Efficiency and Sustainability in Ruminants’ Livestock Systems: A Case Study of Dry-Land Pastures in the Montado Ecosystem

3.1 Grazing systems

Pastures are generally managed by establishing stocking rates, with relatively low grazing pressures, allowing animals to choose their diet [10]. Producers aim to improve the profitability of their production systems while maintaining sustainability and biodiversity. However, they often fail to do so, as examples of this include situations of overgrazing and a decrease in pasture quality. This requires integrating knowledge of the species biology and correctly adjusting management actions [11]. Grazing tends to enable adequate animal welfare and the production of safe and healthy products [12].

Aiming for the most efficient use of the multiple resources available in the Montado, different livestock species can and should coexist through appropriate grazing management. This can be mixed and continuous or intermittent grazing, over an appropriate periodic crop (plots or leaves), rotating the animals through various plots. Grazing by cattle in the cork oak forest plays a very important role in maintaining it, as it counteracts the proliferation and development of shrub species such as cistus (Cistus ladanifer) and sargassum (Cistus salviifolius), among others [13].

Grazing animals are fundamental to the balance of agro-sylvo-pastoral systems, since they transform herbaceous production (dryland pasture) into products of unrivaled quality and contribute to maintaining the balance between herbaceous and shrub layers. On the other hand, the animals also act as firebreaks, thus helping to prevent fires, which damage ecosystems. Added to this are the beneficial effects of maintaining soil fertility and reducing production costs [14].

Continuous grazing (CG) involves grazing the same plot during the grazing season, year after year [15]. According to Teague and Dowhower [16], CG has limitations because it allows for selectivity, with consequences for the heterogeneity of the pasture, with overgrazed and undergrazed areas occurring simultaneously. Overgrazing can lead to soil degradation and loss of biodiversity, while undergrazing can lead to a greater preponderance of less palatable species with lower food value and loss of habitat, with the overlapping of a shrub layer [17].

In CG systems, excessive stomping is detrimental and damages not only the pasture but also the soil [18]. On the other hand, according to Barriga [19], in CG systems, there is a return of nutrients to the soil through feces and urine. CG offers several benefits over rotational grazing (RG), particularly regarding lower infrastructure costs associated with fencing and watering systems. This simplified management approach also allows animals to select their diet, potentially leading to improved performance [8, 15]. Intermittent grazing (deferred grazing on several fences) is also sometimes used, presenting quite attractive productive, ecological, and economic results [11].

RG is a grazing management system where animals are moved between fenced pastures, allowing each area to rest and regenerate before being grazed again [8]. However, most of the time, there is no pre-defined frequency for the animals to remain in each pasture plot. This system, while affording animals a degree of pasture choice, effectively balances animal nutrition with pasture health, allows animals to select their forage while ensuring that their nutritional needs are met without compromising the quality and abundance of the grass [18].

Hussain et al. [20] found that pastures benefit significantly from RG. Their study concluded that biomass accumulation is higher in areas grazed rotationally by sheep compared to ungrazed sites, especially in the spring. On the other hand, Zang et al. [21] noted that grazing under high biotic loads can lead to pasture degradation due to excessive defoliation and trampling, even though it can be effective in controlling weeds. To maintain a balanced pasture-grazing relationship, it is essential to adopt an integrated and well-managed approach. RG enhances animal productivity per hectare through sustainable intensification, achieved not by overgrazing but by employing effective management practices. A possible setback is that it increases labor and additional land could be needed [8]. In semi-arid regions such as the Alentejo in Portugal, RG is recommended, with short grazing periods and long rest periods [18]. In theory, this could work, but in practice, it is not adoptable due to climatic asymmetries in both temperatures and water, both of which limit grass growth. If the grass grows at different speeds, RG at fixed times is not the most appropriate and is therefore not used. If the grazing is rotational, but with variable periods of stay depending on the availability of pasture, it is called deferred grazing (DG). The frequency and duration of grazing in each paddock depend on factors such as forage type, animal stocking rate, and environmental conditions. This management optimizes forage growth, improves soil health, and prevents overgrazing promoting biodiversity, reducing erosion, and enhancing the sustainability of grazing lands.

