Digestibility of a feedstuff can be described as the

amount of feed that can be digested and absorbed by the

animal in relation to consumption. Digestibility is measured

as digestibility coefficient when it is expressed as

percentage and as digestible energy which is most often used

in warm water fishes. Knowledge of nutrient availability is

necessory for effective substitution of ingredients for

formulating low cost diets

Vijayakumaran, M

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• i . IB^^mmf^-mmmmmmifm^mm'im Proceedings of the Summer Institute in Recent Advances in Finfish and Shellfish Nutrition 11 TO 30 MAY 1987 CENTRAL MARINE FISHERIES RESEARCH INSTITUTE Dr. SALIM ALI ROAD COCHIN-682 031 i C\QQhoCC(^ f^ l LA^ — I SUM^ER INSTITUTE IN RECENT ADVANCES IN FINFISH AND SHELLFISH NUTRITION il-30 May, 1987 DIGESTIBILITY OF FEEDS IN FISH AND SHELLiFISH AND METHODS TO DETERMINE DIGESTIBILITY M. VIJAYAKUMARAN Wfedras Research Centre o£ Central Marine Fisheries Research Institute rfedras - 600 105 Digestibility of a feedstuff can be described as the amount of feed that can be digested and absorbed by the animal in relation to consumption. Digestibility is measured as digestibility coefficient when it is expressed as percentage and as digestible energy which is most often used in warm water fishes. Knowledge of nutrient availability is necessory for effective substitution of ingredients for formulating low cost diets. Factors affecting digestibility Digestive coefficients are influenced by several factors and do not remain constant for a given feedstuff or species. Some of the factors affecting digestibility are: - Nutrient ccxrposition of diet. This is a very important factor. Protein digestibility decreases with increase in carbohydrate in the diet in channel catfish, similar observation made in some other fishes also, «* Gastric evacuation time. Lack of time for coitplete digestion and absorption which may result from increased food intake may affect digestibility. r^m-iurifc oit £oc>ri oorxsunxed does not affect protein digestion in fish but may have influence on digestion of lipids and carbohydrates. - Increase in meal size reduces absorption in many fishes probably by reducing the surface area of the meal to be digested in comparison with small food particles. - Crude fibre in diet. It has protective value for other nutrients and also help in easy passage of food through the alimentary canal. * Microorganism in the diet. - Temperature. Increase in temperature increases digestibility in some fish (channel cat fish). In carp, however, no change in protein digestibility over a range of temperature that supports growth, but digestibility reduces when temperature is reduced below this range, - Species diversity Nutrient digestibility in fish and shellfish In general the digestible energy of fishes is 75-80% of the energy consumed and 96-98% of this digestible energy is metabolisable. Digestibility of proteins is affected by the presence of carbohydrates, both in quality and quantity, in fish and shellfish. In Penaeus japonicus 93% of digestion and absorption of glucose and galactose are completed in midgut but only 76-80% of sucrose and glycogen are digested and absorbed in this region. The process is completed in the hindgut. Starch digestibility is very good in warmwater fish and prawns. In common carp 85% of starch is absorbed at a dietary level of 10-60%. Most animal proteins appears to be highly digestible in fish and 9va%ms# as nuch as 99^ of pvot«iti oan l>9 digested in some fishes. Lipid digestibility in fishes varies from 70-94% depending on the nature of fat and water-temperature. Oxi-dised lipids reduces digestibility of diets but oxidised lipids supplemented to the diets do not reduce digestibility of proteins in carp. When dietary fat is low, yeeirling carp secretes endogenous fats into the lumen of the intestine and this may affect estimation of apparent digestibility. In P. japoniis;us fatty acids are easily digested when esterified. Differential absorption has also been observed for minerals. In common carp calcium absorption was more when its level in the diet was increased from 0.09 to 1.24% v^ hile phosphorus was held constant. Mineral absorption is also affected by the mineral base used in the diet fcrmulation. Determination of digestibility Determination of digestibility involves measuring total quantity of nutrients ingested and the amount of corresponding nutrients egested. Amount of nutrients ^^^ Amount of nutrients Digestibility = injested egested _^ Amoun^ of nutrients ingested True digestibility « I - (F-F^) -a'here 1 = Intake, F = Faeces egested and F- = Endogenous Tii^jtabolic f a e c e s . Endogenous materials such as secretion from within .the .intestinal, tract, sloughed epithelial .cells and other materials of metabolic origin also may occur in faeces. It is very difficult to measure such endogenous material and hence apparent digestibility is usually meaaured cather than true digestibility. Apparent digestibility can be measured by direct faecal collection method or by indirect method using inert marker or indicator which can be external (added to the diet) or internal (integral part of the diet). Ideally, an indicator should be totally indigestible and excreted at the same time as the other gut contents. It should hot reduce palatability of food and should be readily deter-mined. Digestibility is determined indirectly by calcu-lating the rate of indicator concentration in dry food and the same rate form the test food. % indicator % indicator AvN,^ v^.