, fecal energy; UE, urine energy; DE, digestion energy; ME, metabolic energy; CH₄-E, CH₄ energy.

²SEM = treatment standard error of the mean.

Nitrogen and energy cycles in annual pasture and native pasture

The N intake per hectare of sheep grazing on annual and native pastures was 249.20 g and 314.33 g, respectively, which were significantly different (P < 0.01; Figure 3). Fecal N output from sheep grazing on annual pasture was 114.60 g/ha, a significant increase of 12.25 g/ha compared with that on native pasture (P < 0.01), whereas urinary N output was 83.31 g/ha, a significant decrease of 2 g/ha compared with that on native pasture (P < 0.01). The N retained of sheep grazing on native pasture increased by 75.36 g/ha compared with that on annual pasture (P < 0.01).

Figure 3.

Relationship between nitrogen intake/gross energy per hectare and nitrogen utilization parameters, energy parameters and methane emissions of grazing sheep (n = 20). The gray words in the figure indicate that there is no significant difference between the two types of pasture (P > 0.05). while the black type indicates a significant difference (P < 0.01).

The GE intake per hectare of sheep grazing on annual and native pastures was 266.14 MJ and 252.29 MJ, respectively, which were significantly different (P < 0.01; Figure 3). The two types of pastures had no significant effect on the excretion of UE, while FE output was lower by 12.25 MJ/ha in annual pasture than in native pasture (P < 0.01). CH₄-E output from sheep grazing on native pasture was 26.02 MJ/ha, which constituted a significant increase of 1.8 MJ/ha compared with on annual pasture (P < 0.01). The DE and ME of sheep grazing on annual pasture increased by 18.23 and 21.15 MJ/ha, respectively (both P < 0.01).

Relationships between forage chemical composition and nutrient digestibility, N parameters, energy parameters, and CH₄ emission

The linear and quadratic relationships between DM digestibility of grazing sheep and forage CP content were positive (r_linear, 0.943; r_quadratic, 0.946; P < 0.01), while the linear and quadratic relationships between DM digestibility and forage EE (r_linear, 0.907; r_quadratic, 0.918; P < 0.01), NDF (r_linear, 0.952; r_quadratic, 0.953; P < 0.01) and ADF were negative (r_linear, 0.927; r_quadratic, 0.950; P < 0.01; Table 6). There were no linear and quadratic relationships between forage CP and EE content and digestibility ADF (P > 0.05), but the remaining linear and all quadratic relationships between forage CP content and nutrient digestibility and N parameters were positive (r_linear, 0.753 to 0.9; r_quadratic, 0.775 to 0.904; P < 0.01). The linear and quadratic relationships between EE content and DM digestibility (r_linear, 0.907; r_quadratic, 0.918; P < 0.01), digestibility OM (r_linear, 0.909; r_quadratic, 0.913; P < 0.01), and digestibility EE (r_linear, 0.820; P < 0.01; r_quadratic, 0.838; P < 0.05) were negative. The linear and quadratic relationships between EE content and N retained (r_linear, 0.872; r_quadratic, 0.882; P < 0.01) and N intake/N (r_linear, 0.881; r_quadratic, 0.849; P < 0.01) retained were negative. There was no significant linear relationship between forage CP and EE content and GE (P > 0.05), while GE was quadratically linearly negatively correlated with CP content (r, 0.667; P < 0.05) and quadratically linearly positively correlated with EE content (r, 0.563; P < 0.05). The linear and quadratic relationships between CP content and DE (r_linear, 0.498; _quadratic, 0.724; P < 0.05) and ME (r_linear, 0.498; r_quadratic, 0.566; P < 0.05) were negative, while the linear and quadratic relationships between CH₄-E (r_linear, 0.847; r_quadratic, 0.847; P < 0.01), CH₄ (r_linear, 0.916; r_quadratic, 0.918; P < 0.01), and CH₄-E/GE (r_linear, 0.737; r_quadratic, 0.819; P < 0.01), CH₄-E/DE (r_linear, 0.713; r_quadratic, 0.804; P < 0.01), and CH₄-E/ME (r_linear, 0.717; r_quadratic, 0.817; P < 0.01) were positive. The linear and quadratic relationships between EE content and DE (r_linear, 0.523; r_quadratic, 0.661; P < 0.05) and ME (r_linear, 0.599; r_quadratic, 0.715; P < 0.05) CH₄-E/GE (r_linear, 0.797; r_quadratic, 0.856; P < 0.01), CH₄-E/DE (r_linear, 0.733; r_quadratic, 0.806; P < 0.01), and CH₄-E/ME (r_linear, 0.784; r_quadratic, 0.853; P < 0.01) were positive. NDF and ADF content of forage were correlated negatively (both linear and quadratic) with nutrient digestibility, N parameters, and CH₄-E parameters (linear: r_NDF = 0.552 to 0.923, r_ADF = 0.621 to 0.907, P < 0.05; quadratic: r_NDF = 0.573 to 0.925, r_ADF = 0.678 to 0.927, P < 0.05), whereas the linear and quadratic relationships of energy parameters (except GE) were positive (linear: r_NDF = 0.544 to 0.798, r_ADF = 0.602 to 0.830, P < 0.05; quadratic: r_NDF = 0.616 to 0.837, r_ADF = 0.625 to 0.833, P < 0.05). Compared with the above correlation degree, the correlation coefficients of forage nutrient content were in the following order: NDF (linear: 0.552 to 0.952; quadratic: 0.573 to 0.953) > ADF (linear: 0.490 to 0.927; quadratic: 0.625 to 0.950) > CP (linear: 0.498 to 0.943; quadratic: 0.566 to 0.946) > EE (linear: 0.523 to 0.935; quadratic: 0.563 to 0.936).