The DG consists of placing high stocking rates on the pasture, with the duration of the grazing period being a function of the available biomass in the pasture. The availability of pasture biomass can be estimated by measuring the height of herbaceous plants. Holechek [15] states that DG allows areas preferred by the animals not to be damaged as much as in CG, in terms of the vigor and production of the plants in these areas. DG can minimize the harmful effects of selective overgrazing, due to the overuse of areas preferred by animals [11]. A disadvantage often pointed out to DG is excessive trampling, due to the high animal loads per hectare. However, according to a study by Serrano et al. [22], as can be seen, no significant differences were observed in soil compaction when comparing two grazing systems with sheep (CG - 1 AUE/ha vs. DG - 2 AUE/ha) in the Montado ecosystem (Figure 5a, b).

4.1 Mediterranean climate: Irregularities vs. climate change

The Mediterranean climate is characterized by hot and dry summers and rainy winters with mild temperatures, being the only climate on the Earth that has the particularity of having a dry summer, typical of summer with high average temperatures for several months, which is associated with a high degree of irregularity in the occurrence of precipitation, both within and between years [28]. It is the only climate in which the maximum annual temperatures overlap with the minimum levels of precipitation. The designation of Mediterranean climate is since the Mediterranean Sea basin is its largest area of influence, although it is also present in California, Chile, South Africa, and Australia [3]. In addition to the irregular annual distribution of rainfall, the Mediterranean climate is also characterized by marked inter-annual irregularity, with the occurrence of years with abundant and scarce rainfall, in a bimodal frequency distribution, which means that the average annual rainfall values are not the most frequent [29]. In the Mediterranean region, annual accumulated rainfall ranges from 300 to 800 mm [30]. As an example, Évora weather station reported an average annual rainfall of 624 mm, a minimum of 203 mm and a maximum of 1186 mm, between 1871 and 2007. The values of average monthly precipitation and average minimum temperature and average maximum temperature for Évora, in the period between 1871 and 2007, are represented in Figure 6. As can be seen, during the summer months (June, July, August, and September), the precipitation values are residual, with the temperature values being the highest.

Lopes et al. [32] highlight that various observations and projections suggest the Mediterranean region and southern Europe, including Portugal, are more vulnerable to climate change compared to northern Europe. Carvalho [2] notes that over a consecutive 10-year period, the cumulative rainfall of 50 mm required to initiate seeds germination, and the agricultural year can occur anytime between September and November. This variability significantly impacts the duration of the favorable autumn period and the productive potential of forage. Similarly, combined rainfall during March and April can range from 40 to 325 mm, influencing the length of the spring season. Notably, April rainfall plays a critical role in the growth and development of pastures.

Prolonged droughts are common in the Mediterranean region and have become more frequent in recent years due to climate change. As shown in Figure 6, between 1871 and 2007, the average monthly temperature was 19.6°C, with a maximum of 24.7°C and a minimum of 16.6°C, for the region of Évora, Portugal [31]. Notably, the average maximum temperature has shown an upward trend, increasing as the years progress toward the present. The irregularities of the Mediterranean climate, exacerbated by climate change, have made dryland pasture production in this region increasingly unpredictable, resulting in severely constrained periods for biomass growth.

According to Ferreira [33], the Mediterranean climate has two unfavorable seasons for plant production: the hot, dry summer and the wet, cold winter in the interior region of Alentejo. Therefore, in a Mediterranean climate, the production of pastures and fodder is very limited by the climatic conditions, in addition to the relief conditions and the physical and chemical characteristics of the soil. It is the only climate in which the maximum annual temperatures coincide with the minimum levels of precipitation. The total annual rainfall is often identical to that of temperate climates, but the concentration of rainfall in the autumn/winter season and the lack of water in the hot seasons cause serious inconveniences for agriculture.

