## 4 Discussion
### Energy Efficiency Measures, Renewable Energy Sources Adoption, and Demand-Side Policies  
Considering the effect that the livestock sector-in general-has on numerous natural resources, and given the increasing scarcity of land, soil, water, and biodiversity, it is evident that the increasing usage of renewable energy sources (RES), Energy Efficiency Measures (EEM), and technologies throughout the world, provide a unique opportunity to farmers to reduce their farms' external inputs by producing their own energy and becoming more self-sufficient. With an initial investment, energy can be harvested from variable renewable energy sources, such as the sun, wind, and water. These sources, in addition to biomass, heat pumps, and geothermal energy, can produce electricity and fuel to adequately cover partly, or exclusively, depending on the size, the on-farm energy demand. In recent years, increasing research has been undertaken to improve the energy efficiency and sustainability of livestock housing. This includes developments that improve structures [62], insulation [66; 67], and climatic systems [68; 69]. RES are increasingly gaining attention as a sustainable energy source in the livestock farm sector [70], under specific circumstances and energy needs. At first glance, one might think that all renewable energy sources can be used efficiently in all types of farms. Nevertheless, their effectiveness is associated with a combination of factors that can be assessed with confidence to conclude whether a RES technology or even an optimal combination of them is/are capable of meeting the needs of the respective livestock farm. The following factors are indicative:  
* Position of the livestock building (e.g., location and orientation);
* Local environment (e.g., climate, elevation and slope, soil water, ground temperature, etc. );
* Characteristics of the buildings that constitute the farm unit (building insulation, existing of ventilation fans, etc. );
* Domestic needs of the farm (e.g., high energy loads for milk cooling, air ventilation, management of the animals' by-products, etc. ); and consumption profile (daily, seasonal);
* Planning policies for RES technologies in rural areas [71]. According to different studies, the location and the orientation of the livestock building are both important in most cases since they can determine which RES technology and in which place of the farm could potentially be applied, thus increasing the efficiency and livestock productivity. In the same line, the local climate, as well as the construction method of the livestock farm, can play a vital role that could potentially lead to a reduction in the final energy consumption of the livestock building. Our results suggested that reducing the reliance on and energy intensity of animal feed, especially imported animal feed, could reduce overall energy use considerably. On the one hand, the EU market for feed is moving towards more locally produced, although a significant deficit in high-protein feed remains, despite a large increase in EU-grown soy and other protein sources. In addition, multiple studies have shown that grass-fed cattle consume less energy than those fed on other types of feed. However, switching to grass-feeding would require significant amounts of arable land and agricultural inputs [49], highlighting the importance of a carefully balanced transition. Since a significant amount of the energy associated with feed is for the production of cereals and oilseeds, finding other feedstocks could reduce the energy intensity of feed. For instance, EIP-AGRI (2020) identified five new feed options for pig and poultry farming that would reduce the environmental footprint of animal feed: bakery products, green biomass (grass/clover), insects, micro-algae, and single-cell protein [72]. It is important to highlight the importance and the potential of livestock manure as a source of organic fertilizer and renewable energy production in the EU.