Apparent digestibility coefficients of nutrients in brewer's and rendered animal by-products for rainbow trout (Oncorhynchus mykiss (Walbaum))

Two experiments were conducted to measure the apparent digestibility coefficients (ADCs) of nutrients, dry matter, and energy in by-products of the brewing industry and in selected animal protein ingredients for rainbow trout. In experiment 1, 500 rainbow trout (average body weight 170.8+/-5.5 g) were stocked in ten 140-L digestibility tanks with 50 fish per tank and two tanks per diet. Yttrium oxide was used as an inert marker in the diets. The high fibre content of brewer's dried grains (BDG) affected the ADCs of dry matter but not of protein or amino acids. Brewer's dried yeast had a higher protein content than BDG, but ADC values for protein and amino acids were significantly lower. The ADCs of phosphorus were similar among brewer's dried products. The BDG high-protein, fraction had marginally higher ADC values for dry matter, protein and amino acids than regular BDG. Conditions in experiment 2 were similar to those in experiment 1. ADC values for spray-dried porcine plasma were over 98% for dry matter, crude protein, crude fat, and gross energy. ADC values for spent hen meal were higher than those of poultry by-product meal or feather meal, with the exception of gross energy. However, spent hen meal was unpalatable. The ADCs in these ingredients were variable, and this variability must be taken into account when these ingredients are formulated into feeds for fish.