Assessing Carbon Emissions from Animal Husbandry in China: Trends, Regional Variations and Mitigation Strategies

Livestock and poultry feeds are predominantly classified into two main categories: roughage and concentrated feed. Roughage, comprising by-products like corn stalks, rice straw, and sweet potato vines, is produced via a single processing stage and results in minimal carbon emissions; therefore, we excluded it from consideration [12]. Concentrated feed primarily consists of corn, soybeans, and wheat. Based on the carbon emissions that result from chemical fertilizers, pesticides, and agricultural films, the energy required to produce concentrated feed grows. The specific calculation equation is as follows:

where ${\mathrm{T}\mathrm{C}}_{\mathrm{f}\mathrm{p}}$ represents carbon emissions produced from feeding grain crops during planting. u represents the types of livestock products, including beef, mutton, pork, poultry, eggs, and milk. ${\mathrm{Q}}_{\mathrm{u}}$ represents the annual output of class u with livestock products (t). ${\mathrm{s}}_{\mathrm{u}}$ represents the grain consumption coefficient of class u livestock and poultry products for class j grain (kg/kg), and ${\mathrm{q}}_{\mathrm{j}}$ represents the proportion of class j grain in the livestock and poultry (%) feed formula, primarily including corn, wheat, and soybean (Table 1). ${\mathrm{e}\mathrm{f}}_{\mathrm{j}1}$ represents the ${\mathrm{C}\mathrm{O}}_2$ equivalent emission coefficient of class j feed grain during planting (t/t) (Table 2). GHG emissions generated from soybean planting are not included in the calculation, as bean cake is a by-product of soybeans’ primary processing [12]. ${\mathrm{t}}_{\mathrm{j}}$ represents the amount of a chemical fertilizer used per unit area of class j grain (kg/${\mathrm{h}\mathrm{m}}^2$), ${\mathrm{m}}_{\mathrm{j}}$ represents the sown area of class j grain (${\mathrm{h}\mathrm{m}}^2$), ${\mathrm{B}}_{\mathrm{j}}$ represents the proportion of class j grain (%), and ${\mathrm{e}\mathrm{f}}_{\mathrm{h}}$ represents the ${\mathrm{C}\mathrm{O}}_2$ emission coefficient of the fertilizer.

Corn, soybean, wheat, and other food crops require a series of processing steps including cleaning, drying, crushing, and mixing with other ingredients, as well as various transportation steps, before being transformed into feed for livestock and poultry. This phase primarily concentrates on GHG emissions generated from energy consumption during transportation and processing operations [12]. The equation for calculating ${\mathrm{C}\mathrm{O}}_2$ emissions associated with the transportation and processing of a feed grain is as follows:

where ${\mathrm{T}\mathrm{C}}_{\mathrm{t}\mathrm{p}}$ represents the ${\mathrm{C}\mathrm{O}}_2$ emission of a feed grain during the transportation and processing stages; ${\mathrm{s}}_{\mathrm{u}}$ represents the grain consumption coefficient of class u-type livestock and poultry products of the unit (kg/kg); ${\mathrm{q}}_{\mathrm{j}}$ represents the proportion of class j grain, primarily including corn, wheat, and soybean, in the livestock and poultry feed formula (%) shown in Table 1; and ${\mathrm{e}\mathrm{f}}_{\mathrm{j}2}$ represents the equivalent emission coefficient of class j grain during the processing and transportation stages.

Gastrointestinal fermentation in livestock and poultry leads to the generation of GHGs, such as ${\mathrm{C}\mathrm{H}}_4$. Rumen fermentation in ruminant livestock, such as cattle and sheep, constitutes the primary source of ${\mathrm{C}\mathrm{H}}_4$ emissions. It accounts for more than 80% of the total gas emitted from the intestines of all livestock and poultry. Non-ruminant livestock, such as horses, mules, and donkeys, and monogastric animals, like pigs, produce negligible quantities of ${\mathrm{C}\mathrm{H}}_4$ as a by-product of gastrointestinal fermentation. Furthermore, the minimal ${\mathrm{C}\mathrm{H}}_4$ emissions produced through the gastrointestinal fermentation process in poultry are not considered in the scope of this study [12]. The equations for calculating ${\mathrm{C}\mathrm{H}}_4$ emissions resulting from the gastrointestinal fermentation processes in both livestock and poultry are as follows:

