The use of leucaena in ruminant nutrition has been widely implemented due to its high CP content, low fiber (NDF and ADF) and presence of some phytochemical compounds that can modulate ruminal fermentation (Montoya-Flores et al. 2020). In this study, the diets offered in both seasons in the SPS were able to increase CP, DM degradability, and GE, and to decreased NDF and ADF content in comparison with the BL diet (Table 4), with greater differences in the rainy season due to the higher supply of leucaena that reached 24% of the DM offer. According to Sarabia-Salgado et al (2020) diets containing leucaena increase animal production both individually and per unit area due to the N fixed by this legume that improves both the productivity and the chemical composition of the diet.
One of the main limitations to increasing the productivity of cattle grazing in low quality tropical forages is the limited supply of fermentable nitrogen (NH3-N) for microbial growth in the rumen, particularly during the dry season (Leng 1990). The main supply of NH3-N comes from the degradation of dietary crude protein in the rumen. CP in leucaena forage is rapidly fermented in the rumen, producing NH3 for microbial protein synthesis (Sarabia-Salgado et al. 2020). Nitrogen intake in cows grazing SPS with leucaena is increased by combining a high-quality legume foliage with a tropical pasture, that improves the N uptake efficiency by rumen bacteria (Paciullo et al. 2011; De Angelis et al. 2021). from which amino acids are absorbed from the small intestine, which in turn are available for milk protein synthesis (Calsamiglia et al. 2010).
The effect of L. leucocephala in reducing enteric CH4 is associated with its condensed tannin (CT) content, which forms complexes with protein (CT-P) and polysaccharides and reduces nutrient degradation in the rumen (Schofield et al. 2001; Stifkens et al. 2022). In addition, some studies propose that CTs promote changes in microbial populations due to their bacteriostatic, bactericidal and enzyme inhibitory effects that modify ruminal fermentation (Harun et al. 2017). In this study, the inclusion of 17 and 24% of leucaena in the BL diet generated a reduction of 7.66 and 11.47% of CH4 emissions for the dry and rainy seasons respectively. Other authors such as Montoya-Flores et al (2020), Piñeiro-Vázquez et al (2018) and Stifkens et al (2022) in Colombia, Mexico, and Australia respectively in in vivo conditions obtained reductions close to 15% of total CH4 emissions when leucaena was included above 18% in diets based on tropical pastures. According to Piñeiro-Vázquez et al (2018), the intake of tropical foliage containing CT in low concentrations (3–6% of DM) improves nutrient intake of ruminant rations, particularly DM, OM, and CP, so moderate intakes of leucaena as given in this study do not affect nutrient intake and degradability. On the other hand, it has been pointed out that doses lower than 2% of CT do not affect DM intake (Beauchemin et al. 2007) and that the molecular weight and structure of L. leucocephala CT are also adequate characteristics that do not affect the degradability of the diet (Barahona et al. 2006).
Regarding emissions from manure, one of the main concerns when using Leucaena is the increase of N2O-N emissions due to the relatively high CP content (> 20%) and the increased cycling of nitrogen through the animals. Since ruminants excrete 75–95% of ingested nitrogen, a high CP diet often leads to increased urinary nitrogen concentration and, under conditions that favor denitrification and nitrification, this can result in increased N2O emissions (de Klein and Eckard 2008). According to the results of this study, although it was a higher offer on N via CP in the SPS diet, the EFs were significantly lower in urine and feces patches for both seasons in this system when compared with the BL. However, due to the higher offer of N and stocking density and the consequent higher total excretion in the SPS, the total N2O-N emissions were higher in this system (Fig. 1). Higher nitrogen concentration in urine leads to increased ammonia volatilization, which further increases N2O emissions (Harrison et al. 2015). Changes in N2O production had minimal effects on whole-farm emissions, as enteric methane emissions were higher and accounted for over 90% of total emissions. These observations are consistent with previous studies documenting emissions from other livestock systems (Alcock et al. 2014; Harrison et al. 2015) and highlight the importance of focusing on enteric methane in mitigation strategies, as enteric methane dominates the emissions profile in a livestock system.
N2O EFs in SPS systems (percentage of N excreted via feces and urine that is converted to N2O-N) were lower probably due to better soil conditions. Chirinda et al (2019) found lower urine patch emission factors at seven sites in South America (0.42 vs. 0.18%) when pastures had higher vegetation cover compared to areas with low cover or degraded pastures. According to these authors, higher emissions in soils with low cover are due to pasture degradation that can stimulate or restrict N losses. For example, low vegetation cover can reduce N sinks for deposited excreta and thus increase vulnerability and N loss through microbial soil and leaching processes (Chirinda et al. 2019). However, low vegetation cover may also be associated with fewer excreted plant roots that decrease microbial activity and N2O emissions (Henry et al. 2008).