4.2 Dry-land pastures

Over the last 35 years, the area of permanent pastures has increased in Portugal, although the majority are natural dry lands without any type of improvement. Furthermore, only 1% of pastures in Portugal is irrigated [7]. The germination and development of pasture plants depends on the characteristics of the soil and the weather conditions. The production of dryland pasture depends on rainfall and temperature, presenting considerable variability between years and within the same year. Normally, in the Mediterranean region, pasture plants begin to germinate and develop with the onset of the rains in autumn, provided there are adequate temperatures and radiation. For general germination occurrence, 50 mm of rainfall is required [2]. During the autumn, there can be a slight peak in production, which added to the scarce winter production, is responsible for producing around 15–35% of the total annual production, however, is not observed when the first significant rains do not occur until later in the year [34].

With the onset of winter and the accompanying drop in temperatures, pasture production ceases entirely. Despite the presence of water in the soil, the extremely low temperatures prevent any green matter from being produced. In spring, however, the combination of adequate water, optimal temperatures, and sufficient sunlight creates ideal conditions for pasture plant growth. During this period, dryland pastures reach their peak production, achieving values of approximately 50 to 120 kg of dry matter (DM)/ha/day. This peak accounts for roughly 65–85% of the total annual pasture production [34].

By the end of May, when pastures generally begin to dry out, there is a very rapid growth of grass, with an increase in dry matter content and a marked decrease in digestibility, energy, and protein [35]. In summer, the pasture dries out completely, leaving the remaining biomass, which, although abundant, has a low food value. To make these agricultural systems economically profitable, the land factor is used, leading to extensive production systems and, if intensification is chosen, irrigation is generally used as a way of increasing productivity.

The permanent pastures currently found in the South-Montado region exhibit very low DM production [23]. According to Carvalho [2], addressing soil acidity and mitigating the toxic effects of aluminum and manganese, along with the application of phosphorus, can lead to a fivefold increase in productivity. Carvalho [2] also emphasizes that dryland pastures must remain the foundation of these systems, as they represent the most cost-effective feed source, and the region lacks sufficient water resources to irrigate the entire area. Enhancing pasture productivity can be readily achieved by correcting soil acidity and applying phosphorus.

Belo et al. [23] refer to studies conducted to support their findings: (a) one study reported DM production values of 800 kg/ha/year in Montado pastures; (b) another study noted that DM production rarely exceeds 1500 kg/ha/year; and (c) a third study observed DM production of 695 kg/ha in autumn/winter and 2014 kg/ha in spring. However, Efe Serrano [36] reports a significantly higher DM production of around 3000 kg/ha/year. In a recent study, Serrano et al. [37] compared pasture DM production under and outside the canopy. They found average DM production values of 437, 1232, 1804, 2751, and 2363 kg/ha under the canopy and 425, 1868, 2987, 3582, and 6191 kg/ha outside the canopy for December, March, April, May, and June, respectively.

Although rainfed pasture is the most cost-effective feed option for ruminants, its nutritional value often falls short of meeting the animals’ production requirements, necessitating costly supplementation. To address these deficiencies, farmers frequently need to cultivate and store fodder supplements or acquire additional external feed resources.

The first step to improve the nutritional value of pasture, is to improve soil fertility. Since the main problem with the soils in the Montado region is acidity and manganese toxicity, this requires the application of magnesium-rich dolomitic limestone [38]. In addition, the recommended process for restoring pastures in the Mediterranean region is to increase soil fertility by applying phosphate fertilizers [39]. On the other hand, the soil under the tree canopy is more fertile than the soil outside the canopy [24], which shows the importance of trees for the grass growth and the production systems.

In a study conducted in the Montado ecosystem, Serrano et al. [37] observed that mean crude protein (CP) levels under the tree canopy varied across months: 22.9% in December, 22.4% in February, 15.9% in March, 11.2% in May, and 8% in June. Outside the canopy, CP values followed a similar seasonal pattern, starting at 21.3% in December and declining to 6.3% in June, with intermediate values of 19.8% in February, 13.5% in March, and 9.8% in May.

Figures 7 and 8 show the percentages of CP and neutral detergent fiber (NDF), respectively, of pasture samples collected throughout the pasture’s vegetative cycle, in plots subject to four different treatments (UC - without application of dolomitic limestone and continuous grazing - 1 AUE/ha; UD - without application of dolomitic limestone and deferred grazing - 2 AUE/ha; TD - with application of dolomitic limestone and deferred grazing - 2 AUE/ha; TC - with application of dolomitic limestone and continuous grazing - 1 AUE/ha).