where ${\mathrm{T}\mathrm{C}}_{\mathrm{s}\mathrm{f}}$ represents the carbon emission produced by the gastrointestinal fermentation of a livestock, and i represents the type of livestock. ${\mathrm{A}\mathrm{P}\mathrm{P}}_{\mathrm{i}}$ represents the class i livestock’s annual average number (head/100). ${\mathrm{e}\mathrm{f}}_{\mathrm{i}1}$ represents the emission coefficient of ${\mathrm{C}\mathrm{O}}_2$ produced by the gastrointestinal fermentation of class i livestock (Table 3). ${\mathrm{G}\mathrm{W}\mathrm{P}}_{{\mathrm{C}\mathrm{H}}_4}$ represents the global warming potential of ${\mathrm{C}\mathrm{H}}_4$ (Table 2). ${\mathrm{H}\mathrm{e}\mathrm{r}\mathrm{d}\mathrm{s}}_{\mathrm{e}\mathrm{n}\mathrm{d}}$ represents the year-end inventory (head/100). ${\mathrm{D}\mathrm{a}\mathrm{y}\mathrm{s}}_{\mathrm{l}\mathrm{i}\mathrm{v}\mathrm{e}}$ represents the livestock feeding cycle (das), and NAPA denotes the rate of livestock slaughter (head/100) in one year.

The ${\mathrm{C}\mathrm{O}}_2$ emissions in an anaerobic environment can be determined as follows:

where ${\mathrm{T}\mathrm{C}}_{\mathrm{f}\mathrm{c}}$ represents the carbon emission generated by the manure management system in an anaerobic environment, and ${\mathrm{e}\mathrm{f}}_{\mathrm{i}2}$ represents the ${\mathrm{C}\mathrm{H}}_4$ emission coefficient of class i livestock in the manure management system (Table 3).

The ${\mathrm{C}\mathrm{O}}_2$ emission in an aerobic environment can be expressed as follows:

where ${\mathrm{T}\mathrm{C}}_{\mathrm{f}\mathrm{n}}$ represents the carbon emission generated by the manure management system in an aerobic environment, ${\mathrm{G}\mathrm{W}\mathrm{P}}_{{\mathrm{N}}_2\mathrm{O}}$ represents the global warming potential of ${\mathrm{N}}_2$O (Table 2), and ${\mathrm{e}\mathrm{f}}_{\mathrm{i}3}$ represents the ${\mathrm{N}}_2\mathrm{O}$ emission coefficient of class i livestock in the system of manure management (Table 3).

Raising livestock and poultry requires substantial quantities of electricity and coal for various operations, including lighting, ventilation, and heating within barn facilities [31,32]. The equation for calculating carbon emissions caused by energy consumption is as follows:

where ${\mathrm{T}\mathrm{C}}_{\mathrm{r}\mathrm{c}}$ is the carbon emissions caused by the energy used to raise cattle and poultry. ${\mathrm{C}}_{\mathrm{i}\mathrm{e}}$ and ${\mathrm{C}}_{\mathrm{i}\mathrm{c}}$ denote the unit consumption expenditure of electricity and coal by class i livestock during its feeding cycle (CNY/head), respectively. ${\mathrm{P}}_{\mathrm{e}}$ and ${\mathrm{P}}_{\mathrm{c}}$ represent the ${\mathrm{C}\mathrm{O}}_2$ emission coefficient of electric energy consumption and coal consumption, respectively (Table 2). ${\mathrm{e}\mathrm{f}}_{\mathrm{c}}$ represents the ${\mathrm{C}\mathrm{O}}_2$ emission coefficient (Table 2).

The carbon emissions produced during the production stage are quantified as follows:

where ${\mathrm{T}\mathrm{C}}_{\mathrm{P}}$ represents the carbon emissions of the livestock production stage, and ${\mathrm{e}}_{\mathrm{t}\mathrm{p}\mathrm{f}}$ represents the conversion of an equivalent into a standard carbon coefficient, as shown in Table 2.