Complementary additivity of the digestibility coefficients of feed ingredients fed to juvenile greenlip abalone (Haliotis laevigata)

When least-cost feeds are formulated, it is assumed that when a range of ingredients are combined in a compound feed, they will behave as if they were fed separately, that is, it is assumed that their nutritive value is additive. This is the basis of least-cost linear computer programs. However, experiments conducted on pigs and rats have revealed that the inclusion of dietary fiber in manufactured feeds may result in a depression of the digestibility coefficients of the constituent ingredients. Thus, feeds that are formulated by adding the digestibility coefficients of each constituent ingredient, derived by the direct measurement of the digestibility of each ingredient when incorporated into base diets, may lead to an overestimation of the digestibility of nutrients in the manufactured feed. A series of isonitrogenous diets containing semolina, fishmeal, barley, lupin kernel meal, and various combinations of these ingredients were fed to juvenile greenlip abalone (Haliotis laevigata) to determine the apparent fecal digestibilities of N, amino acids, and gross energy. The digestibility coefficients of the single-ingredient diets were used to predict the digestibility of N, amino acids, and gross energy in casein in the mixed diets. The calculated digestibility values for casein did not differ significantly between diets. Thus, the apparent fecal digestibilities of N, amino acids, and gross energy of the constituent ingredients were not affected by varying ingredient combinations in the diets. Apparent fecal digestibility values for the chosen ingredients are additive when they are incorporated into compound feeds for abalone and so are good descriptors of nutritive value and can be used with confidence in diet formulations.