As can be seen in Figure 7, throughout the pasture’s vegetative cycle, CP values were always above the threshold required for ruminants (above 7%). Even at the end of May, when the pasture was already dry, CP values were close to 10% in all treatments. The percentage of NDF tended to be higher in the TC treatment (Figure 8), and lower in the UC treatment, where no corrective was applied.

Besides protecting animals from the weather and heat, in agro-sylvo-pastoral systems, trees also act as soil protectors and contribute to increasing soil fertility. Moreno et al. [40] found that the organic matter content under the canopy of holm oaks was around twice as high as that found beyond the canopy. It is very common in the Montado to see plants with greater nutritional value under the canopy in autumn, winter, and spring, as there is greater soil fertility than those that appear outside the canopy, where Rumex bucephalophorus L. appears above all. Usually, this plant is regarded as a bioindicator of soil acidity and Mn toxicity.

Pastures are typically established on low-fertility soils, which are often shallow, making it impractical to cultivate more profitable crops. Moreover, the floristic composition of natural pastures in these areas is often compromised by poor grazing management practices. Improving pastures involves implementing sustainable modifications to their natural floristic composition to enhance both productivity and quality.

The improvement of pastures, Mediterranean or otherwise, thus consists of altering or ‘unbalancing’ bad herbaceous associations by introducing or making species of better forage value prevail [36]. The recommended process for restoring pastures in the Mediterranean region is to increase soil fertility by applying phosphate fertilizers [39]. As a way of improving productivity in extensive livestock production systems, the application of phosphate fertilizers and/or the application of limestone correctives and the introduction of biodiverse pastures are also common practices [25].

The structure and composition of the plant communities that make up pastures are generally affected by grazing [18, 41] and, above all, by selective grazing [42]. Selective grazing occurs when the animal load is low regarding the green mass produced [43]. Faria [44] notes that the shift from sheep to cattle on farms in the Iberian Peninsula brought changes to animal management practices, such as the number of grazing days and animal rotation, as well as alterations in grass structure and pasture floristic composition. Conversely, Zhu et al. [45], in a study conducted in China, found that grazing by cattle, sheep, and goats did not influence the species richness of pasture plants; however, it did significantly reduce plant biomass and increase variability in plant heights. According to Voisin and Lecomte [18] in a pasture sown with Poa pratensis and white clover, the percentages of both vary throughout the year of sowing and subsequent years, depending on the grazing time of each plot and can be as high as 80% clover in grazing systems with one cut per week, down to just 1% clover in systems with only one cut every 12 weeks.

The sustainable intensification of these systems involves, firstly, improving permanent pastures, of which around 90% are considered poor (IEEA 2013 cited by [2]). In a study comparing the productivity of natural pastures with biodiverse, legume-rich pastures in the Montado, Simões et al. [46] observed that the increase in DM production, which in some cases doubled, together with a higher proportion of nutritionally valuable species, allowed the grazing animal load to be tripled.

Following the experimental work referred above, in which the effect of four treatments was evaluated (UC; UD; TD; TC), at the end of spring (end of May) the botanical species belonging to the Poaceae and Fabaceae families were identified (Table 1); of the total of 32 different botanical species identified, 20 belong to the Poaceae family and 12 to the Fabaceae family. The plot where the most plants were identified was the one subject to continuous grazing without application of soil amendment (UC). The plot with the fewest botanical species identified was the one subject to continuous grazing and application of dolomitic limestone (TC). In this last plot, the fewest botanical species belong to the Fabaceae family. The application of dolomitic limestone led to greater development of grasses during the autumn/winter, which were not consumed because the stocking rate was relatively low, thus preventing the penetration of solar radiation into the soil, essential for the germination of legumes. In addition, most of these species are prostrate and therefore did not receive enough light to develop.

