## 2 Materials and Methods  
### Selection Criteria, Search Strategy, and Data Collection  
This study conducts a meta-analysis based on the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) in order to conduct a critical review of the energy use in the livestock sector. In order to be included in this paper, studies needed to meet a number of criteria set out by PRISMA. All studies needed to be peer-reviewed, published, and, when relevant, based on detailed LCA methodologies using one of the main LCA software and up-to-date database values. In a few cases, reports were also considered if they were widely cited, publicly available, and published by well-reputed organizations. Our search strategy followed a number of pre-set steps; the relevant studies were identified through keyword searches through SCOPUS and google scholar, while partners in the AgroFossilFree consortium were also requested to provide relevant studies. Studies were then screened for relevance, and data were only extracted/used if the methodology and results were comparable to other LCA studies. Due to this, over half of the identified studies were discarded due to a lack of a detailed breakdown of energy inputs. Instead, they provided only aggregate energy use figures with little or no methodological justification. In total, 22 papers were included in this review. This process also highlighted that different methodologies assess energy use using different parameters and categorizations, which are often not comparable to one another.  
Energy use data were retrieved from each study and compiled. These data were then compared to other existing studies for verification. This process was chosen to account for the scarcity of accurate energy use data. Comparing the main findings of screened LCA studies with other studies placed results in a wider context, allowing for the identification of patterns and the comparison between different approaches and systems. In addition, this process facilitates the identification of future areas of research.  
An operational definition of energy use in the livestock sector was adopted for this study, including energy from both direct and indirect sources. The boundary of this study was all energy used until the farm-gate. Direct energy use refers to all energy that is consumed in the production of livestock that happens on the farm. The direct energy sources included in this study were energy for all on-farm operations and machinery use, crop processing and feeding, milking processes (milking and milk cooling), manure handling, and animal housing (lighting, heating, cooling, dehumidifying, and ventilation). Indirect energy sources refer to energy that is associated with the production of livestock but is consumed prior to the farm. The indirect energy sources included in this study were the production of fertilizers, pesticides, and energy used in animal feed (includes all the energy used to produce animal feed, including for its raw materials). In practice, almost all studies provide data on animal feed as indirect energy sources. Energy use associated with the production of agricultural buildings, equipment, and machinery was not included; this approach was chosen as many studies do not include these data in their energy use data. In addition, there are considerable issues with measuring this agricultural infrastructure accurately as it is often assigned to other sectors, such as industry, and due to the long-time frame associated with the use of this agricultural infrastructure. Our results section presents these results; for a more detailed data breakdown per livestock category and sources, please see the Appendices A and B.  
Conceptually, this study investigated energy use in the largest livestock sectors in the EU, including bovine (beef and milk), swine/pig meat, and poultry (meat and eggs). Other livestock sectors, including sheep and goat, are mentioned briefly but not discussed in detail due to a lack of sufficient reliable data; these are identified as areas for future research.