THE EFFECT OF DIFFERENT PROCESSING TREATMENTS ON SOYBEAN-MEAL UTILIZATION BY RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

A 12-week feeding trial was conducted with 38 g rainbow trout to evaluate the effect of 20% dietary protein replacement of brown fish meal in a control diet (diet C) by commercially full-fat soybean meal (diet S1 ), full-fat extruded soybean meal (diet S2), solvent-extracted soybean meal (diet S3), and solvent-extracted soybean meal, treated by infrared radiation (diet S4). In a separate trial the apparent digestibility of the diets was measured in triplicate groups of rainbow trout, using an automatic faeces collector. Growth rate, feed conversion ratio and nitrogen retention (%Nl) of fish fed diets S1, S3 and S4 were better than in fish fed S2 and the diet based on brown fish meal. The poorest results were observed in trout fed the diet S2. Energy retention (%El) was not significantly different among the dietary treatments. At the end of the trial there were no significant differences in body composition, hepatosomatic and visceral indexes among experimental groups. Apparent protein digestibility coefficients of the experimental diets were significantly (P<0.05) higher than that of the control diet. There were no differences among experimental groups in apparent dry matter and energy digestibilities, except in trout fed diet S2 which had an apparent energy digestibility significantly lower than in trout fed the fish meal based diet. It is concluded that at the 20% dietary replacement level of brown fish meal by soybean meal improves the growth performance of rainbow trout. However, the commercially extruded full-fat soybean meal did not seem to improve the nutritional value of this plant protein source for rainbow trout.