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Introduction
Tropical and subtropical regions of Latin America, due to their wide territorial extension with lower human population density than Asia and with an accelerated concentration of their inhabitants in the cities, enjoy less limiting agroclimatic conditions than a great part of Africa and Australia in such a way that they are called upon to supply a growing portion of the world demand for bovine, ovine, caprine and buffalo meat, and possibly also for bovine milk. It is predicted that this demand will continue to rise in the next decade (Steinfeld et al. 2006). Forage production, naturally and economically performed, is the basis of the feeding of these domestic herbivores. An essential condition for this potential to be achieved one day is that small, medium and large scale producers strictly apply the agroecological principles in soil management, water care, and sustainable production of forage biomass and biodiversity conservation (Calle et al. 2013). But, in addition, it is mandatory to guarantee the welfare of animals (Broom et al. 2013), and that production systems reduce greenhouse gas emissions, while being more resilient to climate change effects (Montagnini et al. 2013, Gerber et al. 2013 and Chará et al. 2017).
Productive and sustainable silvopastoral cattle rearing
The silvopastoral systems (SPS) are one of the most innovative solutions to meet the livestock challenge of sustainable production. They are a varied group of spontaneous or deliberate arrangements in which perennial woody plants (trees or shrubs), herbaceous or twining plants (grasses, herbaceous legumes and weeds) and domestic animals interact simultaneously (Murgueitio et al. 2015). SPSs that can cover larger areas in less time and with lower labor and capital investment are trees, shrubs and palms scattered in grazing areas, almost always by natural regeneration and with the participation of a large number of propagated species with the help of cattle and wildlife. The cultural change of producers and technicians is essential for them to avoid extreme practices of woody vegetation removal with the use of fire, herbicides and mechanical destruction (Calle et al. 2017 and Murgueitio et al. 2011). On the other hand, in the last forty years, thanks to research and work of farmers, technicians and pioneer entrepreneurs, other silvopastoral models have been developed in different areas of the continent, with more deliberate objectives of increasing productivity of timber trees and associating animals to forest plantations (also in some fruit tree crops) or maximizing forage productivity for greater animal production and stocking rate in the form of forage mixed banks for cutting and carrying or for direct browsing and grazing. The latter are called intensive silvopastoral systems (SPSi), a land use where one or more species in different strata or levels interact in the same space and time. In the lowest (herbaceous stratum), there are forage grasses native from America or introduced, as well as herbaceous legume and other woody herbaceous plants. In the second level or middle layer of vegetation, there are forage shrubs such as Leucaena leucocephala (Lam.) de Wit., of Mimosoidae subfamily; Tithonia diversifolia (Hemsl.) A. Gray, of Asteracea family; or Guazuma ulmifolia Lam., of Malvaceae family, in high density (between 10 and up to more than 40 thousand plants ha-1) destined for cattle browsing. The third stratum (even a fourth is possible) is formed by native or introduced trees and palms for all kinds of uses, especially shade for cattle, fruit offerings, wood and firewood production, which are in dispersed form or in planting lines with a density ranging from 25 to 200 adult trees ha-1 (it varies according to species). In the SPSi, the permanent supply of good quality water for animal intake must be guaranteed in mobile drinkers and balanced mineralized salt (Murgueitio et al. 2015). As a result of all the above, it is possible to increase the stocking rate up to four or more times in front of extensive grazing, favoring a greater meat animal production when passing from 160 kg year-1 to 800 or 1,500 kg year or even more (Solorio-Sánchez et al. 2011). In turn, the implementation of SPSi allows reducing costs compared to intensive grazing systems, by reducing the use of nitrogen fertilizers from 300 ha-1 year-1 or more, up to zero (Rivera et al. 2017). Experiences of producers in the tropics of Mexico show that a genetic component that includes crossbreeding with animals Bos taurus x Bos indicus (50 % or a little more) in the SPSi of leucaena - selected pastures, allows an additional production of meat between 15 and 20 % (Murgueitio 2017). The SPSi also stand out for reducing reproduction seasonality because the cattle have better food in times of critical drought, offer better quantity and balance of essential nutrients (protein, energy, minerals and vitamins) and because they suffer less heat stress with reduction between 6 and 14 degrees Celsius in the annual average temperature of their surroundings. In all SPSs, biodiversity benefits compared to treeless systems (Murgueitio et al. 2011 and Harvey et al. 2013). Thus, by increasing livestock production in smaller spaces, connectivity of forest fragments is also increased through corridors of native vegetation, such as gallery forests along rivers and smaller water courses. In this way, livestock intensification based on natural processes can also play a strategic role in the rehabilitation of degraded ecosystems and provision of environmental services derived from biodiversity and the hydrological cycle (Chará et al. 2015 and Calle et al. 2017).
