The nopal-vegetable system, a suburban agro-ecosystem in the southeastern hills of Mexico City based on the intensive use of manure, is contrasted with the Marais system of nineteenth century Paris, in terms of the energy balance and annual fluxes of mass and macro nutrients. The Mexican system provides about 240,000 tonnes of fresh salad food all over the year, and is based on the use of approximately 3 million tonnes of cattle manure. The energy balance of this agro-ecosystem shows a total energy input of 1325 GJ/ha/year and an output of 47 GJ/ha/year. Estimates of the approximate annual fluxes of mass and macro plant nutrients show a similar trend to that reported by the energy balance. Assessed by the criteria of marketable food and the production of dry matter per unit area of land, the productivity of the nopal-vegetable plantations is high. The system is based on use of renewable natural resources and contributes to soil formation. This suburban agro-ecosystem is critical in order to help reduce the problem of cattle wastes in Mexico City.

An intensive use of manure has been reported in the Marais system of Paris, during the second half of the nineteenth century (Stanhill 1977). The main feature of the Marais was the production of high-quality salad and vegetable crops based on inter- and successional-cropping, supported by the intensive use of horse manure which made possible a winter crop by the heat and CO₂ released during the fermentation of the manure. This system contributed a significant proportion of the fresh food for Paris, with a sufficient excess to supply a valuable export market. At the same time the Marais transformed the major transport pollution of the time into an asset by turning vast quantities of stable manure into a surplus of highly fertile soil (Quarrell 1938). A century later, the nopal-vegetable production in the southeastern hills of Mexico City (Losada et al 1996), represents a cropping system that follows a similar pattern to that of the Marais. It is a system that produces a high-quality Mexican salad based on successional cropping by the intensive use of dairy cattle manure. This production is able to supply the city with an invaluable source of fresh food in the form of the nopal cactus, with a sufficient surplus to satisfy nearby states and the US demand; it performs an important role in containing urban development on the rural outskirts to the south of the city; and it turns vast quantities of dairy cattle manure into fertile soil.

This paper presents the main features of the dairy cattle manure utilization in nopal-vegetable production and attempts to provide an approximate energy and mass flow analysis of the nopal-vegetable system based on the theoretical calculations reported in the paper by Stanhill (1977), with adjustments to actual manure dairy cattle and nopal-vegetable values.

A major proportion of the dairy cattle manure that supports nopal-vegetable production is produced in the suburban stables, located in the outskirts of the city . The main features of this system have been previously reported by Losada et al (1992). The manure produced in the stables is gathered daily and stored together with feed refusals and bedding material until collected by the nopal-vegetable farmers. A mean value for fresh manure production per cow is 25 kg/day.

Collection of the manure from the stable is carried out every three days or once a week, depending on the number of animals and availability of transport. Although dairy manure is a resource with a high demand, the collectors do demonstrate some preferences. These include manure with oat and wheat straw (from the bedding used at milking time) rather than maize stubble, and manure with a high water content. In most cases the manure is offered free to collectors, although there is some evidence that on occasions the dairy producers have to pay in milk for its removal.

Once collected the manure is transported to nopal-vegetable plantations of Milpa Alta in the southeast hills of Mexico City according to landowners’ needs. The main objectives reported by the nopal-vegetable farmers for the use of dairy cattle manure focus on three factors: (a) the manure as a supplier of nutrients and organic matter, (b) the manure as a provider of heated planting beds (hot-beds) and the manure as a source of moisture (water-beds). According to the research results, the extensive use of manure on the nopal plantations demonstrates little seasonal variation, with just some indication of seasonal fluctuations during the period of higher growth rates (May to October) and during the colder months of the year (December to January).

Similar to the figures reported by Stanhill (1977), which identified an average of 6.5 persons/ha, including the owner and his family, employed in the Marais system, the average human labour in nopal-vegetable production was calculated as seven people. For the nopal-vegetable system, the human energy input into cultivation, which included maintenance, pruning and harvesting was estimated at 24.00 GJ/ha/yr (GJ=gigajoule=238,894 kcal, Leach 1975). This figure was based on a labour input of 1.33 MJ/ hr, and a year of 300 working days, each of which averaged 12 hours. The human energy input into the manual manipulation of the manure was estimated at 1.5 GJ/ha/yr, assuming a lower rate of energy input (0.66 MJ/ hr) and a shorter working day (8 hr) appropriate to the different nature of the work.

Heavy dressings of dairy cattle manure are an essential feature of the nopal-vegetable system, providing the fermenting hot-beds or water-beds. Rates of application were reported to be on average 420 tonnes/ha/yr. This represents the output of 46 adult dairy cows/yr. The energy input of the stable manure was 1028 GJ/ha/yr, assuming a value of 3,500 kcal/kg (Bhattacharya and Taylor 1975).

High quantities of inorganic fertilizer are used to support nopal-vegetable production. Rates of application were reported to be on average 1.1 tonnes/ha/yr. The energy input of inorganic fertilizer was calculated as 35 GJ/ha/yr, assuming an equivalent of crude protein of 281% and a factor of 5.72 as proposed in Nehring and Haenlein's (1973) equation for calculating the value of gross energy.

