1. Introduction
The increasing demand of livestock products and the search for an increase of livestock productivity have induced the rapid transition of livestock breeding systems from pastoralism into intensive systems. This search for livestock productivity has been based on the genetic selection of improved breeds to the detriment of indigenous ones adapted to the various agroecosystems and on the change in animal feeding associated to the growing tendency to house the livestock. Extensive livestock husbandry systems and their traditional use of pastures have been substituted by high livestock density or even landless systems with an intensive diet. These systems are dependent on the globalized supply of corn and soybean or other grains produced by export monocultures in lands that could be used to provide human food or occupied by old-growth forests functioning as irreplaceable carbon sinks [
1]. This intensification has led to a separation of livestock and territory, a process that is interrupting nutrient flows and causing soil organic matter depletion in the territories where animal feed is originally produced, often also generating pollution in the place where the livestock is housed [
2]. Direct environmental impacts of this process include groundwater pollution, the increase of anthropogenic greenhouse gas emissions [
3,
4], concentration of polluting waste [
5], and growth of energy consumption [
6]. Indirect impacts, most of them related to animal feeding, are also to be considered, including deforestation, soil erosion, and loss of biodiversity [
7], particularly in the main intensive agricultural regions [
8]. Therefore, the initial hypothesis of the present work is that the global disconnection of crops and livestock from pasturable resources implies a lack of efficiency in the use of natural resources and a greater environmental impact of confined livestock farming [
9].
Within the debate on sustainability, energy is a central and cross-cutting element affecting all economic activities [
10,
11,
12], especially in a context of oil depletion and climate change [
13], that urges a move toward food provision systems based on renewable energy, low energy intensity, and rates of energy return above the unit [
14]. Therefore, the fostering of decision-making processes favoring energy sustainability require to develop new analytical methodologies and indicators that allow understanding and evaluating, in a comparative way, the energy-environmental implications of technology and production-related decision-making within farms. Energy indicators, as well as sustainability indicators, operate to make visible the potential environmental benefits derived from energy efficiency, and consider energy and material flows that are usually neglected. These tools need to be particularly adaptable to management specificities in the agro-livestock systems that are being analyzed [
15].
This paper develops and implements a series of indicators specifically designed for the study of goat farming sustainability (Final energy return on investment—EROI, NR Final EROI, Food/feed EROI, etc.). These indicators allow the energy implications of the different types of management associated with this activity to be assessed and compared, focusing mainly on the role of animal feeding and the use of pasturable resources. The livestock farm, understood as a techno-productive and economic decision-making entity, is therefore the unit of analysis. An agroecological approach is applied that views stockbreeding as part of a complex agroecosystem exchanging energy flows with other natural and social systems [
16,
17]. From this point of view, the socialized output is a fraction of the total output intended to meet human needs mostly through the market. However, the total output is a broader factor that includes internal energy flows, some of them with no direct human-oriented purpose, but potentially contributing to ecological balance, like manure. This agroecological approach allows bringing to light and analyzing the internal energy flows between pastoral agroecosystems and the use of manure, both of which are central elements of the energy and biophysical metabolism of livestock husbandry, even if they have no market value or direct and immediate human-oriented use. In addition, from an ecological economics approach, new indicators are proposed to identify and make visible the avoided energy costs associated with pastoralism (“avoided energy cost of manure” AEC
M or “avoided energy cost” and “avoided energy land cost” of natural pasture AEC
P and ALC
P).
Energy analyses based on indicators, such as the EROI (energy return on investment), allow measuring of the energy efficiency of agriculture. This important analytical tool provides valuable information on the energy cost of producing food and feed [
18,
19]. However, different authors have recently criticized the insufficiency of this conventional approach, which reproduces the cost-benefit logic in terms of energy and treats agricultural systems as unexplored black boxes, ignoring internal energy flows that do not enter the market, but are crucial for the ecological balance of the agroecosystem [
17]. In the case of livestock husbandry, for instance, pastoralism enables using the biomass produced by agroecosystems and largely reincorporating it into the systems in the form of organic fertilizer. These internal energy flows, which are not usually taken into consideration, contribute to maintaining the structure and functions of the ecosystem and, therefore, sustain the flow of ecosystem services [
20]. In particular, manure is a central element of the nutrient flow within the agroecosystem and of the soil structure, despite the loss of energy in the form of heat during the decomposition process. Manure is a crucial contribution to the net primary biomass and energy production of the agroecosystem in different production cycles over time. To overcome economicism, recent agrarian energy studies have suggested the implementation of an agroecological approach to analyze the complexity of the systems’ biophysical interactions, exchange of flows with the environment, and potential environmental benefits, independent of their market orientation, as well as to widen the understanding of the recirculation of internal flows generated by agroecosystems (see [
16]). However, even if some previous studies have analyzed the use of energy in livestock husbandry activities [
21,
22] and in the production of cow milk [
6,
23,
24] or goat milk [
25], very few works have adopted this agroecological approach to evaluate the use of energy in livestock farming systems [
26].
On the other hand, the scarcity of energy resources in economic processes is a central element in the analysis of energy viability. By applying an ecological economics approach [
27], this scarcity can be analyzed in terms of “biophysical opportunity cost”. The concept of opportunity cost, proposed by the Austrian marginalist school at the end of the 19th century, refers to the benefit or monetary revenue that is waived when choosing one use among the different possible uses of a resource. Thus, the biophysical opportunity cost is especially relevant in conflicts between alternative human uses, whether there is a monetary value attached to them or not [
28]. For instance, in energy analyses of agriculture, the solar energy used during photosynthesis is often considered a “free” input, i.e., a resource with no opportunity cost that cannot be depleted or degraded by human use [
29]. This “free-of-charge” concept of solar energy introduces a moderate anthropocentric bias in energy use analyses and allows the EROI to reach a value that is above the unit. Along those lines, the energy input of natural pasture, contributed through grazing, has no biophysical opportunity cost and may also be considered a free input because it is not digestible for human beings and its use for livestock feeding is not competitive in these terms. However, in contrast with the solar flux, an improper management of pastures, for instance through overgrazing, may indeed deplete or degrade the resource by exceeding its bearing capacity. In this sense, pastures should not be considered unlimited and freely available economic resources, but the biomass generated by them and used by the livestock can be conceptualized as a resource without any human biophysical opportunity cost in relation to the food/feed debate.
