1. Introduction

2. Materials and Methods

2.1. Calculation of Animal Husbandry Carbon Emissions

Based on the research of Dai et al., this study classified the carbon emissions generated in the life cycle of animal husbandry into three subsystems according to the characteristics of animal husbandry production, namely, feed grain transport system, energy consumption system, and gastrointestinal fermentation and manure management system [17]. The gases of livestock carbon emissions mainly include CO₂, CH₄ and N₂O [18]. To facilitate the calculation of carbon emissions, all carbon emissions in this study were converted to CO₂-eq according to the global warming potential (GWP).

Table 1. Carbon emission factors of gastrointestinal fermentation and manure management system of livestock and poultry.

Table 1. Carbon emission factors of gastrointestinal fermentation and manure management system of livestock and poultry.

Species	CH4[kg/(head. a)]		N2O [kg/(head. a)]	Reference Source
	Gastrointestinal fermentation	Manure management
Cattle	52.90	3.31	0.85	Zheng et al., (2022) [19]
Mule	10.00	0.90	1.39	Yao et al., (2017) [11]
Donkey	10.00	0.90	1.39
Horse	18.00	1.64	1.39
Pig	1.00	3.50	0.53
Sheep	5.00	0.16	0.33
Rabbit	0.254	0.08	0.02
Poultry	0.00	0.02	0.02

2.1.1. Feed Grain Input System

The system includes two links: the cultivation of feed grains and the processing and transport of feed grains. Soya, maize and wheat in concentrates are the main sources of livestock and poultry feed. As roughage is a by-product of primary processing, its carbon emissions are minimal. It is therefore not included in this study. Corn, soybeans, wheat and other feed raw materials produced from crops must be processed through various processes to become feed, and the carbon emissions from this link should also be included in the boundary of the livestock system. The formula for calculating CO₂ emissions from the cultivation, processing and transport of feed grains is as follows:

$${\mathrm{E}}_{\mathrm{T}\mathrm{Z}}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{Q}}_{\mathrm{T}}·{\mathrm{t}·{\mathrm{q}}_{\mathrm{i}}·\mathrm{e}\mathrm{f}}_{\mathrm{j}1}$$

(1)

$${\mathrm{E}}_{\mathrm{T}\mathrm{Y}}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{Q}}_{\mathrm{T}}·{\mathrm{t}·{\mathrm{q}}_{\mathrm{i}}·\mathrm{e}\mathrm{f}}_{\mathrm{j}2}$$

(2)

Where: E_TZ is the CO₂ emission caused by the cultivation of feed grain; E_TY is the CO₂ emission from feed grain processing and transportation; Q_T is the annual output of T-type livestock and poultry products.; t is the consumption coefficient of unit livestock and poultry products; q_i is the proportion of grain in the feed formula of class i livestock and poultry. ef_j1 is the CO₂ equivalent emission coefficient of type j grain, ef_j2 is the CO₂ equivalent emission coefficient of type j grain processing and transportation (Table 2).

2.1.2. Energy Consumption System

The system includes two links: livestock and poultry breeding energy consumption and slaughtering and processing. In the process of raising livestock and poultry, a large amount of electricity, coal and other energy will be consumed, which will directly or indirectly cause carbon emissions [24]. The amount of CO₂ produced by livestock and poultry feeding can be calculated according to the following formula:

$${\mathrm{E}}_{\mathrm{T}\mathrm{W}}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{N}\mathrm{A}\mathrm{P}\mathrm{A}}_{\mathrm{i}}·\frac{{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}_{\mathrm{i}\mathrm{e}}}{{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{e}}_{\mathrm{e}}}·{\mathrm{e}\mathrm{f}}_{\mathrm{e}}+\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{N}\mathrm{A}\mathrm{P}\mathrm{A}}_{\mathrm{i}}·\frac{{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{t}}_{\mathrm{i}\mathrm{c}}}{{\mathrm{p}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{e}}_{\mathrm{c}}}·{\mathrm{e}\mathrm{f}}_{\mathrm{c}}$$

(3)

Where: E_TW is the CO₂ emission caused by the energy consumption of livestock and poultry production; i is the category of livestock and poultry breeding; NAPA_i is the annual production of category i livestock and poultry; cost_ie is the electricity expenditure per head (only) of type I livestock and poultry; price_e is the unit price of electricity for livestock and poultry breeding; ef_e is the CO₂ emission coefficient of power consumption; cost_ic is the expenditure of coal per head ( only ) of livestock and poultry in category i; price_c is the unit price of coal for livestock and poultry breeding; ef_c is the CO₂ emission coefficient of coal consumption (Table 2).