Table 6.

Correlation coefficients of linear (Lin) and quadratic (Quad) relationships between nutrient digestibility, N utilization, energy parameters, CH₄ emission and forage chemical composition (n = 20)

Forage chemical composition1
Item2	CP		EE		NDF		ADF
	Lin3	Quad2	Lin	Quad	Lin	Quad	Lin	Quad
DMD	0.943**	0.946**	−0.907**	−9.18**	−0.952**	−0.953**	−0.927**	−0.950**
Nutrient digestibility
OM	0.900**	0.904**	−0.909**	−0.913**	−0.923**	0.925**	−0.907**	−0.927**
NDF	0.753**	0.775**	−0.736**	0.756*	−0.552*	−0.573*	−0.621**	0.735**
ADF	—	—	—	—	−0.617**	−0.623**	−0.619**	−0.678**
EE	0.836**	0.852**	−0.820**	−0.838*	−0.697**	−0.709**	−0.743**	−0.824**
N	0.860**	0.871**	−0.854**	−0.863**	−0.872**	−0.872**	−0.839**	−0.848**
N utilization
N intake	0.850**	0.865**	0.882**	0.891**	−0.888**	−0.890**	−0.875**	−0.878**
N retained	0.871**	0.886**	−0.872**	−0.882**	−0.899**	−0.901**	−0.880**	−0.886**
N intake/N retained	0.867**	0.881**	−0.849**	−0.860**	−0.878**	−0.880**	−0.855**	−0.861**
GE	—	−0.667*	—	0.563*	—	—	0.490*	—
DE	−0.498*	−0.724*	0.523*	0.661*	0.544*	0.616*	0.602**	0.625*
ME	−0.498*	−0.566*	0.599*	0.715*	0.614**	0.675*	0.665**	0.680**
DE/GE	−0.582**	−0.708*	0.756**	0.817**	0.659*	0.710*	0.690**	0.702**
ME/GE	−0.692*	−0.810*	0.758**	0.836**	0.766**	0.809**	0.796**	0.802**
ME/DE	−0.722**	−0.823**	0.787**	0.856**	0.798**	0.837**	0.830**	0.833**
CH4-E	0.847**	0.847**	−0.935**	−0.936**	−0.885**	−0.885**	−0.838**	−0.870**
CH4-E/GE	0.737**	0.819**	0.797**	0.856**	0.796**	0.825**	−0.820**	−0.820**
CH4-E/DE	0.713**	0.804**	0.733**	0.806**	−0.778**	−0.810**	−0.804**	−0.804**
CH4-E/ME	0.717**	0.817**	0.784**	0.853**	−0.798**	−0.836**	−0.828**	−0.830**
CH4, g/d	0.916**	0.918**	−0.858**	−0.858**	−0.926**	−0.927**	−0.875**	−0.922**

¹CP, forage crude protein concentration; EE, forage ether extract concentration; ADF, forage acid detergent fiber concentration; NDF, forage neutral detergent fiber concentration.