On the contrary, the use of grains for animal feeding has a clear human biophysical opportunity cost, given that grains can be destined to human consumption. Thus, food/feed competence is one of the central debates around the sustainability of livestock husbandry [
30]. Devoting energy resources that are edible for human beings to livestock feeding reduces the energy efficiency of animal food production as compared to that of agricultural systems. On average, the net energy used by ruminants for maintenance, milk production, and fattening amounts to 41%, 34%, and 25% of the gross energy ingested by them, respectively [
31]. In addition to the energy cost, the production of fodder has a biophysical opportunity cost in terms of territory. The livestock breeding industry is not only responsible for 18% of the greenhouse gas emissions [
32,
33], but it also accounts for 80% of the total anthropogenic land use, with grazing land for ruminants covering about 70% of the global agricultural land, and feed crops occupying 34% of the global cropland [
34].
Finally, another relevant concept to be integrated in energy analyses of livestock husbandry is the “avoided cost”. The avoided cost allows the identification of the benefits derived from choosing one alternative instead of another. In the field of environmental economics, this concept was initially proposed as a monetary indicator [
35] and it has been recently reinterpreted from the biophysical perspective of ecological economics. Thus, for instance, Arto et al. [
36] have measured the emissions avoided by international trade in Spain, while Ruisheng et al. [
37] have quantified the avoided environmental impacts of recycling wood waste in Singapore. The present work suggests applying the concept of avoided cost to manure and natural pasture. The use of manure as fertilizer avoids the environmental cost of using the energy equivalent of inorganic fertilizers [
29]. The energy accounting of manure is, in addition, a measure of the potential environmental benefit of incorporating biomass to the agroecosystem. The use of natural pastoralism avoids the energy costs associated with the production of livestock feed [
38], in addition to reducing pressure on the cropland.
This article has the general objective of proposing new energy sustainability indicators for livestock husbandry, particularly goat farming, according to the above-defined theoretical premises. More specifically, it proposes indicators that allow the highlighting and assessing of the potential environmental benefits derived from the sustainable use and management of manure as organic fertilizer and from animal feeding based on the sustainable use of pastures. Empirical evidence is presented for the three productive goat farms studied. Analyzing these farms, located in the Sierra de Cádiz Natural Park, and presenting different levels of extensiveness makes it possible to demonstrate the analytical possibilities of the sustainability indicators proposed.
4. Conclusions
Including an agroecological economics perspective in the energy analysis of livestock husbandry helps reveal the internal energy flows of livestock systems, which generate important environmental benefits and are usually neglected in conventional analyses. Reformulation of the concept of output shows the relevance of manure in terms of energy, which in the cases analyzed is estimated at 68% of the total output. This contribution of energy and biomass is essential for the balance and productivity of agroecosystems based on forestry and pastoralism. The contribution of grazing represents between 17 and 40% of the cumulative energy demand (CED) depending on the farm’s level of extensiveness, showing how the use of pasturable resources can reduce dependence on external feed, which is, nevertheless, the main energy consumption component in the three cases studied (between 54 and 87% of the CED). This potential positive relation between the use of pasturable resources and energy efficiency must be valued and developed. In the cases studied here, the agroecological economics measurement of energy efficiency shows how the most extensive farm (F3) was 53% more efficient than the semi-intensive farm (F1) in terms of energy and non-renewable energy use. This was calculated through the final EROI and the non-renewable (NR) final EROI. A new measure of energy efficiency based on the final EROI and taking into consideration both manure and grazing is thus required.
The avoided costs associated with grazing and with the organic fertilization linked to it highlight, in terms of energy, the environmental benefits of the interaction between extensive livestock systems and agroecosystems. These two agroecological energy indicators make it possible to evaluate livestock husbandry sustainability by taking into consideration certain aspects of the management that are usually neglected by conventional sustainability analysis. The energy cost avoided by manure in the cases analyzed represents between 15.7 and 26.7% of the NR CED of the farms. The natural pasture consumed through grazing provides between 24.5 and 51.1% of the gross energy required by the livestock, depending on the farm’s level of extensiveness, and the avoided cost of natural grazing is estimated at 35.5, 60.19, and 157.4% of the NR CED of the three farms, from the least to the most extensive one. The reduction of territorial pressure on the cropland resulting from the use of grazing is estimated at between 0.60 and 1.31 hectares per 1000 L of milk. These results point out that the global disconnection between crops and livestock, on the one hand, and pasturable resources, on the other, can lead to inefficiency in the use of natural resources.
The quantitative results of the three cases studied have been used to demonstrate the analytical potential of the new energy indicators proposed. However, this information should be treated with caution and should not be generalized as it is highly context-related. The main objective of this article was to reinforce the idea that it is necessary to widen and sophisticate the conventional vision of energy analyses on agricultural and livestock systems. For this purpose, it is necessary to incorporate the assessment of the internal energy flows associated with grazing and organic fertilization to adequately value the energy efficiency and environmental benefits of livestock farming systems. This work aims at contributing to this objective by proposing new synthetic energy indicators for the assessment of goat farming sustainability.