4.3 Achieving sustainability

Montado plays an important role in carbon sequestration, especially because they are made up of long-lived cork and holm oak trees, which promote carbon storage over very long periods [24]. On the other hand, pastures under Montado cover also play a fundamental role in carbon sequestration. According to Pinto-Correia et al. [5], this system has great potential for accumulating carbon in its arboreal and herbaceous biomass and in the soil. The same authors stated that healthy cork oak forests with reasonable tree cover can sequester between less than 1 and more than 3 tonnes of carbon per hectare every year. Overall, carbon sequestration in the Montado is estimated at 6.7 tonnes of CO₂/ha/year [3].

Balanced management of the Montado also contributes to preventing rural fires. Nowadays this is of great importance in Portugal, as in recent years fires in the centre and north of the country have not only destroyed forests and property but have also killed hundreds of people. As reported by Oliveira et al. [47] agroforestry systems, by reducing the amount of fuel, promoting the formation of horizontal and vertical discontinuities in the vegetation and forming clearings in the landscape, can be a means of controlling the number and severity of fires.

The spatial and temporal heterogeneity of the Montado promotes a substantial wealth of ecological niches [24]. We can find more than 135 species of vascular plants per 0.1 ha in the Montado [48], many of them rare or with protected status such as Narcissus fernandesii, B. cavanillesii, Armeria pinifolia, Centaurea coutinhoi, Halimium verticillatum, Ruscus aculleatus and Narcissus bulbocodium [24].

The diversity of tree cover in the Montado allows for the development of herbaceous and shrubby strata dominated by Mediterranean flora with a high expression of angiosperm species that are pollinated by insects, particularly bees [5]. This is why, cork oak forests are so important for the beekeeping sector, with around 2200 beekeepers in the Alentejo and Algarve regions (southern Portugal) [49]. The value of honey amounts to more than 20 million euros per year, particularly speciality types (monofloral, Protected Designation or Organic Production) [5].

Mushrooms are also present in the Montado ecosystems with great diversity, namely edible mushrooms with economic value, such as Amanita caesarea (Caesar’s mushroom), Amanita ponderosa (heavy amidella), many Boletes species (e.g., B. aereus and B. aestivalis), Cantharellus cibarius (golden chanterelle), Craterellus cornucopoides (horn of plenty), and desert truffles: Terfezia arenaria, Terfezia fanfani, and Terfezia Leptoderma [5].

More than 28 species of fauna with protected status are also found in the Montado and, in addition, the cork oak and holm oak forests are home to around a hundred other animal species, which appear in the annexes to the EU Habitats and Birds Directives [24]. The Montado provides habitat, food and shelter for many species, including many bird species, which are strongly linked to this ecosystem, such as Elanus caeruleus (black-winged kite), Circaetus gallicus (short-toed snake eagle) or Hieraaetus pennatus (booted eagle) [24]. This ecosystem is also of great importance to winter migratory birds such as Vanellus vanellus (Northern lapwing) and Grus grus (common crane).

According to Pinto-Correia et al. [5], of the 71 species of terrestrial mammals (both flying and non-flying) reported in mainland Portugal, over 95% are found in the Montado ecosystem. However, their presence may vary, ranging from occasional and fragmented occurrences to widespread distributions with high abundances. Among the species occasionally present is the critically endangered Iberian lynx (Lynx pardinus).

The shrubby areas and rugged slopes of the Montado also support a diverse array of fauna, including rabbits, hares, lizards, snakes, and wild house mice. These species play a critical role in the food chain, serving as prey for carnivores such as the common genet (Genetta genetta), least weasel (Mustela nivalis), red fox (Vulpes vulpes), and European wildcat (Felis silvestris) [50].

The sustainability of this unique ecosystem relies on pastures composed of self-reseeding annual grassland species. The longevity and persistence of these pastures are closely tied to the soil seed bank, which ensures the renewal of the pasture’s production cycle with the first autumn rains [5]. This natural forage base, complemented by the fruits of holm oak (acorns) and cork oak (lande), along with occasional supplementation from pruned branches, underpins a low-cost production system. When paired with appropriate stocking rates and effective management, this approach greatly reduces costs compared to intensive or poorly managed systems with excessive stocking densities, which are heavily dependent on preserved feeds such as hay, straw, haylage, silage, and concentrates.