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 by soya-derived protein in large rainbow trout fed practical, high-energy diets with and without supplemental phytase. A 2 × 2 factorial arrangement with two soya levels (0 and 69.4% of the dietary protein from soyabeans) and two phytase levels (0 and 1200 U/kg) was used. Soya protein was derived from soya protein concentrate (SPC) and soyabean meal (SBM) with a SPC:SBM protein ratio of 4:1. Diets were formulated to contain 36 and 28% CP and fat, respectively, and supplemented with lysine and methionine, but not with P. Consequently, dietary P contents were 10.5 and 6.9 g/kg for diets without and with soya-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, soya-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 conversion efficiency, which averaged 0.90. Percentage bone ash was significantly lower in fish fed soya 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 soya without supplemental phytase. Phytase supplementation did not significantly increase bone ash of fish fed soya diets. P load significantly decreased from 8.5 to 4.6 g/kg weight gain due to the partial replacement of FM protein for soya protein. Algal availability of P was lower with soya-based diet than with FM-based diet (23 vs. 35%), and lower in faecal matter of fish fed soya-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 soya proteins for low-pollution diets, without compromising weight gain or feed efficiency in large rainbow trout fed practical diets.