| Bioconversion of Organic Residues for Rural Communities (1979) |
|The role of ruminants in the bioconversion of tropical byproducts and wastes into food and fuel|
Nutritional limitations in the use of tropical by-products and waste
It is becoming commonplace to plead for the development of appropriate technology when we talk of the problems of developing countries and the transfer of technology from the advanced countries However, there is a strong argument in favour of this philosophy in the general field of the feeding of ruminant animals on tropical feeds and wastes.
The justification for this approach can be seen in the many results obtained in the feeding of tropical crops and byproducts to ruminant animals, where performance levels have usually fallen short of expectations in comparison with nutritional results obtained in temperate climates.
The majority of by-products and agricultural wastes of tropical origin are characterized by the following factors: a. The carbohydrate component consists of highly soluble elements (sugars) that have a high rate of degradability in the rumen. It also has elements of very low solubility (cell wall components) with low rates of degradation by rumen microorganisms. In particular, most tropical byproducts and wastes do not contain starch, which is an important component of most feeds fed to livestock in temperate zone countries. b. In addition, most tropical by-products and wastes contain low levels of both total nitrogen and true protein. The final effect arising from the combination of these two sets of characteristics is a low voluntary intake by the animal and this, in turn, leads to a low rate of productivity.
The nature of the carbohydrate fraction in tropical feeds, as described above, and the low nitrogen levels that necessitate incorporation of quite high levels of non-protein nitrogen, means that the digestion process in the animal must be principally by the route of fermentation by rumen micro-organisms. This is because the sugars are immediately fermented when they enter the rumen because of their solubility, and because the animal secretes no gastric enzymes capable of hydrolyzing cellulose and associated cell wall components. Obviously, non-protein nitrogen, too, can only be converted into protein by fermentation processes.
One would imagine that such a situation would correspond closely to the environmental niche for which ruminants were evolved. In fact, it has been found that when the nature of the ruminant's diet is such that it is obliged to pass through the rumen fermentation process, and the only "feeds" presented for digestion in the small intestine are the bodies of rumen micro-organisms, the voluntary intake of feed (by the animal) is abnormally low, and as a consequence the level of animal performance rarely exceeds maintenance.
When voluntary intake is low, rumen dilution, or turnover rate, is also low. This, in turn, reduces the rate and efficiency of microbial growth and, therefore, of microbial protein synthesis. Thus, with most tropical feeds and byproducts, we have a poor performance syndrome caused by low voluntary intake and low efficiency of rumen function.
Results of recent research have shown that three factors play determinant roles in such dietary situations. The most important factor is "by-pass" protein. This is protein of dietary origin that is not, or to only a small extent, degraded by the rumen micro-organisms and arrives intact at the duodenum, where it is digested by gastric enzymes to its component amino acids that are then absorbed. The value of such by-pass protein lies less in its role as a direct source of amino acids than in the effect that it has on over all voluntary intake and rumen function. Thus, the higher the amount of by-pass protein, the greater is voluntary intake. Furthermore, both rumen turnover rate and total flow-out of the rumen also increase; ipso facto, efficiency of rumen microbial protein synthesis is increased.
The second factor is the roughage characteristics of the diet. These presumably act through direct effects on rumen motility and rumen contractions, and/or by providing a superior "ecosystem" in the rumen. Evidence for the former is in the increase in intake and rate of animal growth when sugar-cane leaves, employed as the sole roughage in a liquid molasses diet, were chopped into large pieces instead of being ground finely (1). Substantiation for the improved "ecosystem" in the rumen are the observations that both intake and digestibility on a basal sugar-cane diet were improved when a highly digestible, but low-protein, roughage in the form of banana stalk was included in the diet (2).
The third factor is "by-pass" energy. The evidence here is less conclusive; however, there are sufficient observations to justify belief in the basic effect. Starch as a component of the dietary supplement, and glucose infused directly into the duodenum, have been the principal mechanisms used to effect the by-pass. It has been known for some time that some starch sources (e.g., maize) are not rapidly degraded by rumen micro-organisms, and that a considerable portion of the starch passes on to the duodenum (3). These observations were made on temperate zone, cereal-based diets and little practical importance was attached to the findings.
The digestion process in the ruminant, outlining the separate pathways for fermentable and by-pass nutrients, is shown in Figure 1. The quantities of by-pass nutrients required will depend upon the rate of productivity for the particular trait under consideration, whether this is growth of tissue, as in production of meat, or the secretion of milk.
Figure 2 shows how these requirements for by-pass nutrients are related to the particular stage of the production cycle of the animal. The neo-natal calf and the high-producing milk cow represent two stages in the production cycle when there is highest demand for by-pass nutrients. Modern systems of cattle production, as developed in the temperate zone countries, emphasize these requirements by the early weaning of the calf and by encouraging very high rates of lactation yield, and have led my colleagues and me to propose that in tropical countries, where by-pass nutrients are expensive, it is desirable to modify the management system so that both early weaning and high individual milk production are discouraged in favour of more moderate, less specialized approaches (e.g., 4, 5).