|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|
Research on sugar-cane and its by-products has provided ample confirmation of the role of by-pass nutrients in the feeding of ruminants in the tropics.
The importance of by-pass protein was established in Cuba in the late 1960s, when it was demonstrated that, on a basal diet of molasses consisting principally of sucrose and reducing sugars with small amounts of soluble non-protein nitrogen supplemented with urea and low-quality forage, the rate of animal performance was a curvi-linear function of the amount of true protein fed (Figure 3). Only proteins of low rumen degradability were effective (e.g., fish meal, torula yeast, and soybean meal), indicating that they were acting by by-passing the rumen fermentation (6).
Work in Mexico (7) showed that similar relationships could be demonstrated with diets based on sugar-cane (which is mainly sucrose and cell wall carbohydrates), and could be ascribed to the by-pass effects of the protein in the supplement (8).
On this diet, the role of by-pass starch also became evident. The data in Table 2 (9) show how feed utilization efficiency was improved by feeding maize containing by-pass starch, but not by molasses when iso-energetic amounts of the two supplements were fed with derinded sugar-cane. It has been shown that when rice polishings containing 15 per cent protein, 13 per cent lipids, and 40 per cent starch were added to a sugar-cane diet, at least half of the starch passed unchanged to the duodenum (10). Furthermore, glucose entry rates on the same diet were directly related to the amount of rice polishings given (11).
TABLE 2. Effect of Adding Maize Grain or Molasses (at 1 per cent of Liveweight) to a Basal Diet of Derinded Sugar-cane and Protein Supplement, in the Three Trials Using Holstein Steers
|Control||Improvement over control (%)|
|Gain in liveweight (kg/day)|
|Feed conversion *|
|Trial 1||9 1||8||- 16|
|Trial 3||9.9||15||- 15|
Source: James (9).
* Feed dry matter consumed/gain in liveweight.
Further evidence for effects of by-pass starch (maize) on feed conversion rate in sugarcane diets has been obtained. Leng showed that infusing glucose directly into the duodenum of lambs receiving a sugar/straw diet led to better feed conversion, provided that by-pass protein was also given (12).
In summary, therefore, we can conclude that, in order to maximize animal productivity on tropical by-product and waste feeds, small amounts of preformed by-pass nutrients, specifically protein, must also be provided, and attention must be given to the roughage characteristics of the feed.
It is important to stress that these nutrient/dietary characteristics act rather like catalysts, in that they create their effect by enhancing the rate and efficiency of microbial activity within the rumen. A theoretical example of this is given in Table 3.
TABLE 3. Effect of By-pass Protein on Efficiency of Synthesis and Total Daily Production of Rumen Microbial Protein on a Sugar-Cane-Based Diet
|Without by-pass protein||With 150 g/day by-pass protein|
|Turnover rate, times/day||1.5||2.5|
g N/kg CHO fermented
|Microbial protein, g/day||243||575|
It can be seen in this example that giving 150 g/day of by-pass protein fed to an increased production of 332 g/day of microbial protein arriving at the duodenum.