A comparison of online phytase pre-treatment of vegetable feed ingredients and phytase coating in diets for Atlantic salmon (Salmo salar L.) reared in cold water

In this trial with Atlantic salmon (initial weight, 142 g) raised in 8 °C seawater, the effect of coating or online pre-treatment with phytase was investigated. Four diets were produced: one fishmeal (FM) diet and three diets — SPC, SPC-P and SPC-PPT — in which 60% of the protein from fishmeal was replaced by soy protein concentrate. The SPC diet did not include phytase; the SPC-P diet was coated with phytase; and in the SPC-PPT diet the vegetable ingredients were incubated with phytase online during feed production. In the latter two diets, phytase was added at a level of 2900 phytase units (FTU) per kg of wheat and soy protein concentrate. Each diet was fed to three groups of 55 fish for 76 days. Growth was not significantly affected by the dietary treatments. The apparent digestibility of protein did not differ significantly, and most of the protein was digested in the pyloric intestine. The digestibility of ash and phosphorus (P) was significantly higher for the pre-treated SPC-PPT diet than for the SPC and the SPC-P diets. The concentration of phosphate in plasma was significantly higher for the FM diet than for the other diets, while fish fed the SPC-PPT diet had a significantly higher concentration of plasma phosphate compared with the SPC and the SPC-P diets. The content of ash, P, calcium (Ca), magnesium (Mg) and zinc (Zn) in whole fish and in vertebrae was generally higher in salmon fed the FM diet than in those fed the three SPC diets. The SPC-PPT diet resulted in a significantly higher content of ash, P and Mg in vertebra than the SPC and the SPC-P diets. In conclusion, online phytase pre-treatment was efficient in improving mineral availability, while phytase coating was inefficient for salmon reared at 8 °C.