The Central Food depot of Mexico City (Central de Abastos) is the largest market in the world, receiving 24,000 tons of food products daily, some 40% of the Mexican national harvest. It generates 800 tons of waste per day, mostly organic, of which 100 tons are used as a forage source to feed the 2500 dairy cattle maintained in urban stables in the east of Mexico City, constituting an significant flow of otherwise waste material. A wide range of products are used as feed for the cows, including parts or whole of the following: broccoli, cauliflower, lettuce, carrot, corn, pumpkin, cabbage, turnip, radish and sugar beet. In turn, excrement from the dairy stables is removed weekly and used entirely for crop production in peri-urban areas. The most significant crop by far is the nopal (Opuntia ficus indica) grown on terraced fields in Milpa Alta, in the south-east of the city with an annual production of over 200,000 tons of leaves. Some 75% of these are consumed within Mexico City, the rest in neighbouring states (Puebla, Hidalgo, Mexico, Queretaro, etc), and a small proportion exported to USA and Japan. Dairy manure is applied for organic matter and nutrients as well as a source of moisture and heat counteracting the effects on both the lack of water or frosts on the plant. Up to 600 tons of fresh manure may be applied per hectare, and the crop produces leaves weekly, all the year round. 

Nutrient flow between vegetable production, Central de Abastos, dairy stables and nopal fields (figures in kg/ton dry matter unless specified)
 

*based on mean input and content of the six principle crops used in urban dairy cattle feed          **based on manure application rate of 400 tons/ha/yr

Introduction

The use of solid organic wastes (vegetable rejects, manure and others) in peri-urban agriculture has recently been advocated by some researchers for its potential to reduce the problems of refuse disposal in urban centers along with generating employment and increasing the production of locally grown agricultural produce (Brock, 1999). This closed loop nutrient system is ideally suited for urban areas for its benefits of reducing the reliance on external inputs and supporting waste management and recycling (Smit and Nasr, 1992). In this study, milk production systems in the East of Mexico City utilize a large quantity of organic refuse for feed from the Central de Abastos, which includes fruit and vegetables. Dairy stables produce manure and straw, which are delivered in a fresh form to the agricultural zones of the Southwest of the city. Manure is an essential input in the production of the nopal vegetable that is an important component of the diet of the population of Mexico City. The objective of this paper is to analyze the recycling of nutrients (N, P, K) in this integrated model of production. 
 

Components of the integrated production system

Vegetables and fruit production

The production of vegetables in Mexico is an important economic activity in national agriculture representing the 9th and 6th place in terms of production and exportation respectively, at a global level (SAGAR, 1999). The total surface area of vegetables in Mexico is 544 000 hectares (SAGAR, 1999) which is distributed among 11 states in the center and north. Elevated productivity is the result of the application of high technology inputs in irrigated agriculture which includes the use information technology, biotechnology (in vitro propagation), greenhouses, laser (leveling of land) and agro-chemicals. Demand for vegetables in the urban centers is represented by a level of consumption reported as 72kgs per capita (SAGAR, 1999). Table 1 shows the inputs (inorganic fertilizers) in six crops that are used for human and animal consumption. 

The Central de Abastos and its role as waste producer. 

The Central de Abastos is the largest commercial center in the world covering some 328 hectares and operating 365 days of the year. It consists of 2000 warehouses with the capacity to hold 155 000 tons and receives 24 000 tons of organic products daily which constitutes around 40% of the national harvest (See picture Number. 1). It is estimated that the daily movements leave some 800 tons of organic waste of which 100 tons is used as a source of forage in the stables situated close to the market in the east of the city (See pictures numbers 2 and 3). Producers also take advantage of excess fruits or vegetables that have lost their freshness for human consumption and use these as feed in the stables (See picture number 4)
 

Picture 1: An aerial view of the Central  Food depot of Mexico City (Central de  Abastos de la Ciudad de México).	Picture 2 : Rejected vegetable area in the  central de abastos of Mexico City
Picture 3 : Dairy stable owner packing his  truck with cauliflower leaves in the central  de abastos of Mexico City	Picture 4 : Rejected carrots of the central  de abastos of Mexico City used to feed  dairy cows of Iztapalapa.

Table 2 illustrates the values of N, P, K reported for the six components of the waste, which are most commonly used in the diet of the animals.

Milk Production in the Delegation of Iztapalapa 

The remaining dairy cattle of the delegation are the result of systems that have evolved with the pressures of urbanization and also those cattle displaced from neighboring milk basins of Coapa and Xochimilco. Production systems are characteristic of that which combines the family house with the rearing of animals, and the majority of milk producers dedicate themselves to this activity while others combine this with other sources of income (salaried work, domestic work, commerce, etc). Average number of animals in each stable is 27 and of these 85% are adult animals in production and the rest bulls, heifers and young (See picture number 5). The predominant breed is the commercial Holstein and the animals are housed using simple recycled materials or concrete where they spend nearly all their life cycle (See picture number 6). Cleaning of the stable is carried out daily and the manure is collected along with the waste of the feeds and bedding material. The organic material is delivered to the South of the city, where it is the principal source of nutrients in the cultivation of Nopal. The feeding system of the cattle is integrated with the vegetables of the Central de Abastos, the food industry wastes (stale tortillas, stale bread, biscuits and soybean shell) along with common forages and feeds (alfalfa, maize, wheat bran). Animals are ordained by hand twice daily (See picture number 7) and the majority of the milk is sold directly from the stable to local residents while the leftover is processed into cheese and sweets (creme caramel and cajeta principally).  Table 3 presents the macronutrients N, P, K in the feed given to the animals. 

