Low fishmeal diets for Atlantic Salmon, Salmo salar L.,using soy protein concentrate treated with graded levels of phytase

The experiment aimed at determining the efficient use of phytase (Phy) in Atlantic salmon diets that had low (4.5%) fishmeal and contained 60% soy protein concentrate (SPC). Phytase was either included at 250, 500, 1,000 or 4,000 U Phy kg-1 diet or the SPC was pre-treated prior to making diets using 250, 500 or 1,000 U Phy kg-1 SPC. Fish were fed the experimental diets for 12 weeks, and there were no differences in survival among treatments nor were there differences in growth performance between the phytasepre- treated SPC diets. Feed intake and weight gain were significantly lower for diets supplemented below 1,000 U Phy kg-1 compared to all other diets. Apparent digestibility (AD) of phosphorus was significantly lower without the use of phytase (45.43 +- 2.06%) than for all other treatments. AD phosphorus increased from 55.70 +- 1.81% at the lowest phytase supplementation (250 U Phy kg-1) to 80.87 +- 2.12% at the highest (4,000 U Phy kg-1). There was no difference in AD phosphorus between the diet with the highest supplementation (4,000 UPhy kg-1) and the pre-treated diets. There were no differences in whole-body dry material, crude protein or total lipid, whereas bone ash was significantly lower for diets supplemented below 1,000 U Phy kg-1. Ash and phosphorus in the whole body and bone increased with increasing added phytase. At and above an inclusion of 1,000 U Phy kg-1, bone ash (51.26 +- 0.12% bone weight) and bone phosphorus (11.21 +- 0.04% bone weight) reached concentrations that were no different to the pre-treated diets. In conclusion, phytase improved Atlantic salmon's growth performance fed low fishmeal diets containing SPC, and at least 1,000 U Phy kg-1 diet was required to have the same effect as pre-treatment of SPC with 250 U Phy kg-1 SPC.