Digestibility of diets containing different soybean meals in Atlantic cod (Gadus morhua);: comparison of collection methods and mapping of digestibility in different sections of the gastrointestinal tract

The purpose of the experiment was to evaluate effects of partial replacement of fish meal with soybean meal on digestibility in Atlantic cod and effects of biotechnological processing of soybean meal. Other objectives were to find out if apparent digestibility in cod was affected by age, temperature and/or faecal sampling methods, and to map digestion and absorption patterns along the intestinal tract for different age classes and diets. A control diet with fish meal as the main protein source (FM), a diet with 24% of crude protein from conventional extracted soybean meal (SBM), and a diet with 24% of crude protein from a bioprocessed soybean meal (BP-SBM) were used. Yttrium oxide was added as an inert marker for digestibility determination. The diets were fed to duplicate groups of 534 g (1-year-old) and 1750 g (2-year-old) cod in seawater. Faeces were stripped from the distal-most section (distal chamber) of the intestine after 48 and 84 days of feeding. In addition, intestinal contents were removed by dissection after 84 days. The dissected intestine was divided into 5 sections and contents were analysed separately. Dietary soy did not alter the viscosity of the digesta. The replacement of fish meal protein with soybean protein reduced the apparent digestibility of crude protein, fat, energy and almost all amino acids, independent of type of soybean meal processing. Larger fish had higher digestibility of starch, ash and several amino acids than smaller fish. A temperature decrease from 9.1 degrees C at feeding day 48 to 6.8 degrees C at day 84 resulted in lower digestibility of starch, ash and several amino acids. Absorption of amino acids was quantitatively terminated in the distal part of the mid intestine when feeding the soy-containing diets, but continued into the distal chamber when feeding the FM diet. Absorption of fat was completed in the proximal part of the mid intestine when feeding the soy-containing diets, but continued into the middle part of the mid intestine when feeding the FM diet. Soy did not affect absorption pattern of starch along the intestinal tract. Faecal sampling by dissection resulted in higher apparent digestibility of macronutrients than stripping in the 2-year-old cod, and higher apparent digestibility of protein in the 1-year olds. (c) 2006 Elsevier B.V. All rights reserved.