[Editor’s note: It is presumed the authors found that the major fertilizer used was urea (46% N) and they therefore expressed this as protein and finally as gross energy. It would perhaps have been more appropriate to have calulated the energy used to manufacture the urea in the factory. However, this is a minor isssue compared with the major objective of demonstrating the useful recycling of waste in an urban environment].

Straw from the bedding provided during milking is incorporated into the manure. The average percentage of straw present in the manure was found to be 5%, with an application of 22 tonnes/ha/yr. The energy input of the straw was 209 GJ/ha/yr , taking a value of 2,500 kcal/kg for the gross energy.

Other inputs include propagation (which is carried out by rooting mature, fleshy leaves) and the incorporation into the soil of green organic matter (pruned leaves). The nopal-vegetable plantation has a lifetime of 10 years and an average propagation of 33 tons of mature leaves. Energy input was calculated to be 3.4 GJ/ha/yr. Values reported for the green organic matter from pruning gave an average application of 11.2 tonnes/ha/yr and an energy input of 11.7 GJ/ha/yr. Transporting of manure from stables to nopal-vegetable plantations amounted to an average of 55 trucks/ha/yr. The distance between the stables and the fields was estimated at 20 km with an average value of 5 km/litre of gasoline, giving a final value of 8 litres of gasoline for the round trip. The energy input for transport was calculated as 28 GJ/ha/yr, assuming a value of 15,000 kcal/litre of gasoline.

Yields of fresh crop were converted to both dry matter and gross energy. The average crop yield of marketable fresh leaves was 45 tonnes/ha/yr containing 3.6 tonnes of dry matter and an energy output of 47 GJ/ha/yr.

Summarized values for the energy balance are presented in table 1.

Table 1: Energy balance for the average nopal-vegetable plantation (GJ/ha/yr)
Input		Output
Human labour	24	Crops	47
Manure	1028
Inorganic fertilizer	35
Straw	209
Miscellaneous	43
Total	1325		47

High quantities of N, P and K are added to nopal-vegetable plantations in the form of manure, inorganic fertilizers and green organic matter. Mean values for the inputs of the three macro nutrients were of the order of: 2152, 312 and 515 kg/ha/yr of N, P and K, whereas the values reported for the output of these nutrients were 33, 4 and 80 kg/ha/yr respectively. Summarized values for the macro nutrients balance are presented in table 2.

Table 2: N, P, K balance on nopal-vegetable production (kg/ha/yr)
	Input	Output
N	2152	33
P	312	4
K	515	80

Assessed by the criteria of marketable food and the production of dry matter per unit area of land, the productivity of the nopal-vegetable plantations is high. The food marketed from each hectare could supply 12 persons with their total calorific requirements per day, and 10 persons with their daily protein requirement. These values contrast with those reported by Stanhill (1977), with the Marais system demonstrating values of 15 and 54 persons for energy and protein supply. The poor comparison of the nopal-vegetable system with that of the Marais might be explained by the low protein content of nopal-vegetable previously reported by Losada et al (1996).

As in the Marais system, nopal-vegetable producers are interested primarily in maximizing their financial returns, and it is to this end that their cropping systems have concentrated on high-value, rainy and winter season crops (June-February) and have neglected (by means of pruning) lower-value spring crops (March-May). Due to this economic emphasis, the total volume of production per year is below its potential.

The efficiency of the nopal-vegetable plantations in terms of energy is low. The ratio of food energy produced to that used in its production was 0.03, which is higher than the value reported in the Marais system (0.007) in which most of the energy gain was obtained by spent hot-beds known as "terrau".

The significance of the energy flow of the nopal-vegetable system does not lie so much in the absolute levels of energy output and input, but rather in the fact that most of the energy input is primarily of biological origin, representing a renewable resource in contrast with the conventional industrialized crop production systems of Mexico which are highly dependent on non-renewable fossil fuel sources.

The nopal-vegetable system provides a profitable solution to the problem of Mexico City sub-urban dairy waste disposal. Government estimates of the number of cows in these sub-urban systems are 60,000 cows (SARH 1989).With this number of animals it would be possible to produce sufficient manure to sustain 1,233 ha of nopal-vegetable plantations, which would constitute 25% of the total area of nopal in Milpa Alta.

Estimates of the approximate annual fluxes of mass and macro nutrients show a similar trend to that reported by the energy balance. The ratio of food macro nutrients produced to that used in its production is 0.015, 0.012 and 0.15 for N, P and K respectively, indicating a significant loss of nutrients. A certain amount of N is lost to the atmosphere as NH₃, and there are further losses from leaching of N (and K ). But as the precipitation of the region is relatively low (700 mm/yr, García 1989) losses through leaching do not provide an adequate explanation. It is therefore possible to suggest that most of the macro nutrients in fact remain in the soil as reported in the data for the "terrau" of the Marais system.

The crop production system practiced by the nopal-vegetable producers of Milpa Alta add a further and very important trophic level to sub-urban ecosystems, profitably recycling the energy and nutrients produced as the waste disposal of dairy cattle herds. The positive aspects of this sub-urban agro-ecosystem constrast sharply with the lack of similar technology to support other crops in order to help reduce the problem of waste in one of the largest and most polluted cities in the world.

The authors wish to thank the producers of the nopal-vegetable from Milpa Alta and the authorities of the Universidad Autónoma Metropolitana (Autonomous Metropolitan University) for facilities given to do the research.

Received 1 October 1996