4.1. Methodological Challenge: Accounting for the Consumption of Natural Resources
The present study is one of the first to include the emissions resulting from the consumption of natural resources in the assessment. In traditional Iberian systems, the excretion of nutrients associated with the consumption of natural resources contributes to CC, AC, EU and LO impacts. This contribution applies to the different outdoor stages. For CC, AC and EU the contribution of natural resources corresponded to on-farm emissions of fatteners montanera and to one part of the on-farm emissions of the other outdoor stages (sows, growers and fatteners cebo campo). For LO, the contribution of natural resources was associated to both the surface of dehesa occupied by fatteners montanera during approximately 100 days and the surface of grasslands used for the other outdoor stages (sows, growers and fatteners cebo campo).
In systems depending on the consumption of natural resources by outdoor pigs, like the traditional Iberian production system, it appeared relevant to consider the acorn and grass intakes in the life cycle perimeter. Indeed, the contribution of fatteners
montanera reached at least 5% of CC, 6% of EU and 10% of AC (
Figure 3a). The final contribution of the consumption of natural resources was higher because there was also contribution from the other outdoor production stages. The fatteners
montanera accounted for at least 79% of LO (
Figure 3a) because of the large surfaces used (between 0.2 and 1.11 pigs per hectare).
When Iberian farms have both
montanera and
cebo campo systems, the contribution of natural resources to impacts decreases due to greater dependence on compound feed. In these farms, the contribution of fatteners
montanera (depended on natural resources) was reduced to half for CC, AC and EU impacts in comparison with farms with only fatteners
montanera but remained predominant in LO impact (
Figure 3b).
Depending on the type of natural resource consumed by pigs during each productive stage, the emissions may vary. Consumption of acorns should result in low emissions due to their low N and digestible fibre content and high organic matter (OM) digestibility [
34]. Consumption of grass should have higher contribution to emissions, because of its higher digestible fibre and crude protein content, i.e., 22.20% of CP compared to 5.88% of CP for acorn [
34]. Therefore, when animals consume grass, higher N excretion and emissions of NH
3, NO
X and NO
3 are expected. Additionally, because of its higher digestible fibre content, higher enteric CH
4 emissions are expected [
37,
41]. Grazing by the animals is related to enteric fermentation [
3]. Therefore, it was relevant to account for natural resources in the LCA of the Iberian traditional pig production system, since these resources are consumed during all production stages. Indeed, although
dehesa can be considered a natural ecosystem without intervention from farmers, N and OM ingested from acorns and grass are provided to the environment in a form (urine and faeces) that is expected to result in higher volatilization than natural degradation on the ground. Therefore, we have recommended the inclusion of emissions derived from the consumption of natural resources in the perimeter of an LCA when dealing with systems with outdoor pigs, because it contributes to about 10% of CC, AC and EU.
If the intake of natural resources had not been included in the assessment (
Table 5), no emissions of NH
3, N
2O, NO
X, NO
3, CH
4 would have been considered in fatteners during
montanera. Additionally, emissions estimated of NH
3, N
2O, NO
X, NO
3 would have decreased by 40% in growing pigs. In the case of fatteners
cebo campo, the decrease in these emissions would have represented only 10%, due to a higher intake of compound feed per day and a decrease in grass intake [
15]. Furthermore, the emissions calculated of enteric CH
4 and CH
4 from the faeces and urine deposited would have also decreased in these production stages. The reduction of emissions in sows was not calculated since lactating sows have access to natural resources only in some farms (farrowing in huts). Consumption of natural resources was not considered for piglets, because of the post-natal development of the digestive system [
45].
4.2. Environmental Impacts of Iberian Traditional Pig Production Systems vs. Other Pig Production Systems
This study provided the first life cycle assessment of traditional Iberian pig production. Several authors estimated environmental impacts of pig production (
Table 6). Conventional, organic and traditional systems have already been assessed through LCA [
4]. Therefore, in this section we compare the environmental impacts of pig production in traditional Iberian systems with conventional, organic and traditional systems investigated in the literature. We also provide insights on the potential mitigation of impacts through the consumption of natural resources by free-ranging pigs.
