4.1 Effect of livestock mobility
Under the conditions analyzed in this study, mobility of livestock is a strategy that promotes climate change mitigation in semi-extensive farms (Fig. 3), reducing the carbon footprint of lamb meat by about one third (Table 3). This is achieved mainly as a result of a substantial improvement of lamb productivity, and an optimal utilization of available forages through grazing of natural and semi-natural grasslands, which minimizes the needs of external feed resources.
Significantly lower consumption of forage and concentrates for sheep and lambs was observed in transhumant farms (Table 1), involving important GHG savings related to the embodied emissions in bought feedstuffs. These emissions are mostly linked to N2O from fertilization, CO2 emissions from agricultural activities requiring fossil fuel consumption, like crop cultivation, processing, and transport, and also GHG emissions associated to direct land use change (LUC) processes, mainly due to CO2 released through deforestation for soybean cultivation in South America.
Conducting seasonal transhumance allows to reduce these feed inputs, and its embodied GHG emissions, by adapting ruminant husbandry to the natural productive cycles of upland and lowland grassland ecosystems, which in the Mediterranean context complement each other throughout the year. In the Iberian Peninsula, natural upland grasslands mostly grow on mountainous areas with high slopes, making cultivation unfeasible. Cold conditions limit plant growth during most of the year, so forage can only be grazed around summer months. In contrast, lowland Mediterranean rangelands go through a summer dry period and maximize plant growth in spring and autumn, with some plant growth also in winter (Manzano Baena and Casas 2010). Still, unbalanced distribution of herbage production along the year implies a management problem for grazing-based livestock systems. Savanah-like landscapes (i.e. dehesa), where grasslands are combined with scattered trees, help to mitigate this issue by: i) extending the grass growing season under the canopy and ii) providing a source of food for harsh periods (e.g. acorns, browsed leaves) that ruminants can utilize as a supplementary resource (García de Jalón et al. 2018).
Livestock mobility also provided positive effects with regards to herd/animal productivity (Table 1), ultimately leading to a higher ratio of lambs sold per ewe (STAT:0.90, THT, THH:1.11–1.12). Transhumant farms showed a significantly lower lamb mortality rate (Table 1), together with an extended longevity of the adult ewes, reflected on lower requirement of annual replacement rates (STAT:17%, THT, THH:14%). A similar pattern was observed by previous studies analyzing static and mobile flocks in the area (Ramo et al. 2018). These differences are attributed to the animal handling provided by transhumant management, which allows animals to graze outdoors continuously along the year, while protecting them from extreme temperatures through seasonal mobility. The negative effect of low ambient temperature on sheep farms is well-known. Cold weather environment is a crucial factor increasing perinatal lamb deaths (Horton et al. 2018), and it also affects lambs rearing process by reducing average daily gain while increasing feed consumption, ultimately leading to a reduction in protein and feed efficiency ratio (Ames et al. 1977).
Differences in animal management among systems also impact direct consumption of fossil fuels (STAT:5.0 kg/ewe, THT: 1.5 kg/ewe, THH:1.2 kg/ewe). Increased diesel use in static farms was attributed to energy demand linked to machinery for cleaning operations and preparation of feed (forage/concentrate) rations.
Farms applying transhumance by truck or on hoof showed very similar results in their CFs, and in most of the parameters studied, although some differences were identified. Transhumance by truck showed a higher diesel consumption than on hoof, which is associated to the road transport of the animals requiring an extra input of fuel. A higher need of concentrates for adult ewes and lambs was observed too (THT: 103 kg/ewe, THH: 98 kg/ewe). Transhumance by truck reduces the time animals are on the move but it involves extending the stay in the upland area during several weeks, so to limit damage to vegetation in the southern rangelands happening through extended grazing pressure (Carmona et al. 2013). This implies an additional consumption of external concentrates. In contrast, farms conducting transhumance on hoof start their journey earlier, taking advantage of the available grazing areas they find along the traditional paths or “cañadas”. The width of these paths, of up to 75m, and the daily displacement of about 24km, provides to the animals the necessary food and time for intake and rest, thus maintaining an adequate body condition (Ramo et al. 2018).