Improved soil cover and pasture management also contribute to maintaining or increasing soil organic matter (SOM) (Aryal et al. 2018), which plays a key role in determining the response of N2O emissions to urea deposition. Increasing SOM increases cation exchange capacity, reducing the concentration of NH4+ in the soil solution which, in turn, reduces NH3 in the soil solution and the associated inhibition of NO2- oxidation, thereby reducing urea-derived N2O emissions (Breuillin-Sessoms et al. 2017). The SPS studied in this evaluation had adequate grazing managements ensuring greater soil cover.
Similarly, the lower CH4-C emissions from manure in SPS are possibly due to a higher diversity of microorganisms in the soil, a higher soil structure due to the presence of trees and shrubs in the grasslands, and a higher soil cover. Cubillos et al (2016) reported in a study that included SPS of different ages, that these arrangements have a significantly lower potential for anaerobic processes (between 15 and 20%) compared to monoculture systems, and similar those observed in forested areas. Rivera et al (2018) found that some silvopastoral systems can present negative CH4 fluxes and similar to those presented in forests. On the other hand, overgrazing without time for grass recovery increases the risk of soil compaction. Compaction in turn reduces soil porosity and pore continuity, decreases aeration, restricts plant growth, and increases soil CH4 emissions, characteristics usually found in conventional systems (van Groenigen et al. 2005).
On the other hand, SPS can modify emissions in feces due to the presence of dung beetles that limit the interactions of manure with mineral soil, restricting substrates for nitrification and denitrification processes (Slade et al. 2016). Studies in the same region of the Cesar River Valley have shown that SPS have higher abundance, diversity, and activity of beetles than treeless pastures (Montoya-Molina et al. 2016). According to Slade et al (2016) the presence of beetles in livestock systems reduces N2O emissions by 14.7% and CH4 emissions by 17%.
Finally, in terms of GHG emission intensities and balance, the results of this study coincide with those reported by Gonzales-Quintero et al (2021) and González -Quintero et al (2022) in systems with different levels of intensification and orientation in Colombia. These authors found that intensification generates lower emissions per unit of product and that CH4 emissions from enteric fermentation represent more than 90% of total emissions in systems with low use of external inputs such as fertilization and feed. The inverse correlation between GHG intensity and productivity of beef and dairy cattle production systems has been documented by several studies in Latin America (de Léis et al. 2015; Gaitán et al. 2016; González-Quintero et al. 2021), indicating the strong influence of increased beef and milk production on GHG dilution. This study contributes to these findings, as considerable GHG mitigation potentials were found to be achievable by less productive systems when they increase their productivity via implementation of more complex and efficient systems.
The contribution of animal sources, such as enteric fermentation and excretions deposited on pasture, to total GHGs was high, a common finding reported for grazing livestock systems where CH4 and N2O are the most critical GHGs requiring mitigation actions (González-Quintero et al. 2021). The results found in this study also coincide with those reported by González-Quintero et al (2022) who concluded that the contribution to total GHGs from off-farm emissions from the manufacture and transport of inputs is low in systems with limited productivity compared to more intensive cattle systems.
The emission intensities found in this work are in the range of values reported by González-Quintero et al (2021), who in an analysis of 1,313 farms found emissions between 2.1 and 4.2 kg CO2 eq per kg of FPCM, and between 9.0 and 18.3 kg CO2 eq per kg of LWG exported. These authors associate the lower emission intensities in dual-purpose systems to practices such as: the use of improved pastures and implementation of SPS, use of pasture rotation and electric fences, weed control and adequate fertilization management.
The above analysis provides insights into possible technological changes and management options that can increase productivity parameters and improve the environmental performance of dual-purpose systems. For the correct establishment of policies aimed at supporting mitigation and adaptation actions for the livestock sector in tropical conditions and under grazing, it is important to know the relative profitability for the implementation of SPS systems as a mitigation measure. It is also important to advance in the determination of carbon sequestration in bovine systems since their potential could be underestimated.
In a meta-analysis, Feliciano et al (2018) found that systems using controlled grazing practices and appropriate grass species can increase average aboveground carbon sequestration between 2.29 to 6.54 Mg/ha/year. Likewise, López-Santiago et al (2018) reported that systems with L. leucocephala associated with M. maximus contain higher aboveground C (19.6 ± 1.6 Mg/ha) and belowground biomass (7.7 ± 0.90 Mg/ha) compared to tropical deciduous forest and monoculture pastures in Mexico. With these values, negative carbon systems could be raised, since for this study an emission of 2,194.1 and 6,372.4 kg of CO2 eq/year was estimated for the BL and SPS systems, respectively. Another aspect to consider could be the evaluation of scenarios with the release of areas where the most suitable areas for cattle ranching could be intensified and other areas could be released for conservation; this approach would bring greater environmental benefits such as carbon sequestration, connectivity, conservation of water sources and increased biodiversity, among others (Calle et al. 2014; Calle 2023).