In Iberian pig production systems, CC, AC, EU and LO values (
Table 4) are higher than those obtained in conventional systems (
Table 6). Indeed, conventional systems are commonly based on high productivity, while extensive systems promote carcass quality over quantity [
46]. CC in conventional systems ranges between 2.22 and 2.89 kg CO
2 eq [
9,
24,
47,
48,
49,
50,
51] while EU ranges between 0.014 and 0.023 kg PO
43− eq [
9,
24,
47,
49,
50] and AC between 0.058 and 0.063 molc H
+ [
52]. Iberian pig systems had higher CC, AC and EU impacts than conventional systems, in both farms with only fatteners
montanera (3.40 kg CO
2, 0.091 molc H
+ and 0.046 kg PO
43− eq, respectively) and farms with fatteners
montanera and
cebo campo (4.36 kg CO
2, 0.110 molc H
+ and 0.057 kg PO
43− eq, respectively). This was due to the lower feed efficiency of the traditional breeds [
53]. LO in this study (31.60 and 43.01 m
2·year) was lower than LO in a conventional system reported in the literature [
9,
47,
51]. It resulted from land surfaces required for fatteners
montanera [
23]. CED in Iberian farms with fatteners
montanera and
cebo campo (31.60 MJ) was higher than CED in conventional production, due to large quantities of compound feed distributed during growing and finishing periods to fatteners of
cebo campo. However, CED in Iberian farms using only
montanera (20.65 MJ) is approximately equivalent to the highest values (16.22 and 20.80 MJ) in conventional systems [
9,
24]. Environmental impacts of Iberian pig production in traditional systems are reduced when using only the
montanera system for finishing (vs.
cebo campo). Therefore, relying on the ability of Iberian pigs to consume acorns and to valorise them with compensatory growth allows these systems to reach environmental impacts close to those of conventional systems despite lower growth potential.
Iberian systems have CC, EU and LO impacts (
Table 4) rather greater than organic systems (
Table 6), since these systems exhibit CC values between 2.20 and 2.92 kg CO
2− eq [
8,
9,
10], EU values from 0.016 to 0.029 kg PO
43− eq [
8,
9,
47] and LO values from 6.9 to 9.87 m
2·year [
8,
9,
47]. Halberg et al. [
8] additionally assessed a free range organic system, and calculated a CC value (3.32 kg CO
2 eq) close to those of Iberian farms with fatteners
montanera (3.40 kg CO
2− eq). A long productive cycle with high slaughter weight and a low feed efficiency [
17,
53] were the main causes of higher impacts in Iberian systems than in organic systems. In contrast, the CED value obtained (
Table 4) in this study was near the CED value in organic systems (18.08 MJ [
9]) when finishing pigs were fed only with natural resources. In addition, Basset-Mens et al. [
47] obtained higher CC and CED values in the organic system than Iberian farms for fatteners
montanera but lower than Iberian farms with fatteners
montanera and
cebo campo.
In recent years, environmental impacts of traditional pig production were estimated by several authors [
9,
11,
12,
13,
14,
15]. The values (
Table 6) vary greatly due to different farm managements and breeds. For this reason, CC, EU and CED impacts in Iberian systems are within the range of values found in the literature (3.03 to 9.35 kg CO
2 eq, 0.026 to 0.054 kg PO
43− eq and 14.40 and 35.60 MJ, respectively). Traditional indoor systems had lower EU values, while traditional outdoor systems like the Iberian one obtained the highest EU values. Traditional mixed systems (indoor and outdoor) obtained EU intermediate values. Indeed, the higher efficiency of intensive systems needs less inputs per functional unit than extensive systems [
3]. Finally, LO values in Iberian systems are higher than LO values in other traditional systems (6.43 to 22.70 m
2·year) primarily due to the use of a large surface in the fattening period. A traditional Corsican system [
13] obtained the lowest CC, EU, CED and LO values (1.47 kg CO
2 eq, 0.012 kg PO
43− eq, 7.70 MJ and 2.14 m
2·year, respectively) of the assessed traditional systems.
Consumption of natural resources (chestnuts, acorns and grass) in LCA was only previously considered by Monteiro et al. [
15] who calculated the emissions from grazing in different systems using local breeds. Espagnol and Demartini [
13] did not considered the natural resources in the LCA perimeter when assessing the environmental impacts of Corsican pig production systems. Environmental impacts obtained in traditional Iberian systems (
Table 4) were in line with the values obtained with other European local breeds [
15]. However, values obtained in this study were higher than the ones obtained by Espagnol and Demartini [
13] for the Corsican system.
One of the causes for high impacts in Iberian pig systems is that Iberian pig protein deposition is potentially lower than in modern highly selected breeds [
53]. Additionally, the animals reared in free-range use the calories to cover energy requirements for maintenance and the energetic cost of grazing [
32]. As a result, the feed efficiency was also lower in this system. The use of natural resources from the
dehesa [
17] together with the ability of Iberian pigs to shell acorns [
54] may compensate the lower feed efficiency when animals are fed with natural resources. Indeed, when the ratio of production from
cebo campo to production from
montanera increased, the different environmental impacts increased and vice versa (
Figure 4). This was due to greater use of compound feed in
cebo campo. Furthermore, a low distribution of compound feed per day during the growing period in Iberian pigs causes compensatory growth in
montanera, which increases feed efficiency and contributes to reduced N excretion and environmental impacts. According to Stanley et al. [
55], the management of fatteners
montanera (extensive fattening based on natural resources in a large area with rotational management) provides environmental benefits (such as soil C sequestration and other ecosystem services) and lower environmental impacts per kilogram carcass weight. Therefore, the
montanera system generates lower total meat production, although it is produced with greater environmental benefits than the
cebo campo system.