## 4 Discussion  
### Energy Use  
Our review confirmed and illustrated some important trends in energy use in livestock systems. Our results, in line with other studies, clearly indicated that animal feed is the largest energy-consuming activity in livestock systems [21]. While the energy needs of the livestock sector are diversified, on-farm energy use is concentrated in housing and manure management. Electricity is generally used for lighting, feeding, and milking systems, while fossil fuels associated with direct energy inputs are often used for manure management and heating. Switching to more renewable electricity and thermal sources could invariably help reduce the amount of fossil energy used for on-farm activities.  
There are a number of reasons why feed is generally the largest energy-consuming activity. From an energy perspective, the production of animal products is inherently inefficient (depending on feed to product conversion ratio) due to metabolic and heat processes. In practice, this means that the majority of energy consumed by animals is expended at maintaining and renewing its cell structure rather than the production of animal 'products' [21]. In addition, the production, transport, storage, and processing of animal feed is, to a large extent, dependent on cereals and other open-field crops, which are produced using an intensive process that consumes large amounts of energy associated with diesel, fertilizers, and pesticides [61].  
It is clear that beef production in the EU consumes by far the most energy per kilogram, followed by pork and broiler production. Geographically, various studies suggested that energy use in livestock housing is likely to be considerably higher in northern Europe as compared to southern Europe due to additional heating required due to climatic conditions [7; 62]. Studies also suggested that livestock grown on locally produced feed requires considerably less energy input than livestock dependent on non-locally produced feed [63; 64]. This suggests that there are likely to be strong links between production systems, feed use, and geographic location; the investigation of these links is an important area for future research. According to the data discussed previously, electricity and fossil fuels constituted the main energy sources for the necessary operation of livestock farms and maintaining the proper conditions within their facilities. However, there is no clear overview of the specific proportion of energy sources in livestock; it is generally accepted that diesel use is the largest energy source. This is similar to other areas in the world, for instance, in the US, according to the US Energy Information Administration, the main source of direct energy consumption occurs in the form of distillate fuel accounting for over 50% of total energy inputs, followed by propane, electricity, and natural gas [65].  
Considering the increasing research interest, obtaining a clearer view of the specific proportions of energy sources in the livestock sector may prove invaluable. The tendency of future works towards the energy analysis based on the primary energy approach, in order to assess the share of primary energy from renewable and non-renewable sources, will be essential to evaluate how the transition toward cleaner energy mixes will affect the sustainability of livestock production [66]. That distinction would considerably improve the assessment of the environmental sustainability of livestock production, but for that to be possible, we believe that a more detailed recording of energy use per activity in the livestock farm facilities should be considered a prerequisite.