Digestible Energy and Carbohydrates in the Nutrition of Tilapia (Oreochromis niloticus Linn.)
The apparent digestibilities of a series of tropical feeds were evaluated in feeding trials with tilapia. A standardised method of measuring feed digestibility is reported and microassays are described which enable analysis of energy, nitrogen and chromium in the small faecal samples (c. 150 mg) obtainable from individual fish. From regressions of the digestibility data on chemical measures of feed composition, several statistical models were produced which can be used to predict the digestible energy (DE) of plant-product feeds to an accuracy of +1.5 MJ/kg DM. The single most accurate predictor of DE was neutral detergent fibre, although this was only applicable to certain feed types. For a wider range of feeds, DE was more accurately predicted by multiple regression equations which included measures of dietary protein, available carbohydrate and fibre. Factors affecting feed digestibility were also investigated. With soybean meal, it was shown that DE was significantly greater when this feed was given to tilapia as a component of a complete diet than when it was given on its own. DE was also significantly greater after 15 weeks of feeding than after 1 week. However, the DE of soybean was not affected by its level in a diet over a range fron 20% to 60% and there was no interaction between the effects of inclusion level and feeding duration. Soybean protein digestibility was independent of inclusion level and feeding duration. DE values of carbohydrate-rich feeds were generally higher than corresponding values for other species of farmed fish. In growth trials, the carbohydrates in these plant-product feeds were shown to be effectively utilised to spare dietary protein energy for growth, and it was demonstrated that the maximum tolerable level of dietary carbohydrate was at least 40%. The energy value of different carbohydrates was related to their level in a diet and the number of saccharide units in the molecule. Thus, at low dietary inclusion (10%), glucose spared more protein energy in diets than sucrose, dextrin or starch. At higher levels (40%), the situation reversed and dextrin spared more protein than the lower molecular weight carbohydrate. At all inclusion levels, dextrin was a more available source of energy than starch. Increasing levels of dietary fibre depressed DE, food utilisation, carcass fat and growth, and it is recommended that fibre levels should not exceed 10% in practical diets for tilapia.
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