Location of the study area

The state of Sinaloa is located within the Northwestern Coastal Plain, which directly borders the Sierra Madre Occidental. Geographically, Sinaloa is located in the northwest of Mexico, bordering the states of Sonora and Chihuahua to the north, Durango to the east, Nayarit to the south, and the Pacific Ocean and the Gulf of California to the west. This research was carried out in Southern Sinaloa, in the town of Los Pozos, Rancho Las Flores, in the El Rosario municipality, located between 22°47'35" to 25°30'00" N and 105°11'16" to 106°03'02" W; the climate in the area is warm sub-humid, with summer rains and marked dry seasons. The average annual temperature is 22 °C, and the average annual rainfall is 827 mm (^{Inegi, 2017}).

Data collection instrument

The information was obtained through technical and economic monitoring on a monthly basis, on the first days of each month, and whenever the technical aspects of the components required it. The follow-up was carried out from January 2017 to November 2019. A cooperating producer and a technician from El Rosario helped in this activity.

The planning, implementation, monitoring and evaluation of the ISS included the following stages: 1) identification and delimitation of the ranch where the ISS was to be implemented; 2) visits to verify the establishment and management of the ISS formed by Leucaena leucocephala variety Cunnimghan and Coastcross II Bermuda grass in the months of January to June 2017; 3) visits to verify the proper functioning of the perimeter fence system and the installation of the electric fence for the division of paddocks throughout the year 2017; 4) visits for the adaptation of the production area, which amounts to 20 ha of sprinkler irrigation (it should be noted that in the study area the fresh water groundwater layer is 2 m deep); 5) visits for the design of troughs in the paddocks; 6) visit for the establishment and operation of the sprinkler irrigation system; 7) support and identification for obtaining credit for 200 calves for pre-fattening, and 8) visits for the supervision of pre-fattening management and technical assistance for the integrated management of the implemented components.

Technological components and production costs

The components that integrated the module with the ISS were the following: establishment of Leucaena and a Coastcross II Bermuda grass prairie (Cynodon dactylon) both managed by sprinkler irrigation system for the pre-fattening of calves, whose initial weight is approximately 180 kg, to half fattening (330 kg) in 150 days. The weight of the animals was recorded only at the beginning of grazing and at the end of the pre-fattening period. Spray irrigation was performed with 60 m diameter canyons at the stages of establishment and management of the ISS; the frequency was 7 to 10 days, with an irrigation sheet of 5 to 7 cm, a duration of 6 to 8 hours, and changing each canyon twice or thrice per day, depending on the time of year.

The land where the ISS was implemented is flat, with a sandy-textured soil with a poor organic matter content; it was previously used for producing coconut trees and, for a few years, green chili peppers.

8 kg ha^-1 of Leucaena seeds were mechanically planted in furrows, which were 1.8 m wide. Weed control was mainly of wide leaf species and annual grasses. The initial population of Leucaena was four plants on average per linear meter, equivalent to 22 000 plants ha^-1 (i.e. 5 500 linear meters by four plants).

For the establishment of the Coastcross II Bermuda grass prairie, the central part of those furrows that had Leucaena was marked and planted with vegetative material; eight days of work per hectare and 1 500 kg of vegetative material were used. This activity was carried out when Leucaena reached a height of 80 cm.

In order to obtain the economic information of the components of this endeavor, all the costs and revenues involved in the establishment and maintenance of the technological components were considered. The costs of establishing the ISS were accounted for during the first year, as follows:

Investment costs that included four sections: 1) land to establish the ISS; 2) establishment of Leucaena; 3) establishment of the prairie; and 4) development of management corrals.

The costs of establishing Leucaena included land preparation, planting and fertilization, pest and weed control. After its establishment, the maintenance cost for fertilization and weed control was considered from the second year. In the case of the prairie, the costs of land preparation, sowing and fertilization, pest and weed control were included. From the second year on, the maintenance costs consisted of the cost for fertilizer use and for weed control.

Other items integrated in the economic evaluation were the operating costs of the ISS, such as the concepts related to the feeding (mineral salts) of the cattle, labor, health, electricity, maintenance of equipment, fuels and lubricants, as well as the insurance of the vehicle and costs of earrings for the calves.

