Comparison of white flakes and toasted soybean meal partly replacing fish meal as protein source in extruded feed for rainbow trout (Oncorhynchus mykiss)

An experiment was carried out to study the nutritional value of solvent extracted, toasted soybean meal (SBM) and solvent extracted, untoasted soy “white flakes” (WF) partly replacing high-quality fish meal (FM) in extruded diets for rainbow trout. Particularly, we wanted to find out if feed extrusion provides sufficient heat to reduce the soybean trypsin inhibitor activity (TIA) to a tolerable level in diets for trout. Three diets were produced: a control diet with 490 g FM kg− 1 (FM diet), a diet with 290 g FM and 250 g SBM kg− 1 (SBM diet), and a diet with 290 g FM and 250 g WF kg− 1 (WF diet). Each diet was fed to three groups of trout for 63 days. The TIA levels in the SBM and WF diets after extrusion were 0.6 and 2.1 mg (g dry matter)− 1, respectively. Feed intake was lower (P < 0.05) in trout fed the SBM and WF diets than in those fed the FM diet, resulting in reduced growth and poorer feed conversion. The diets did not significantly affect whole-body content of dry matter, protein and fat, or carcass yield, total gut weight, or weight of pyloric, mid and distal intestine. Trout fed the WF diet had higher stomach weight than trout fed the FM diet. Liver weight, plasma cholesterol and triacylglycerol were lower in trout fed the SBM and WF diets than in those fed the FM diet. Apparent digestibility of nitrogen was lower for the WF diet than for the FM diet and intermediate for the SBM diet. The SBM and WF diets caused lower digestibility of fat and most amino acids and lower nitrogen retention than the FM diet. Moderate morphological abnormalities were found in the distal intestine of trout fed the SBM and WF diets. Bile acid concentration in chyme was lower in fish fed the SBM and WF diets than in fish fed the FM diet, except for the proximal pyloric region and the posterior distal intestine. Trypsin activity in chyme of the distal intestine was higher and leucine amino peptidase activity in intestinal wall tissue was lower in trout fed the SBM and WF diets than in trout fed the FM diet. In conclusion, both SBM and WF had a negative influence on growth and feed conversion compared with the FM diet. Extrusion did not influence soy-related challenges such as reduced fat digestibility, and morphological and enzymatic changes. However, extrusion was sufficient to lower TIA in the diets to an acceptable level for rainbow trout.