Energy consumption in the process of livestock and poultry slaughtering and processing also needs to be included. Livestock and poultry must be disinfected, pack-aged and processed into livestock products in order to be sold as commodities in the market. Carbon emissions will also be generated in this process. The calculation formula is as follows:

$${\mathrm{E}}_{\mathrm{T}\mathrm{R}}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{Q}}_{\mathrm{T}}·\frac{{\mathrm{M}\mathrm{J}}_{\mathrm{u}}}{{\mathrm{e}}_{\mathrm{n}}}{\mathrm{e}\mathrm{f}}_{\mathrm{e}}$$

(4)

Where: E_TR is the CO₂ emission from livestock and poultry slaughtering and processing; Q_T is the annual output of T-type livestock products; MJ_u is the energy consumption of slaughtering and processing of animal products per unit of u; e_n is the calorific value of one degree of electricity; ef_e is the CO₂ emission coefficient of power consumption (Table 2).

2.1.3. Gastrointestinal Fermentation and Feces Management System

The gastrointestinal tract of livestock and poultry plays a role under anaerobic conditions. The rumen fermentation of ruminants produces a large amount of CH₄ gas, which is the main source of CH₄ emissions [25]. The calculation formula of CH₄ emissions from gastrointestinal fermentation of livestock and poultry is as follows:

$${\mathrm{E}}_{TD}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{A}\mathrm{P}\mathrm{P}}_{\mathrm{i}}·{\mathrm{e}\mathrm{f}}_{\mathrm{i}1}$$

(5)

Where: E_TD is the CH₄ emission from gastrointestinal fermentation of livestock and poultry; i is the category of livestock and poultry breeding; APP_i is the annual average feeding amount of i-type livestock and poultry; ef_i1 is the CH₄ emission factor of gastrointestinal fermentation of livestock and poultry (Table 1).

The degradation of livestock and poultry manure under anaerobic conditions will produce CH₄ gas. At the same time, the nitrogen contained in the manure will undergo nitrification and denitrification, and the nitrogen contained in the protein in the manure will be converted into N2O gas. [26,27,28,29] Calculating the carbon emissions of the manure management system includes the following two steps.

$${\mathrm{E}}_{TQ}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{A}\mathrm{P}\mathrm{P}}_{\mathrm{i}}·{\mathrm{e}\mathrm{f}}_{\mathrm{i}2}$$

(6)

$${\mathrm{E}}_{TK}=\sum _{\mathrm{i}=1}^{\mathrm{n}}{\mathrm{A}\mathrm{P}\mathrm{P}}_{\mathrm{i}}·{\mathrm{e}\mathrm{f}}_{\mathrm{i}3}$$

(7)

Where: E_TQ and E_TK are CH₄ and N₂O emissions from livestock and poultry manure management system, respectively. i is the category of livestock and poultry breeding; APP_i is the annual average feeding amount of i type livestock and poultry; ef_i2 and ef_i3 are CH₄ and N₂O emission factors of livestock manure management system (Table 1).

2.1.4. Total Animal Husbandry Carbon Emissions

According to the life cycle assessment method, the total GHG emissions from the livestock sector are calculated using the formula:

$${\mathrm{E}}_{\mathrm{T}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}={\mathrm{E}}_{\mathrm{T}\mathrm{Z}}+{\mathrm{E}}_{\mathrm{T}\mathrm{Y}}+({\mathrm{E}}_{\mathrm{T}\mathrm{D}}\text{•}{\mathrm{G}\mathrm{W}\mathrm{P}}_{{\mathrm{C}\mathrm{H}}_4}+{\mathrm{E}}_{\mathrm{T}\mathrm{Q}}•{\mathrm{G}\mathrm{W}\mathrm{P}}_{{\mathrm{C}\mathrm{H}}_4}+{\mathrm{E}}_{\mathrm{T}\mathrm{K}}\text{•}{\mathrm{G}\mathrm{W}\mathrm{P}}_{{\mathrm{N}}_2\mathrm{O}})+{\mathrm{E}}_{\mathrm{T}\mathrm{W}}+{\mathrm{E}}_{\mathrm{T}\mathrm{R}}$$

(8)

Where: E_Total is the total carbon emissions of animal husbandry life cycle measured by CO₂ equivalent; E_TZ, E_TY, E_TD, E_TW and E_TR represent the production of feed grain planting, feed grain processing and transportation, livestock and poultry gastrointestinal fermentation, livestock and poultry breeding energy consumption, livestock and poultry slaughtering and processing, respectively. E_TQ and E_TK are CH₄ and N₂O emissions from the fecal management system; ${\mathrm{G}\mathrm{W}\mathrm{P}}_{{\mathrm{C}\mathrm{H}}_4}$ is the global warming potential of CH₄, taking 21, ${\mathrm{G}\mathrm{W}\mathrm{P}}_{{\mathrm{N}}_2\mathrm{O}}$ is the global warming potential, taking 310 [22].

3. Results

4. Conclusions and Policy Recommendations

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