Data analysis

According to ^{García (2008)}, the financial evaluation is of a business type and for profit; it has the objective of measuring the efficiency of the capital provided to finance a project; therefore, it determines the financial capacity, the profitability and the return on capital proposed in the investment. On the other hand, the economic evaluation consists in making a comparison between the resources used and the expected benefits, in order to determine the good allocation of resources (^{Rosales, 2007}); in this type of evaluation, the return of the benefits and better life conditions across the society (at country level, as well as the environmental and other benefits) resulting from the investment made are determined.

The financial evaluation of the ISS was carried out using the methodology of financial analysis, in which the indicators proposed by ^{Gittinger (1982)} were estimated: Net Current Value (NCV), Internal Rate of Return (IRR), and Benefit-Cost Ratio (BCR). The evaluation considered a time horizon of 10 years. The real and risk-free discount rate utilized was 9 %, which was used by the commercial banks in the region of study during the year 2019 (Table 1). The economic assessment considered the environmental benefit of carbon sequestration of US$10 per hectare (^{Ávila et al., 2001}) (Table 2). In addition, the ISS was compared with studies conducted in Southern Sinaloa in order to identify other positive externalities: biodiversity, nitrogen fixation, and soil conservation. The cost of possible negative externalities (use of agrochemicals, fertilizers, contamination of water tables) was omitted in this research. However, it should be noted that with the use of Leucaena, a high nitrogen fixation is achieved; therefore, the use of fertilizers is reduced.

Description of the ISS

The ISS was established in an area of 20 ha, with 10 calves per hectare (1 800 kg live weight ha^-1) and an initial average weight of 180 kg, in a pre-fattening period of 150 days. Management included intensive rotational grazing, with paddock rotation each day and one to two days of occupation. The grazing area was 20 m², with a fodder supply of 60 kg (day/calf), and a resting period of 30 to 45 days. The animals ended up with an average weight of 330 kg. The sales price considered for a standing calf was $44.00 kg^-1 in order to estimate the final income from the ISS.

Financial evaluation of the ISS module

Table 3 shows the fixed investment, in Mexican pesos, required for the implementation of an intensive silvopastoral (ISS) module of pre-fattening calves under the irrigation modality.

The infrastructure was depreciated at different periods of useful life: 20 years (irrigation network, electric network, management corral and scale), 15 years (perimeter fence, electric fence, electric transformer), 10 years (irrigation cannon, starter, irrigation motor pump, cattle trailer), and 5 years (feeders and drinkers). The total amount of investment was $5 114 100.00 pesos, with a 78 % contribution from the producer, since he owned the agricultural area where the ISS was implemented.

The operating costs or variable costs of the project were $1 947 400.00 Mexican pesos in 2019. These included the purchase of 200 calves of 180 kilograms per pre-fattening cycle, at a price of $7 920.00 ($44.00 kg^-1). The purchase of the animals amounted to 81.3 % of the costs; the second place (8.9 %) corresponded to the production of fodder; the third place, to labor and, finally, to the purchase of medicines and vaccines (Table 4).

Fodder production in traditional systems represented an important economic outlay for the small producer. However, in Sinaloa, approximately 5 million tons of agricultural waste are generated annually that can be used to feed livestock (^{López et al., 2018}). This abundance of waste material is more accessible to large companies, which allows them to obtain greater profits from the production of beef under the stable system, through the reduction of feed costs in the pre-fattening and fattening of their livestock.

At a 10-year period of the project, an updated income flow of $ 44 160 964.46 and an updated expenditure flow of $ 36 555 172.32 will be obtained (Table 1). In the economic evaluation, the income derived from carbon capture was included, thus obtaining an updated income flow of $ 45 312 013.12 and an updated flow of expenses of $ 36 555 172.32 (Table 2). From the financial point of view, the project is profitable with a NPV of $ 7 605 792.14, an IRR of 27.9 % and a B/C ratio of 1.21. In the same way, under the economic analysis, a NPV of $ 8 756 840.81 Mexican pesos was obtained, an IRR of 30.4 % and a RB/C of 1.24 (Table 5), that is, due to the effect of carbon capture in the SPPi, the NPV would increase by 1.1 million pesos, while the profitability of the project would increase by 2.5 %.