^ «*. ^ ^^ „^^ .•,••u^  T 4i.,. in food in faeces Apparent digestibility _ -p,p, .^n v o£ nutrient. y^ indicator % indicator in faeces in food The most popularly used indicator is chromic oxide, Cr-O . Other indicators used are Polythene markers, radio-active phosphorus (P-^), silica. Hydrolysis Resistant Organic Matter (HROM), Hydrolysis Resistant Ash (HRA), crude fibre, titanium (IV) oxide and metalic iron particles. Faeces collection is very important in digestibility estimations. Faeces should represent quantitatively the ingested residue of the food consumed. Main problem in £a<vr;osi 00.1 lection is the leaching of nutrients in water. Faeces oan T>o. collected by sacrificing the animal, by stripping the int..3ti.n«^  arvi from the rearing tank after egestion. * • o • • 1, Direct faeces collection method Materials needed; - Specially designed aquaria for faeces collection. Troughs (closed system) can Ise usdd in its absence but faeces should b« collected frequently if the latter is ned. . - Reagents and equipments required for protein, lipid and carbohydrate determination. - Hot air oven - Feeding trays, polythene tubes for siphoning out the faeces, aerators etc. Procedure The feeding trials should be conducted for 10-15 days. First the animals are to be acclimatised to the rearing conditions and to the test diet. Before the start of the experiment the animal has to be starved for one or two days for complete gut evacuation. Feeding with test diets should last at least 10 days and faeces should be collected at regular intervals, washed with little quantity of distilled water if the fish is reared in saline water, and dried in an hot air oven at SCC. After the conpletion of the experiment weigh the total faeces collected and determine the nutrients in dry faeces. Calculation - „ Apparent digestibility » ^ ^ ^°° where I = Nutrient intake and F = Paeoal nutrient • • 6 • • 2. Chromic oxide indicator method / The escperimental design is same as in direct faeces collection method. The test animals should be fed with the test diet without chromic oxide for about a week before start of the ejqjariment. Determination of chromic oxide content The method involves oxidation of organic material in the Sample using concentrated nitric acid followed by oxi-dation of insoluble (green) chromium III in chromic oxide to soluble (yellow) chromium IV, which is then determined spectrophotometrically. Reagents - Cone, Nitric acid and Perchloric acid Apparatus •- Kjeldahl digestion tubes - Heating mantles - Gloves and Goggles for protection against acid fumes - 5 ml graduated pippette with filling bulbs - 100 ml volumetric flasks - Spectrophotometer - Cuvettes. Procedure 1. Weigh accurately 50-100 mg of sample into a piece of foil 2. Transfer sample carefully into a numbered Kjeldahl flask and reweigh the foil. 3. Add 5 ml cone. HNO_ washing the sample down from the i^J--3 of the flask. • • / • • 4. Place the flask on a heating mantle and boil gently for 20 minutes. Do not allow to boil it dry, if it is add more HNO^. 5. Turn off the mantle and allow to cool, 6« Add three ml o£ perchlorle acid to fi«sk vXth cmro* 7. Boil in the mantle until colour changes from green to yellow orange or brown and continue to boil for a further 10 minutes. 8. Allow the flasks to cool - if the colour reverts to green it must be reboiled. 9. Cool the flask to room tenperature and then carefully wash the contents into a 100 ml volumetric flask v/ith distilled water and make upto 100 ml. 10, Transfer an aliquote to a spectrophotometer cuvette and read optical density at 350 nm with distilled water as blank. Calculation - 1 Weight of sanple = A .^ . jj J ,^_ , OD of sarrple Concentration Chromxc oxide in the sample « ^ ^ ^^ standard*=X OD of standard X % Chromic oxide in sample = — x 100 A * Known concentration of chromic oxide should be treated in the same way as the sample for standardisation. Calculation - 2 Weight of aajpple « Amg Optical density « Y Y-0.0032 Weight of chromic oxide « ' ' • • • in sample (mg) 0,2089 X % chromic oxide in sample » — x 100 A ..8 REFERENCES Clive Talbot 1985. Laboratory methods in fish feeding. Xnt P^ter, Tytler and Peter CalCfW (Eds) Fish Energetics, New Perspectives, Croomhelm, London. Furukawa, A. and H. Tusukahara 1966. On the acid digestion method for the determination of chromic oxide as an index substance in the study of digestibility in fish feed. Bull. Jap. Soc. Scl. Fish,, gg: 502-508. Leonard A Maynard and J.K. Loosli 1971. Animal Nutrition McGraw Hill, London, National Research Council 1983. Nutrient requirements of warm water fishes and shellfishes. National Academy Press, Washington D.C. 

Digestibility of feeds in fish and shellfish and methods to determine digestibility

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Digestibility of feeds in fish and shellfish and methods to determine digestibility

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