²ADF, acid detergent fiber; BW, body weight; CH₄, methane; CH₄-E, CH₄ energy; GHG, greenhouse gas; CP, crude protein; DM, dry matter; DE, digestion energy, EE, ether extract; FE, fecal energy; GE, gross energy; ME, metabolic energy; N, nitrogen; NDF, neutral detergent fiber; OM, organic matter; UE, urine energy.

³Linear relationships.

⁴Quadratic relationships; “**” means “P<0.01”; “*” means “P<0.05”

Discussion

DM intake and nutrient digestibility

There was no significant difference in DM intake between sheep grazing on annual and native pastures based on daily or metabolic BW, although the nutritional composition of the annual pasture was better than that of the native pasture (Tables 1 and 2). It is possible that the grazing plots had sufficient pasture, the native pasture had rich species diversity (Figure 2), and the sheep grazing sheep on the native pasture could obtain similar nutritional requirements as those on the annual pasture by selective foraging to choose good forage or organs (Cuchillo Hilario et al., 2017; Fan et al., 2023), so there was no significant difference in the DM intake of grazing sheep between the two pastures. The NDF content of forage promotes the normal rumen function by stimulating livestock chewing (Schulze et al., 2014). The act of chewing reduced the size of forage and increased surface area for microbial attachment, thereby improving the efficient degradation of forage nutrients by microorganisms (Adesogan, et al., 2019). The forage with a higher NDF content tends to fill the rumen to a greater extent, leading to reduced chewing activity and surface area for microbial attachment, thereby reducing the degradation of forage in rumen (Vinyard et al., 2018; Adesogan, et al., 2019; Xu et al., 2022). This in turn results in the digestibility of nutrients in the native pasture of grazing sheep being significantly higher than that in the annual pasture (Table 2). Both linear and quadratic relationships between forage NDF and ADF content and all nutrient digestibility parameters involved negative correlations, which was probably partly explained by the high cellulose content of forage limiting the digestion and utilization of other nutrients due to the difficulty of degradation in the rumen (Cui et al., 2023). This would in turn contribute to the fact that the digestibility of all nutrients in grazing sheep was significantly higher in annual pasture with higher NDF and ADF. Second, CP concentration in forage is the main limiting factor affecting nutrient digestibility, and when the dietary CP concentration is below the minimum threshold level (7%), it inhibits the activity of rumen microorganisms in ruminants, directly leading to decreased dietary intake and digestibility (Stergiadis et al., 2015b; Yang et al., 2018). In this study, the linear and quadratic relationships between CP content in herbage and digestibility of DM, OM, NDF, ADF, EE, and N were positive, and the CP content in both grasslands was higher than 7%. Therefore, the nutrient digestibility of sheep grazing on native pasture with high CP content was higher than that of those on annual pasture. The correlation coefficients between ADF and NDF content of forage and OM digestibility were found to be significantly higher than those of other nutrient digestibility parameters, whereas the lowest correlation coefficient was found between the NDF content of alpine meadow forage and OM digestibility of grazing Tibetan sheep (Yang et al., 2018), and there was no significant correlation between the ADF content of forage and OM digestibility in cows fed fresh grass diets (Morgan and Stakelum, 1987). The effect of forage NDF and ADF content on the variability of nutrient digestibility implies that hemicellulose plays a crucial role in the effect of nutrient digestibility on grazing livestock in saline meadows; however, the mechanism behind this effect requires further investigation.