Silvopastoral cattle rearing may mitigate climate change
Global climate changes, as a consequence of human activities, especially the use of fossil fuels, urbanization and exaggerated patterns of consumption, are processes that occur more rapidly and exceed the capacity of governments and societies to carry out profound changes that are needed. Therefore, extreme expressions of climate are increasingly worrisome in the world and are more often manifested as severe droughts, devastating hurricanes, torrential rains with floods and landslides, cold fronts and rising sea levels (SOCLA 2014).
The International Convention of the United Nations on Climate Change (UNFCCC) clearly defines the importance of mitigation and adaptation of states and all sectors of world society to climate change.
For mitigation, it is essential to reduce greenhouse gas (GHG) emissions, which include carbon dioxide (CO2), nitrogen dioxide (N2O) and methane (CH4) (Gerber et al. 2013).
Advances in research with SPS and SPSi on mitigation begin to be published with data from the region (Ibrahim et al. 2010 and Harvey et al. 2013). Thus, in several experiments, it is concluded that models with Leucaena leucocephala and improved grasses have the ability of reducing GHG emissions in bovine systems (Solorio-Sánchez et al. 2011). These determinations include CH4 production by in vivo (Molina et al. 2015a and Molina et al. 2016) and in vitro (Huang et al. 2011, Molina et al. 2013 and Rivera et al. 2015) enteric fermentation. Determination of carbon footprint and GHG balance (Naranjo et al. 2012, Harrison et al. 2015 and Rivera et al. 2016) and the measurement of gas flows from grasses and bovine excretions.
Regarding enteric emissions of CH4, Molina et al. (2016), with diets based on African star grass Cynodon plectostachyus with an inclusion of approximately 25 % of Leucaena leucocephala cv. Cunningham, offered to heifers of the Colombian breed Lucerna (Bos taurus), found that these emissions can decrease to 15 % per kg of consumed dry matter (CDM), from 43.6 to 37.7 L of CH4/kg of CDM. In addition, there was less energy loss due to CH4 production in the diets with shrub species. Molina et al. (2015a) found similar results when evaluating enteric methane emissions in response to a 24 % inclusion of Leucaena leucocephala in diets based on Cynodon plectostachyus and Megathyrsus maximus. In both evaluations, although the animals in the systems with leucaena consumed from 15 to 20 % more DM, total emissions per animal only increased by 3 % on average without significant differences. These results are due to the fact that, with the inclusion of leucaena, total NDF levels decrease and this reduces methane emissions (Archimède et al. 2011). Another aspect that affects CH4 emissions when leucaena is offered to animals, is the presence of condensed tannins (Barahona et al. 2014). These components possibly inhibit growth of Archaea in the rumen and their effect depends on the chemical structure and its quantity (Archimède et al. 2011 and Huang et al. 2011).
On the other hand, the inclusion of Leucaena leucocephala results in a lower carbon footprint of meat and milk produced in SPSi. Rivera et al. (2016) reported that an SPSi with leucaena was reduced in 12 % the kg of CO2 - eq emitted in the production of one kg of milk corrected for fat and protein (MCFP) in relation to an intensive system based on star grass, irrigation, supply of concentrated foods and fertilization under tropical conditions (2.05 vs. 2.34 kg of CO2-eq, respectively). In turn, in the system with leucaena, the GHG emissions associated with the production of one kg of fat and protein were reduced in 19 and 23 % (42.3 vs. 54.9 CO2-eq/kg and 47.3 vs. 58.3 CO2-eq/kg, respectively). Rivera et al. (2016) concluded that systems based on Leucaena leucocephala reduce the intensity of emissions thanks to its high productivity, diet quality, low dependence on external inputs (fertilizers and concentrated feed) and high stocking rate. In the same way, Naranjo et al. (2012) estimated that, under conditions of high shrub density and presence of trees in grazing areas, the balance between emissions and removals can be negative, thanks to the fact that, in SPSi, between 17 and 32 of CO2/ha/year can be captured and emitted about 12 of CO2/ha/year.
