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In industrialized countries to optimize pig production means to maximize weight gain with diets formulated on a least cost basis. It is generally assumed that improvements in economic performance will be associated with increased rate of animal productivity. However, in most developing countries this strategy is not appropriate because of the high costs of conventional feed ingredients (maize, soybean meal and fish meal), many of which are imported. This creates problems of balance of payments at national level and dependence on credit at the level of the producer. Attempts to produce the ingredients locally (principally maize and and soya beans) are constrained by high prices of machinery, seeds and agrochemicals, all of which usually have a content of imported components. Moreover, in most developing countries, cereal grains such as maize are utilized primarily in the human diet, which elevates the price and makes it difficult to include them in feed for animals. Another issue is that the climate and soils in tropical latitudes are not favourable for maize cultivation, which results in lower yields and higher costs of cultivation. The final point is that pig production is often not considered as a component of the farming system, but rather as a specialized activity producing only meat. . By contrast, in an integrated system, pigs are multipurpose animals producing fertilizer and energy as well as meat.
The implication of the above analysis is that in tropical latitudes there is a need to develop feeding systems which make maximum use of plant species which are adapted to, and exploit, local ecosystems. The use of sugar cane as a replacement for cereals as the energy component of the diet, for both ruminant and monogastric live stock (Preston and Leng 1987), is an example of such a strategy. However, the application of this feeding system to pig production, initiated in Colombia in the '80s (Sarria et al 1990), was constrained by the dependence on soybean meal (usually imported) as the source of protein. This problem of protein supply was also experienced in early experiments with sugar cane and its derivatives for ruminant production (eg: the use of fish meal to supplement sugar cane molasses in Cuba (Preston and Willis 1974); and of rice polishings to supplement chopped whole sugar cane in Mexico (Preston et al 1976). However, the ability of ruminants to deal with fibrous feeds facilitated the search for alternative sources of protein in the form of foliages from trees and shrubs (Leng 1997), which are available in a wide range of species that grow naturally in tropical latitudes.
The idea of using protein-rich foliages as a replacement for soya and fish meals in practical pig diets was first applied in Vietnam with the use of fresh duck weed (Lemna spp) as the only protein source in pig diets in which sugar cane juice provided the energy (Rodríguez and Preston 1996). The leaves from the cassava plant (Manihot esculenta), initially in sun-dried or ensiled form, were shown to have potential to replace up to 30% of the protein in diets for growing pigs based on cassava root meal (Bui Huy Nhu Phu et al 1996). Fresh foliage of water spinach (Ipomoea aquatica) gave good results as the only source of supplementary protein in diets for young growing pigs in which the energy was from broken rice (Ly 2001), and was shown to have a synergistic effect on pig growth rates when mixed with fresh cassava leaves in broken rice diets (Chhay Ty and Preston 2005). Sweet potato leaves were used successfully as a protein source in pig diets in Vietnam (Le Van An et al 2003) as were the leaves of the mulberry tree (Morus alba) (Phiny et al 2003).
Two factors facilitate the use of leaves from tropical trees and shrubs as protein sources in pig diets. The first was the demonstration that the conventional protein requirements for pigs (eg: NRC 1988) could be reduced by up to 40% (Speer 1990) when the protein in the diet had the required balance of essential and non-essential amino acids, subsequently referred to as the "ideal protein" (Wang and Fuller 1989). Leaf proteins, being composed mostly of enzymes necessary for growth of plant tissue, have an amino acid balance that resembles the "ideal" protein. Thus diets in which the protein comes exclusively (basal diet of sugar cane juice) or mainly (cassava roots and broken rice) from leaves of plants can be compounded with lower protein levels (up to 40% less) than when cereal grains such as maize or sorghum are the source of energy, as more than 50% of the protein in such diets is from the cereal component with an imbalanced array of amino acids (Speer 1990). The second factor relates to the capacity of the pig to consume and digest fibre. Leaves from trees, shrubs and crop plants are relatively high in fibre hence the advantages of using energy sources, which are low in fibre (sugar cane juice, cassava roots, sweet potato tubers, banana fruit and broken rice all fall into this category). Use of these feeds as energy sources creates "space" in the diet for protein supplements relatively high in fibre as is the case of leaves from trees, shrubs and crop plants.