Methane emission

Daily CH₄ emissions were directly affected by livestock DM intake (Jonker, et al., 2017; Dall-Orsoletta, et al., 2019), whereas the DM intake of grazing sheep in the two pasture types was consistent; therefore, the difference in CH₄ emission of grazing sheep could be interpreted as the difference in the nutritional value of forage in the two pasture types (Table 3). CH₄ is produced by hydrogen as a byproduct of the fermentation of carbohydrates, especially structural carbohydrates (Seshadri et al., 2018). Consequently, higher nutrient digestibility in native pasture leads to higher carbohydrate degradation, resulting in more CH₄ to remove hydrogen from the rumen environment (Fraser et al., 2015). The low fiber concentration and high CP concentration in the diet contributed to the reduction in enteric CH₄ emissions (Niu et al., 2016), but the results of this study showed that NDF and ADF concentrations in pasture were negatively correlated with CH₄-E in both linear and quadratic relationships in annual and native pastures, and positively correlated with forage CP concentration. This has been due to a confounding effect of forage NDF and ADF concentrations, rather than a physiological function (Zhao et al., 2016b). This phenomenon may also have been caused by the abundance of forage species in native pasture, and secondary metabolites in some forage may promote the production of methanogens or promote methanogenesis, resulting in higher CH₄ emissions from the native pasture. Forage EE concentration was reported to be an important factor in determining CH₄ emissions from livestock in salinized meadows (Xie et al., 2023), and the concentration of EE in forage is negatively correlated with CH₄ emissions or CH₄-E output. This may be because EE competes for hydrogen with methanogens through the hydrogenation process, indirectly reducing CH₄ emissions (Boadi et al., 2004). This can be used to explain the higher CH₄ emissions in native pasture with low NDF and ADF concentrations and high CP concentrations. The relationship between forage CP content and retained N and CH₄ emissions of livestock exhibited a strong positive correlation (Xie et al., 2023). The retained N and retained/N intake in livestock are higher in a high forage CP of native pasture compared with annual pasture, resulting in increased CH₄ emissions. FE, UE, and CH₄-E are important energy losses in livestock metabolism (Shi et al., 2023). FE, UE, and CH₄-E in native pasture are significantly higher than in annual pasture, so livestock in annual pasture improve energy utilization by reducing energy loss primarily in the form of feces, urine, and CH₄ emissions.

N utilization

Sheep grazing on annual and native pastures had similar DM intake, but native pasture was associated with significantly higher N intake, probably due to its higher N concentration (Table 4). The significant increase in fecal excretion in annual pasture was due to the fact that it had low N digestibility and the forage was excreted in the feces before it had been fully digested (Potts et al, 2017). The low fiber concentration of native pasture was more readily broken down into carbohydrates that were digested, absorbed, and utilized by grazing sheep (Xie et al., 2023), while carbohydrates provide energy for protein synthesis by rumen microorganisms, increase the rate of ammonia capture by rumen microorganisms, and balance the rate of ammonia production, thereby increasing the efficiency of N utilization (Seo et al., 2013). Therefore, both NDF and ADF concentrations in forage were negatively correlated with N utilization parameters. Based on metabolic weight, daily urinary N excretion was significantly higher in native pasture, in contrast to the lower urinary N/N intake (Table 4). This may have been due to the fact that grazing sheep selectively feed on high-quality forage or organs in native pasture, and high-quality forage may increase the rumen partial outflow rate, allowing less time for rumen microorganisms to ferment digestive fluid, resulting in excess ammonia N production and subsequent reduction in urinary N excretion ( Fan et al., 2023). Low forage CP concentrations in annual pasture may limit the tricarboxylic acid cycle in grazing sheep to reduce the rate of energy synthesis and rumen microbial ammonia capture, indirectly increasing the rate of N excretion (Spring et al., 2020). All of the above factors resulted in significantly lower retained N of the annual pasture than that of the native pasture. Fecal N and urinary N excretion represents a considerable N loss, which may be influenced by a number of dietary and animal-related factors (Waldrip et al., 2013). The environmental pollution caused by urinary N excretion is mainly due to its rapid hydrolysis into ammonium under the action of urease (Dong et al., 2014), along with the high organic load and large amount of N in feces (Jain et al., 2022), both of which cause the pollution of groundwater and surface water by agriculture-derived (Govarchin et al., 2023). Therefore, in practical production, for sheep grazing production systems, there is a need to take into account not only digestibility, growth performance, and CH₄ emissions, but also the management and application of sheep excrement (Montes et al., 2013). The grazing sheep excreted more total N per hectare in annual pasture than in native pasture, in contrast to which they retained the least N (Figure 3). Therefore, native pasture is more suitable for livestock grazing from the perspective of ecological environmental protection and sheep N utilization.

