Influence of dietary soy and phytase levels on performance and body composition of large rainbow trout (Oncorhynchus mykiss) and algal availability of phosphorus load

A feeding trial was designed to evaluate the influence of partial replacement of fish meal (FM) protein for soy-derived protein in large rainbow trout fed practical, high-energy diets with and without supplemental phytase, A 2 x 2 factorial arrangement with two soy levels (0% and 69.4% of the dietary protein from soybeans) and two phytase levels (0 and 1200 U kg(-1)) was used. Soy protein was derived from soy protein concentrate (SPC) and soybean meal (SBM) with a SPC:SBM protein ratio of 4:1. Diets were formulated to contain 36% and 28% crude protein and fat, respectively, and supplemented with lysine and methionine, but not with phosphorus (P). Consequently, the dietary P contents were 10.5 and 6.9 g P kg(-1) for diets without and with soy-derived proteins, respectively. Three replicate groups of fish per treatment were hand-fed once daily to apparent satiety for 24 weeks. Fish grew from 0.25 kg to an average of 2.02 kg, with soy-fed fish being significantly larger at the end of the trial. Phytase had no influence on weight gain of fish. Dietary treatments did not affect feed efficiency, which averaged 0.90, Percent bone ash was statistically significantly lower in fish fed soy diets than in fish fed FM diets (means 56.4 and 57.7), but weight performance and whole body composition analyses did not confirm the modest P deficiency in fish fed soy without supplemental phytase, Phytase supplementation did not significantly increase bone ash of fish fed soy diets. P load significantly decreased from 8.5 to 4.6 g P kg(-1) weight gain due to the partial replacement of FM protein for soy protein. Algal availability of P was lower with soy-based diet than with FM-based diet (23% vs. 35%), and lower in fecal matter of fish fed soy-based diets than in fish fed FM-based diets (9% vs. 27%). Results from this study show that a significant part of FM can be replaced by soy proteins for low-pollution diets, without compromising weight gain or feed efficiency in large rainbow trout fed practical diets. (C) 2000 Elsevier Science B.V. All rights reserved.