Pastures

Pastures are forage crops defined as “a type of grazing management unit enclosed and separated from other areas by fencing or other barriers, and devoted to the production of forage for harvest primarily by grazing” (Allen et al., 2011). Thus, these crops are used directly by domestic and wild animals.

 

Avrey (2001) estimated that less than 40% of the earth’s surface could be used for agriculture, 11.5% is arable land, and 26% is classified as pasture. Similar percentages for cropland and pasture were estimated by Ramankutty et al. (2008). Most of the pastures are grazed by herds bred for meat and dairy production, which means employment and source of income for the local economy of some regions (Lemaire et al., 2005). Because animals use plant biomass directly on pastures to feed, grazing is considered the cheapest and simplest way to feed herbivores.

 

Pastures are classified, according to their origin, as natural (or primary) if present above the treeline, spontaneous (or secondary) if their vegetation formed spontaneously in a previously deforested area, or artificial if created by seeding, re-seeding or overseeding (Ziliotto et al., 2004). Pastures also differ according to their persistency (annual, pluriannual, or perennial), species number (monophytic, oligophytic or polyphytic), and the presence or absence of shrubby/tree species (herbaceous, shrubby, or wooded) (Ziliotto et al., 2004). For example, natural pastures located just above the treeline are composed of herbaceous species mixed with shrubs, while at higher elevations the plant communities consist only of herbaceous species. In spontaneous pastures, along with herbaceous species, there may often be shrubs and trees and their encroachment is dependent on the management strategy of the area..

Ecological functions

 

Together with forage production, pastures provide several ecological functions. The conservation of unique biodiversity, the regulation of physical and chemical fluxes in ecosystems, the mitigation of pollution, and the preservation of landscapes have been recognized as the most relevant (e.g. Gibon, 2005; Lemaire et al., 2005; Pornaro et al., 2017). Moreover, the role of grasslands in carbon sequestration and in the mitigation of greenhouse-gas emissions coming from breeding is important (Soussana et al., 2010).

 

Grasslands have high inherent soil organic matter content that supplies plant nutrients, increases soil aggregation, limits soil erosion, and also increases cation exchange and water holding capacities (Miller and Donahue 1990). According to an estimate of Lugato et al. (2014), grasslands in Europe store 5.5 million tons of carbon in the first 30 cm of soil. Pasture capacity to store carbon is linked to climate conditions and soil characteristics (Stevenson and Cole, 1999).

Pastures are characterized by high biological diversity due to the richness of plant species.

 

Pastures are generally composed of complex plant communities, mainly grasses (Poaceae) and, to a lesser extent, legumes and other forbs (Troxler, 1990; Scehovic, 1991; Pornaro, 2012). Their botanical composition largely depends on altitude, gradient, and climatic and edaphic factors (Jeangros et al., 1999).

 

However, differences in botanical composition also occur at a finer spatial scale, especially in mountainous regions where various site conditions, such as terrain slope, differ at a scale of meters. Botanical composition of pastures is the result of grazing management and environmental factors such as temperature (Buxton and Fales, 1994; Van Soest, 1994; Ziliotto et al., 2004), water deficit (Halim et al., 1989; Ziliotto et al., 2004), solar radiation (Lechtenberg et al., 1971; Buxton and Fales, 1994), and soil nutrient availability (Buxton and Fales, 1994; Ziliotto et al., 2004; Gibon, 2005).

Therefore, the herbivores’ and plants’ productive cycles are strictly linked in a grazing system.

 

The botanical composition of vegetation as well as the quantitative and qualitative characteristics of herbage mass are strongly influenced by the environmental characteristics and the grazing system.

 

These ecosystems are vulnerable to different kinds of degradation caused by climate change and human activities. Considerable researches reveal that grassland ecosystems are under degradation, which is becoming worse, especially in arid and semi-arid places (UNCCD, 1994; Conant et al., 2001; Safriel et al., 2005; Veron et al., 2006). Gang et al. (2014) described pasture degradation as the reduction of net primary production, and found that, in 2010, 49.3% (14,010,000 km2) of pastures in the world is degraded due to over-exploitation of soil, growth of population, and global warming or climate changes (Harris, 2010).

 

Other degradation phenomena are related to soil compaction, erosion, and decreasing in pastures fertility (Cox and Varpama, 2000). The main cause of pasture degradation is due to over or undergrazing. The effect of overgrazing has negative effect on plant vegetation, vegetation ground cover, and soil structure (Hiernaux et al., 1999; Manzano and Návar, 2000; Van de Ven et al., 1989). On the other hand, as an effect of the under-grazing, pastures are subjected to reforestation (Conti and Fagarazzi, 2004).

Pastures are generally located in areas not suitable for intensive crops, with steep slope or not easily accessible, where the soil is often shallow and poor in fertility (Godfray et al., 2010).

 

Most pastures are continuously grazed throughout the season. However, continuous grazing results in the lowest possible pasture yields since the forage is not allowed to recuperate between grazing (Undersander et al., 2002), or since the animals can select the feed leaving the worst and oldest plants that become weeds on the canopy.

 

For a more useful use of biomass, rotational grazing is preferred. Under rotational grazing, only one portion of the pasture is grazed at a time while the remainder of the pasture “rests.” Thus, the grazing surface is divided into smaller areas, and livestock is moved from one area to another when the biomass has been completely grazed (Undersander et al., 2002).

 

Resting grazed areas allows plants to renew energy reserves, rebuild vigor, and deepen their root system. However, since the environment-plant-animal system composing pastures is a complex structure, the management is challenging and cannot be restricted to rigid schemes. Sustainable management should be ensured to maintain all pasture benefits. First, they must be used continuously and uniformly to preserve high-quality vegetation from weeds invasion. The utilization pressure must also be targeted at conserving the floral richness. Lastly, the management impact on the environment should take into account (soil and water).

References



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