The results show that the proposed ISS is profitable and generates an increase in animal productivity (gain in weight of the calves, which start at 180 kg and are sold after 150 days, with a weight of 330 kg), and therefore better income conditions for producers. In addition, if greater productivity is obtained, it has an efficient use of natural resources that is relevant to the adaptation and mitigation of the effects of climate change. In this regard, SSPs are a fundamental tool for achieving this (^{Buitrago-Guillen et al., 2018}).

Some studies indicate that, with the passage of time and as the use of ISS components increases, there is a more abundant animal production, through a greater availability of plant biomass, more leaf litter production that favors improvements in organic matter content, nutrient recycling and soil fertility (^{Crespo, 2008}). Other authors have identified that AFSs can fix and store 12 to 228 t^-1 C, including soil organic carbon, which represents between 20 and 46 % of the C sequestered in primary forests (^{Andrade and Ibrahim, 2003}; ^{Beer et al., 2003}).

According to the results of this research, one of the most important limiting factors for the implementation of ISS is, admittedly, the lack of available credit and public policies that encourage their establishment and use; another restriction, without a doubt, is the identification and implementation of technological components suitable for the geographical and climatic conditions where they are intended to be established. In addition, in the state of Sinaloa there is a high concentration and purchase of calves by beef companies, which control the production chain and ultimately set the price of standing animals throughout the year. These conditions limit the development of small farmers who start beef calf production at a small scale.

In the study region, the price of calves and livestock in general has lost 24 % of the value reached in 2015 (from $58.00 kg^-1 to $44.00 kg^-1 in 2019). In contrast, the price of the carcasses continues to rise and therefore, the price of meat to the public has also increased; all of this without generating a direct impact on the small producers who engage in this productive activity.

The production of meat with ISS, however, can be an alternative for the improvement of productivity and profitability of small producers, as well as, an alternative of environmental sustainability, in which the institutional actors (research and teaching centers and government) must have a more direct participation.

Sustainability of ISS

In the study region, there is ample experience in the generation of technology for the production of fodder (^{Cuevas-Reyes, 2019}). One of the strategies followed is to acquire technologies through the establishment of agricultural modules. Based on three years of work, the module established with ISS in southern Sinaloa identified an environmental impact of 10 to 35.7 t of carbon sequestered per hectare, in addition to increased nitrogen fixation of up to 500 kg ha^-1 per year; as well as greater conservation of both soil and biodiversity, and less contamination of groundwater, which in the study area is located at a depth of 2 m (Table 6).

These results are consistent with other studies that point to advantages regarding the establishment of ISS, in comparison with monocultures. In that sense, a research carried out in Cuba, in which a silvopastoral system (Panicum maximum Jacq and Leucaena leucocephala) was compared to a monoculture (Panicum maximum meadow), identified in the former a gradual increase, through time, of 54. 4 to 65.3 t ha^-1 of carbon stored in the soil (CSS); in contrast, the monoculture system showed signs of deterioration and a decrease in CSS over time, from a concentration of 60.4 to 43.7 t ha^-1 (^{Lok et al., 2013}).

The ISS consist of technologies that contribute to the removal and reduction of greenhouse gases (GHG), through the sequestration or sequestration of carbon in the plant biomass and soil. Some authors indicate that the ISS are an alternative that enables livestock in tropical regions to adapt to climate change and participate in GHG mitigation: "as the establishment of ISS can remove up to 26.6 t of CO₂ equivalent ha^-1 year^-1" (^{Cuartas et al., 2014}).

Agroforestry systems, including ISS, offer proven strategies for carbon sequestration, soil enrichment, biodiversity conservation, and improved air and water quality not only for the owners of the forest resource, but also for society at large (^{Shibu, 2009}). In short, they entail a greater benefit for the producer and the environment. Therefore, as ^{Alonso (2011)} points out: "the implementation of an incentive payment for the generation of environmental services could change the perspective towards the use and management of tree species in livestock systems, given their effect on the conservation of agro-ecosystems”.