|Bioconversion of Organic Residues for Rural Communities (UNU, 1979)|
|The role of ruminants in the bioconversion of tropical byproducts and wastes into food and fuel|
Having identified the nutritional constraints associated with the use of tropical byproducts, it becomes possible to plan efficient systems for converting these into animal products such as beef and milk. This task also has been facilitated considerably by the recent findings that both by-pass protein and desired roughage characteristics can be provided by such protein-rich tropical forages as the legume shrub Leucaena leucocephala, the leaves of the banana plant, and the aerial forage part of the sweet potato (2, 13, 14). Cassava forage is also effective with one specific tropical by-product, namely molasses (15).
The likely input-output relationships involved in a rural bioconversion unit using cattle can be appreciated from the data in Table 4. It is assumed that a single family unit has 1.5 ha of land and that it will derive its disposable income from the sale of bananas, milk or cheese, and beef. The cattle population comprises five adult cows, four calves, and four steers/heifers. The cattle are confined throughout the year in an open-side building fitted with a partially slatted floor so that the faeces and urine fall directly into a channel below the floor, which, in turn, connects with the inlet of a biogas digester. After partial digestion, the final effluent is pumped onto the crops as a fertilizer (see Figure 4). The same equipment is used to apply irrigation water. The biogas produced from the unit (about 5.7 m³/day) is calculated to be sufficient to provide for cooking in the house and to drive the irrigation/effluent pump and forage chopper.
The cattle are fed a mixture of sugar-cane (high biomass per unit area), and banana forage - i.e., the residue after harvesting the fruit, as the energy source, and sweet potato forage for by-pass protein. No roots are harvested and the plant is managed as a perennial. Some additional nonprotein nitrogen such as urea, and phosphorus-rich minerals and salt are purchased; these are the only imports into the unit. Both the banana forage and sweet potato forage provide desirable roughage characteristics.
The advantages of this particular system are that it:
a. provides a good level of disposable income for the family;
b. is likely to be in energy balance (except for the energy cost of the urea and minerals);
c. avoids erosion by using perennial forages and by recycling organic matter;
d. relies to a minimum on imported fertilizer, yet represents a high level of plant nutrient application because of recycling;
e. reduces environmental contamination; and
f. uses a minimum of land area (1.5 ha), yet produces both a cash crop and animal products.