Still, during the journey, ewes expend a significant amount of energy by walking. In our model, this was captured by applying a higher coefficient for computing energy requirements during the travelling periods. This aspect, together with differences in feed quality, are the main factors leading to slightly higher CH4 emissions from enteric fermentation in the farms conducting transhumance on hoof compared to by truck (THT:13.5 kgCO2/kg LW, THH: 13.7 kgCO2/kg LW), which in the end resulted in a very similar overall value of the CF from the two transhumant managements.
The production system determines the profile of GHG emissions obtained in CF, with transhumant herds showing a higher contribution of CH4 in comparison to static herds. Increased use of concentrates in intensive/static systems reduces enteric CH4 emissions due to improved feed digestibility, but it involves increasing CO2 and N2O contribution linked to fossil fuel consumption and crop cultivation. A similar trend has been reported in previous studies (Vigan et al. 2017; Ripoll-Bosch et al. 2015). Climatic implications of these GHG profiles must be carefully analyzed, especially when analyzing dynamic scenarios, due to the different behavior of long-lived pollutants (i.e. CO2, N2O) versus short-lived (i.e. CH4) (del Prado et al. 2021).
Establishing comparisons among LCA studies of livestock systems is difficult, as methodological choices and modelling approaches have a strong influence on the results. Overall, the CFs estimated for all farms in our study are within the ranges reported for sheep systems in Spain (Ripoll-Bosch et al. 2013), but also for sheep systems in other Mediterranean (Ibidhi et al. 2017) and Northern European (Morgan-Davies et al. 2021) contexts. For the same region as our analysis, Ripoll-Bosch et al. (2013) reported a CF value from a grazing-based system of 25.9 kgCO2eq/kg lamb LW (compared to 26.3 kgCO2eq/kg lamb LW for static extensive farms in this study) and of 19.5 kgCO2eq/kg lamb LW from a zero-grazing system. Hence, according to our results, the CF estimated for transhumance systems (18.1 kgCO2eq/kg lamb LW) is in a similar range (or lower) to the equivalent intensive systems. This is in accordance with the conclusions of Vigan et al. (2017), which calculated similar CF values for intensive and transhumant systems in a French Mediterranean context.
In addition to this, transhumance can further promote climate mitigation linked to carbon sinks, by practicing extensive grazing throughout the year, and allowing system extensification. When accounting for C sequestration, low stocking rates have been associated to decreased carbon footprint of livestock products from extensive systems, even lower than equivalent intensive systems (Batalla et al. 2015). This is of particular importance in Mediterranean savanna-like agroforestry landscapes (‘dehesas’), where in some cases, carbon sequestration has been estimated to compensate all GHG emissions from ruminant farms, leading to negative CF values (Reyes-Palomo et al. 2022).
Mobile pastoralism and transhumance – particularly on hoof – is known to provide additional benefits to the environment. These range from the promotion of plant, insect or scavenger diversity to wildfire and erosion prevention (Manzano-Baena and Salguero-Herrera 2018). Mobile livestock is also key for climate change adaptation by acting as an effective dispersal vector of seeds, and it also preserves pollinators by grazing times adapted to plant phenology, with tangible effects on the genetic pool of plants (García Fernández et al. 2019).
Although not considered in the present paper, previous studies have pointed out the importance of considering these other functions in LCA approaches. When including valuation of ecosystem services in the economic allocation of sheep farms, Ripoll-Bosch et al. (2013) showed that the most extensive grazing-based system resulted in the lowest values of CF. Accordingly, we prospect that, if multifunctionality could be properly accounted and captured, transhumance on hoof should result in lower environmental impacts than calculated by current methodologies.