Regarding emissions in meadows with the presence of leucaena, Harrison et al. (2015) found that these gas flows may be lower than conventional systems with similar supply of N in the diet. Under tropical dry forest conditions, Rivera et al. (2015) found lower losses of N deposited in bovine manure and urine in the form of N2O in an intensive SPSi than in a conventional system (p = 0.002). Thus, in the SPSi, only 1.37% of the excreted N was emitted via manure compared to 1.77 % emitted in the conventional system, while, in the case of urine, the emissions were 3.47 vs. 0.3 % for the conventional system and SPSi, respectively. When observing grassland flows, Rivera et al. (2015) found that the emissions of a system with leucaena are similar to those measured in a forest (p> 0.05) and much lower than those of a system of grasses in single crop with irrigation and fertilization (p=0.001). In the results of Rivera et al. (2015), the high fertilization intensity of the conventional system (420 kg of N2/ha/year) and irrigation, undoubtedly favored the conditions for increasing denitrification processes and, as a consequence, the net emissions of N2O towards the atmosphere.
The reduction of methane emissions with Tithonia diversifolia is also researched since this species also has low fiber levels. Molina et al. (2015b) and Donney's et al. (2015) evaluated the effect of inclusion of foliage of this shrub on methane production in conventional grass diets. Although there were no differences in the daily emissions of CH4 (p = 0.351), the emissions per kg of weight gain were reduced from 22.3 kg of CO2-eq kg-1 in a diet based on brachiaria or bitter grass Urochloa decumbens to 4.89 kg of CO2 -eq kg-1 when Tithonia diversifolia (p = 0.002) was included (Molina et al. 2015b). On the other hand, Donney's et al. (2015) found that the inclusion of 20 to 25 % of the same shrub in the diet reduced in 10 % the in vitro production of CH4 per kg of degraded matter in diets based on kikuyo grass Cenchrus clandestinus and up to 15 % in brachiaria or bitter grass-based diets.
Public policy actions for mitigation with silvopastoral systems
To achieve remarkable effects in reducing GHG emissions, large-scale actions are required with the strong support of the state. For this reason, research, such as the ones mentioned above, should be used quickly in the design of public policies (Gerber et al. 2013 and Acosta et al. 2014). In Colombia, recently, a viability analysis was carried out to replace grasslands degraded by silvopastoral systems, as well as improved management, as proposed in the profile of Nationally Appropriate Mitigation Actions better known as NAMA. For the actions selected in the NAMA of Sustainable Livestock, it was demonstrated that the country has a reasonable potential to increase domestic meat production in 25 % and milk in 30 %, while releasing six million hectares towards other uses such as ecological restoration, conservation of wild areas or agroforestry systems. Emissions (mainly methane and nitrous oxide) from the production process would decrease 23 % for each kilogram of protein. If productivity gains were used for avoiding deforestation of more land, and if farmers planted forests on two of the six million hectares of reduced grazing lands, mitigation of land use could prevent or compensate 1.4 billion tons of carbon dioxide in 15 years. This level of mitigation, which combines livestock improvement with forest protection, could achieve the goal of reducing emissions in Colombia by 2030 (MADS 2015 and Lerner et al. 2017).
Livestock can be a sustainable and mitigating activity of climate change if it is transformed into intelligent management of soil and water conservation, as well as being carried out in mixed grasslands with intense presence of trees and shrubs, capable of transforming solar energy into animal feed and these, in turn, have genotypes better adapted to the new climate and to infectious and parasitic diseases (Murgueitio 2017 and Chará et al. 2017).