Picture 5 : The typical herd in  Iztapalapa stables of Mexico City.

Picture 6 : Housing of dairy cows  in Iztapalapa.

Picture 7 : Milking time in  Iztapalapa stables, to the East of  Mexico City.

Nopal Production in Milpa Alta

Milpa Alta forms part of a mountainous zone in the south of Mexico City. The altitude ranges between 2470 and 2750 meters above sea level and hill slope gradients between 20 and 60%. Soils are volcanic in origin, dark brown and the texture is clay-sand to silt sand (Picture 7). Although the area has some deep soils (>50cm) most are in the range of between 25-50cm (INEGI, 1994). Mean annual temperature is 16°C and average annual rainfall between 600 and 800 mm distributed between May and October. Frosts in the higher parts are frequent between December and January (García, 1973). Natural vegetation in the wooded areas is dominated by various pine species by various species of pines. Annual and perennial grasses (Graminae) and shrubs (Cardoso and García, 1982) form Flora (See picture number 8. A landscape of Milpa Alta, a peri-urban area in the south of Mexico City).

The Nopal vegetable (Opuntia ficus-indica) originates from this area, and was domesticated for human consumption before the arrival of the Spanish (Rojas, 1991). There are some 5000 hectares of this perennial crop with a production of 200 000 tons of leaves of which 75% is consumed in Mexico City and the rest of the neighboring states (Puebla, Hidalgo, México. Querétaro, etc) while a small part is exported principally to the USA and Japan (Rivera et al, 1994 and Losada et al, 1996a) (picture 9). The production system is intensive based on a average distance between the plants of 20 cm and the corridor between the furrows of 1.2m. The majority of smallholdings are between 0.3 and 0.6 hectares, and larger ownership around 1 hectare is rare. In most of the cases, tasks in the field including weeding, pruning, fertilizing, disease treatment and harvesting are done by hand using local tools (Rivera et al, 1994). The nopal vegetable has three periods of production related to productivity, which directly affect the prices in the market. These are the winter between December and January that limits production due to low temperatures and frosts resulting in higher prices; the spring with higher production and reasonable prices: in the summer with the arrival of the rains production is high and low prices.

Production of Nopal leaves has been calculated at 750, 1750 and 5500 cladiodos (leaves) per hectare corresponding to winter, spring and summer respectively. Average age of the plant is between 5 and 20 years determined by the productivity other social and economic factors (See pictures 9 to 13)
 
 

The Nopal vegetable  (Opuntia ficus-indica)	Picture 9 : Nopal-vegetable cultivation area in Milpa Alta, in the lowlands of the  extinct volcano Teuhtli.
Picture 10 : Dealing with nopal-vegetable  in Milpa Alta	Picture 11 : A typical plot of nopal-vegetable in Milpa Alta.
Picture 12 : Heavy dressings of cow's  manure from Iztapalapa,  used in the  nopal fields of Milpa Alta.	Picture 13 : Packing nopal-vegetable to  be sent to the local market place or  to the central de abastos of Mexico City.

All the small holdings use cattle manure as a source of nutrients and the application rate is reported as intensive with some 600 tones of manure for each hectare annually assisting the producers to achieve a weekly harvest throughout the year. Other wider benefits of the organic manure have been indicated by producers which include providing a source of water and heat demanded during the dry season and frosts respectively (Losada et al, 1996b). Some chemical fertilizers are used to supplement the inorganic nutrient content. Composition of the manure from the stables is outlined in tables 4 and 5. 

Nutrient flows in the integrated production system 

The close association between the milk production systems of the internal basin and the peri-urban agricultural systems which is represented by the example of the nopal constitute without doubt a model that fits the sustainable conditions desired for the large urban centers. The macro-nutrient balance obtained in part shows the inputs of the organic products of the Central de Abastos and the stables and lastly the large contribution to the nopal fields (Table 6) which confirms the presence of a closed nutrient cycle using these components.
 

The efficiency of energy and macronutrients used in the field has been reported in previous work (Losada et al 1996, see table 7). 

The results are not impressive in terms of bio-mass produced, however the principal contribution is in the recycling of energy of biological origin which constitutes a renewable natural resource in contrast with the conventional systems of production which are dependent on non-renewable fossil fuels. A second contribution is the importance of macronutrients in the formation and improvement of soils in the medium term, which indirectly conserves the environment with the maintenance of agriculture in the zone.

Conclusion

Notwithstanding the system Central de Abastos-dairy stables-nopal cultivation seems to be in equilibrium and holistic, there exist the danger that the complexity of this type of model and its components are misunderstood by authorities or policy makers. This situation might lead to the design of wrong policies regarding waste management or the rearing of animals in urban centers. It must be understood that these components, animals and waste management is essential to the model and thus, policies must be orientated towards its promotion.
 

Acknowledgements

The authors would like to acknowledge the assistance of the UK Department for International Development (DFID) and the Food and Agriculture Organization (FAO) for partial funding that made this research possible, To the authorities of the Universidad Autonoma de México for the facilities provided, and Messrs J. Rangel, P.Hernandez and G.Alonso for the data collection.
 
 

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