Effects of extrusion and expelling processing, and microbial phytase supplementation on apparent digestibility coefficients of nutrients in full-fat soybeans for rainbow trout (Oncorhynchus mykiss)

An experiment was conducted to evaluate the effects of extrusion and expelling processing, and microbial phytase supplementation on apparent digestibility coefficients (ADCs) of dry matter, crude protein (CP), amino acids, and minerals in full-fat soybeans (FFSB) for rainbow trout. Raw, extruded, and expelled soybeans were used. Microbial phytase (Natuphos 5000 l) was added to extruded soybeans at 200, 400, 600, 800, and 1000 FTU/kg diet, and to expelled soybeans at 200 FTU/kg diet. They were each mixed with a casein-gelatin reference diet at a 30:70 ratio to determine their respective ADCs. Nine diets were made and a total of 900 fish (initial mean body weight 170.8 +/- 5.5 g) were stocked into 18 150-l tanks with 50 fish per tank. Two tanks were assigned randomly to each diet. Fish were fed their respective diets for 1 week before fecal collection began. Fecal collection by hand-stripping was performed every 3 days for three collections. Yttrium oxide was used as inert marker. Results showed that extrusion processing increased ADCs of CP and sulfur but reduced ADCs of magnesium and total-phosphorus significantly compared to ADCs in raw soybeans (P < 0.05). Phytase supplementation in extruded soybeans increased ADCs of magnesium, total-phosphorus, phytate-phosphorus, manganese, and zinc significantly (P < 0.05). Phytase supplementation in expelled soybeans increased ADCs of CP, sulfur, total-phosphorus, and phytate-phosphorus significantly compared to ADCs in raw soybeans (P < 0.05). The optimum dosage of phytase used to effectively releasing phosphorus and other minerals in extruded full-fat soybeans is approximately 400 FTU/kg diet for rainbow trout. (C) 2003 Elsevier Science B.V. All rights reserved.