4.2 Effect of considering natural baseline emissions
Current GHG accounting methods, as reflected in the IPCC guidelines, exclude wild ruminants from GHG estimates, as these are considered a natural source of emissions, and therefore, not anthropogenic. Similarly, from an LCA perspective, wild herbivores can be categorized as “naturally occurring biotic resources” (Crenna et al. 2017), and therefore, computed as elementary flows entering the system from the ecosphere, which implies, for example, excluding direct emissions (e.g. enteric CH4) of wild ruminants when assessing the environmental impact of deer meat (Fiala et al. 2020).
In the present study we attempt to adapt a similar approach for the case of domestic animals that are managed mimicking the ecosystem functions and production patterns of wild herbivores in nature. Taking into account that transhumant livestock is occupying their ecological niche and displacing wild herbivore populations, and that it is fulfilling similar ecosystem functions, it is therefore reasonable to only consider as anthropogenic the transhumance-triggered emissions that depart from the natural baseline level. In order to account for this baseline effect, we subtracted the corresponding natural emissions from the displaced wild ruminants grazing in the equivalent area from the total farm GHG emissions.
As a proxy estimation of the biomass of wild herbivores in Mediterranean grasslands, we used the reported population density of red deer in a public hunting preserve, with similar characteristics of the savanna-like ecosystems grazed seasonally by transhumant sheep flocks. Average population density in this site was 32.9 deer/km2, within the range found in other studies in the Iberian Peninsula that reporting > 30 deer/km2 in Spain (Perea et al. 2014) and up to 40 deer/km2 in Portugal (Silva et al. 2014). We estimated a biomass density of wild herbivores of 4814 kg/km2. This was slightly lower but close to the natural baseline of herbivore biomass (5173 kg/km2) calculated by Fløjgaard et al. (2022) for Faia Brava (Portugal), a natural reserve representative of Mediterranean ecosystems.
In comparison, our estimations indicate higher biomass densities (5775 kg/km2) for transhumant sheep grazing Mediterranean grasslands. Supplementation with forages and concentrates allows to keep biomass densities above the natural carrying capacity of the ecosystem, which has been observed not only for livestock but for red deer populations in the same study area (Carpio Camargo et al. 2021). In addition to this, mobility may also affect significantly the biomass density of herbivores. Seasonal movements in pastoralism mimic the typical patterns previously used by wild ruminant during migrations, as a strategy to take advantage of different natural pasture resources along the year (Manzano and Casas, 2010). Currently, landscape fragmentation, and confinement, either in protected reserves or hunting preserves, drastically restrict these movements for wild herbivores, thus limiting their population density.
Considering herbivores grazing in nature as an elementary flow entering the system, and therefore, not an anthropogenic source of emissions, has a crucial effect on the impact assessment of products derived from them. As a result, the meat from hunted ungulates has been pointed out as a sustainable alternative to conventional meat from livestock ruminants due to its low environmental footprint (Fiala et al. 2020). In our study, when subtracting the estimated natural baseline emissions to the GHGs accounted for transhumant sheep, the CF of lamb meat is reduced by almost 30% (Table 5), reaching absolute values that are quite below those reported for intensive lamb production systems in Spain. Furthermore, in other contexts, applying a similar approach to extensive ruminant systems could have even more relevant effect. For example, in Africa, where higher biomass densities of wildlife are estimated (Flojgaard et al. 2022), traditional low-input pastoral systems relying only on natural grasslands, could be close to climate neutrality if considering baseline emissions, especially when implementing complementary mitigation options for improving herd and grazing management (Gerber et al. 2013).
Table 5
Profile of GHGs for the average carbon footprint of 1 kg of lamb meat (LW:liveweight) by farm typology: static (STAT), transhumance by truck (THT) and transhumance on hoof (THH). Results with and without considering natural baseline emissions from wild herbivores in Mediterranean grasslands ecosystem.
| Without baseline | | With baseline |
GHG contribution | STAT | THT | THH | THT | THH |
CH4 (%) | 62% | 76% | 77% | 73% | 74% |
CO2 (%) | 25% | 13% | 12% | 18% | 17% |
N2O (%) | 13% | 11% | 11% | 9% | 9% |
Carbon footprint (kg CO2eq/kg LW) | 26.3 | 18.1 | 18.1 | 12.9 | 12.9 |