## 2 Digital Twins Concept Integration in Livestock Farming
### The Digital Twins Concept in Livestock Farming  
The livestock sector is undergoing rapid development on a worldwide level as a result of the growing demand for animal products, driven by the need for food safety [44] due to population increase and rapid urbanization, as well as stimulating prosperity in the industrial world and most developing countries [45]. In addition to the potential benefits associated with the growing development of the livestock sector, a number of challenges seem to arise from operational, socio-economic, environmental, and sanitary issues [30]. That which is characteristic of livestock production systems (at present) is their intensive nature, which is associated with both greater volumes of inputs (i.e., farm size, manpower, financial resources, etc.) and outputs per production holding. In this perspective, the number of holdings is declining, while the number of animals reared in closed, environmentally controlled, automated facilities is rising [46]. Moreover, vertical integration is often embedded in the view that a single company can be in charge of the overall production system (e.g., from the feed mill to the processing plant), which is configured to achieve maximum output with all of its components running consistently at full load [47]. Figure 2: The basis of the digital twins concept. Therefore, whatever disorder may occur at any stage of the production chain will lead to a direct impact on both the upstream and downstream levels, with direct implications not just for animal welfare but for humans and the wider environment as well. This fact is aligned with the emergent notion of \"One Welfare\", an approach which, as reported in [48], gives recognition to the immediate and implicit associations between animal welfare and the well-being of people and the broader environment. In response, the entire digitization of livestock production systems is often suggested [17] in the context of Agriculture 4.0 and, consequently, Agriculture 5.0 [5; 7]. According to this approach, computationally enabled technologies [8], incorporating smart sensors, WSNs, AI and ML, Edge computing, big data analytics, the IoT, and other cutting-edge technologies, with DTs among them, are considered to be of great importance. To this end, when integrating the DTs concept in livestock farming, the physical state accounts for a physical system that embodies the composite and dynamic environment of livestock facilities, with the boundaries of the DTs specified [49]. Herein, it has to be noted that the physical system is a fundamental component of a DT since the lack of it implies that the physical state merely represents a model [50]. To this end, the physical system involves the fundamental features of the incorporated physical objects and devices [51]; it can be comprised of an entire object with individual components found in a physical environment, a single component of the object [50], or an individual property of the object that is linked to other objects. In the case of livestock production systems, the physical system may indicatively consist of an entire facility, including animals, holdings, machinery, equipment, and feeding strategies, as well as a number of animals or just an animal body [52]. Finally, monitoring and sensing devices [53] are incorporated into the physical system so as to collect the data required. Accordingly, the virtual state accounts as a representation of the physical state model and data. By involving data mining, processing, and analytics by means of AI and ML techniques, the virtual state provides feedback for supporting decision-making and controlling the physical system so as to foresee any obscure matters and optimize its operation [54]. Finally, the twinning, which enables the interconnection between the physical and virtual state, accounts for the transmission of data from the physical system to its virtual twin, as well as for the reverse information feedback [55] to the livestock production system.