Energy utilization

The main sources of energy loss include UE, FE, CH₄-E, and body heat. FE is the largest contributor to energy loss, accounting for about one-third of the total energy intake (De et al, 2013; Zhao et al., 2016b). Annual pasture is an improved salinized cultivated pasture, whereas native pasture is a natural salinized pasture (Ning et al., 2020). As a result, native pasture is more saline than annual pasture (Table 5). This also results in grazing sheep having to drink more water in native pasture (Gabr et al., 2023), which promotes livestock excretion and indirectly increases the excretion of FE and UE per day or hectare of pasture (Figure 3). The linear and quadratic correlations between forage NDF and ADF concentration and DE, ME, and GE were negative, which was inconsistent with other studies. This discrepancy may be due to the high species diversity of native pasture, and grazing sheep’s need to search for quality pasture or organs by walking further, leading to higher energy consumption (Cuchillo Hilario et al., 2017; Xie et al., 2023) and thus reducing their DE and ME. The variety of annual pasture was single, the grazing sheep’s selective feeding was weak, and the energy consumption was also low (Yang et al., 2019). The linear and quadratic correlations between forage N concentration and ME/DE, ME/GE, and DE/GE were negative, and the output of N in urine but not feces increased proportionally with the increase in N intake (Dijkstra et al., 2013; Todd et al., 2015). This in turn caused ME to decline faster than DE or GE, resulting in lower ratios of ME/DE and ME/GE. Saline meadows provide about 50% of forage for livestock worldwide (Field et al., 2014); thus, it is particularly important to develop more accurate CH₄ emission data for these regions. CH₄-E/GE values in annual pasture and native pasture were 0.09 and 0.10, respectively, which were both higher than previously reported values of CH₄-E/GE in lambs fed fresh ryegrass and native pasture (0.067 and 0.054, respectively) (Fraser et al, 2015; Zhao et al., 2016b), possibly due to animal type, environmental factors, and pasture variety, among others (Yan et al., 2010). Thus, measuring CH₄ emissions using pasture and animal breeds in specific areas can improve the accuracy of CH₄ emission estimates, although many basic CH₄ emission factors have been proposed for grazing sheep or grazing on freshly mowed grassland (Yan et al., 2010; Zhao et al., 2016b), contributing to more rational management of grazing systems.

Conclusion

The CH₄-E/GE values of Hu sheep × thin-tail Han sheep rams grazing on annual pasture and native pasture were 0.09 and 0.10, respectively. The feed intakes of grazing sheep in the two pasture types were similar. From the perspective of environmental protection and N utilization of sheep, grazing sheep had higher nutrient digestibility and N utilization in native pasture. In terms of energy utilization, annual pasture was associated with better energy utilization and lower excretion of FE, UE, CH₄, and CH₄-E. Therefore, this study provides basic data for sheep grazing on salinized meadows, on which further research is needed to develop a more rational grazing mode for sheep.

Glossary

Abbreviations

ADF

acid detergent fiber

BW

body weight

CH₄

methane

CH₄-E

CH₄ energy

CP

crude protein

DE

digestion energy

DM

dry matter

EE

ether extract

FE

fecal energy

GE

gross energy

GHG

greenhouse gas

H₂SO₄

sulfuric acid

ME

metabolic energy

N

nitrogen

NDF

neutral detergent fiber

OM

organic matter

SF₆

sulfur hexafluoride

UE

urine energy

Conflict of interest statement. The authors declare no real or perceived conflicts of interest.