## 3 Materials and Methods  
Herein, the applications of the recently emerged DTs technology in livestock production systems are identified and studied for improving their outputs and enhancing their sustainability [9]. For this, a systematic review was conducted based on the methodology proposed by other systematic and bibliometric reviews, as per [26, 57, 58] and according to the PRISMA guidelines [59]. Moreover, the methodology introduced in [60] was taken into account since it is considered quite efficient for identifying related research sources based on rigid quality criteria [61]. On such a basis, the review was conducted according to five phases: (a) the determination of the research questions, (b) the selection of the information sources, (c) the planning of the search methodology, (d) the definition of the eligibility criteria, and (e) the selection of contributions. * **RQ1**. What are the later research trends in the application of the DTs technology in livestock production systems? - **RQ2**. In which types of livestock production systems have the DTs technology been applied? - **RQ3**. In what domains of livestock farming does the DTs technology find application? - **RQ4**. What are the potential benefits and current open issues of incorporating the DTs technology in livestock production systems? * _Information Sources Selection._ The search was conducted in the Google Scholar, Scopus, and Web of Science (WoS) academic research databases. Contributions indexed in ScienceDirect, IEEE-Xplore, and SpringerLink were indirectly included, as these are also contained in the aforementioned databases. * _Search Methodology Planning._ The search of the literature was performed based on a combination of keywords closely related to the determined research questions. The principal keyword selected was the term \"digital twin\" associated with the key terms \"livestock\", \"livestock production\", \"smart farming\", \"application\", \"animal\", \"dairy\", \"cattle\", \"swine\", \"pig\", \"pigsty\", \"husbandry\", \"welfare\", \"energy management\", \"environmental control\", \"supply-chain\", \"meat\", \"food supply\", \"use case\", and \"case study\". These keywords were associated with the Boolean operators \"AND\" and \"OR\" so as to provide the most accurate results. * Type of contribution: For the purposes of this review, peer-reviewed journal articles and conference papers published in proceedings that introduce applications of the DTs technology in the livestock farming sector are included. Comparative studies and literature reviews were excluded. - Year of publication: Given the recent emergence as well as the development of the DTs technology in the livestock sector, a 5-year prospecting period was defined from 2018 to 2023; hence, outdated information that could be irrelevant was excluded. - Region: Worldwide contributions were included. - Language: The literature search was limited to contributions in English. - Status of publication: Only accepted and published contributions in indexed journals and conference proceedings were included. * _Contributions Selection._ At first, contributions were selected based merely on the digital twins topic, resulting in 334 papers, among which 114 were indexed in Google Scholar, 94 in Scopus, and 126 in the Web of Science. However, considering that the topicin question refers to the application of the DTs in livestock farming, the number of the selected contributions was eliminated in 87 based on the queries generated by the association of the defined keywords. In this respect, this was considered to be the first step of selection, that is, identification. Among the contributions selected during identification, 48 were indexed in Google Scholar, 16 in Scopus, and 23 in the Web of Science. Then, by screening the titles, abstracts, and keywords of these contributions, the duplicate versions, as well as those with almost identical titles and irrelevant or insufficient content related to the main criteria, were omitted.